DENDRONIZED POLYMERS

Introduction

Practically all dendrimers (1–5) known today have cores with a few functional
groups to which the corresponding number of dendrons (dendritic wedges) are
attached. The fact that these dendrons are connected to one another by a small,
almost dot-like molecule results in considerable steric congestion around the core.
This congestion is a unique structural feature and has led one to view dendrimers,
specifically those of high generation, as molecular boxes or containers (6,7) and
as entities that assume a spherical shape to which a “surface” can be assigned.
The term surface may, of course, only be applied with care. Since their discovery
some 20 years ago, dendritic macromolecules have stimulated an almost explo-
sive research effort and many synthetic, analytical, and application-related issues
have been addressed (1–5). Even industrially applicable syntheses were developed
(8–11). During this stormy process, research has virtually exclusively focused on
dendrimers with small cores, in spite of a U.S. patent entitled “Rod-like Den-
drimer” in 1987 (12,13). There the dendrimers are like (1) with polymeric cores
and are proposed as being useful in the production of molecular composites and
as crystallinity modifiers for polymeric materials. It took some years more be-
fore first (published) steps were undertaken to obtain these dendrimers (14,15).
It is immediately apparent that they not only complement dendrimers with small
cores under structural aspects but as a consequence of the structural differences
they should also have unique properties. A simple reason for this slow develop-
ment may be that in the beginning of dendrimer research spherical dendrimers
were simply considered a more appropriate and perhaps more important target.
As judged by the increasing number of publications, this view is presently under-
going a rapid modification. Another reason may be seen in a reluctance to begin

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working on a seemingly more complex area than small core dendrimers where
chemists are already being confronted with considerable synthetic and analytical
difficulty.

This article tries to draw a comprehensive picture of what has been done

in the field of dendrimers with polymeric cores, putting emphasis on synthetic
issues, on accurate molecular weight determination, and, finally, on the aggrega-
tion behavior of these intriguing macromolecules both in the bulk and at solid
and fluid interfaces. First attempts to manipulate individualized dendronized
polymers on surfaces are also described. The article starts with a small chap-
ter summarizing some of the ideas why these make up a challenging project
and where these unusual molecules are expected to have impact on the natural
sciences.

The macromolecules treated here may be considered as either dendrimers

with polymeric cores or alternatively dendronized polymers (or polymers with ap-
pendent dendrons) depending on whether one sees them from the vantage point
of an organic or macromolecular chemist (Fig. 1). The first view is somewhat puz-
zling, because dendrimers are normally considered monodisperse, which they cer-
tainly are not (16), whereas polymeric dendrimers are intrinsically polydisperse.
At best the polymeric core may have a narrow molecular weight distribution. Not
considered here are polymers which do not carry dendrons at every repeat unit
(r.u.) but at a few ones only, eg, at both termini (17–22), as well as works on

Fig. 1.

Design of a dendronized polymer whose dendritic layer is so densely packed around

the backbone that it is stretched out and a molecular cylinder with a “surface” is generated.

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137

surface coatings with dendrons or hyperbranched molecules (23–28). The refer-
ences to dendrimers with dot-like cores are kept at a minimum and the reader is
referred to pertinent reviews, monographs (1–5,29), and other articles. Reviews on
dendronized polymers are available (30–32). Attempts to stiffen polymer chains
by attaching flexible, straight oligomers to each repeat unit which lead to the in-
teresting class of bottle-brush-like polymers (33–41) are not covered in this article
because of space limitations.

Dendronized Polymers

Dendronized polymers are formally comb polymers (42), with the combs’ teeth
being dendrons. What makes them so unique that one does not normally refer to
them as such? Mainly it is the complex interplay of dendron size, distance between
dendrons, and backbone flexibility/rigidity, together with several interactions (eg,
dendron/dendron, dendron/backbone, solvent/dendron). All these factors sum up to
a slower decrease of density on going from the interior to the exterior than in com-
mon comb polymers where the teeth are linear chains. This unusual density pro-
file, which for spherically shaped dendrimers has been controversially discussed
(43–46), leads to new and rather fascinating properties of otherwise conventional
polymers. The first issue to mention here is their shape and its dependence on
the substitution with dendrons. Depending on the dendrons’ structure, size, and
attachment density along the backbone, conventional polymer backbones such as
polyacrylate or polystyrene can attain conformations all the way from random-coil
to fully stretched linear: A flexible, cooked spaghetti-like polymer can be converted
into a rigid (high bending modulus) rod just by proper substitution with large den-
drons. This stiffening of the backbone is caused by steric repulsion between the
pendent dendrons. For this reason the whole matter is referred to as shape-control
by implementation of steric strain. In this sense steric strain may be compared
to hydrogen bonding and

π,π-stacking as the main shape-determining factors in

proteins or DNA. In the extreme case, which will be described later, the dendritic
layer around the polymer backbone is so dense that a macromolecule can turn
into a molecular cylinder with defined dimensions: Its length is determined by
the degree of polymerization and its diameter is roughly two times the dendron
extension. In this case the dendrons spread away from each other to keep steric
repulsion between them at a minimum.

Dendronized polymers can, however, also be designed so that there is an at-

tractive interaction between dendrons that leads to shape control. This requires
two features: (1) dendrons with mesogenic properties and (2) their loose attach-
ment to the backbone which leaves the dendrons sufficient freedom to find the
optimum packing. This kind of shape control driven by the dendrons’ mesogenic
properties can lead to highly ordered materials in the solid state as will be de-
scribed later.

Let us come back to rigid polymers with a tightly packed dendritic layer

around the backbone. What is the importance of such cylindrical objects (Fig. 2
and 3). Contrary to conventional polymers, their diameter is of the order of a
few nanometers rather than tenths of a nanometer. As a consequence, the rigid-
ity is so high that the persistence length of an individual chain can be a few tens of

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(a)

(b)

(c)

(d)

rub

anneal

push & react

bend

Fig. 2.

Characterization, manipulation, and assembly of rigid dendronized polymers: (a)

determination of bending moduli of individual macromolecules in dependence of factors
like nature of backbone, generation of the attached dendrons, and the dendrons’ molecular
structure; (b) manipulation of individualized macromolecules on surfaces in order to use
them for covalent nanoconstructions, eg, by inducing linkages photochemically; (C) Self-
assembly of ordered arrays; and (d) generation of large oriented areas by rubbing or one-
dimensional flow for applications, eg, in polarizers, polarized emitters, or as templates in
LC-applications.

(a)

(b)

(c)

(d)

M

M

M

M

M

M

M

M

M

M

M

M

M

M

M

M

M

DNA

e

−

e

−

−

+

Fig. 3.

Further potential applications of rigid dendronized polymers: (a) Use of conjugated

polymers with dendritic shells for photoluminescence or charge carrier transport. The black
line indicates a conjugated backbone. (b) Usage as nanometer-sized supports for catalysis
when surface charged with catalytically active centers (m). (c) Generation of hybrid ma-
terials by, eg, wrapping of positively charged dendronized polymers with DNA, which is
a negatively charged polyelectrolyte. (d) Amphiphiles with a lengthwise segregation of
the polar and nonpolar domains (left) should possibly self-aggregate into supercylinders
(right) which during growth can accept amphiphiles of different lengths and nevertheless
be monodisperse regarding the diameter.

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139

nanometers. Thus, exceptionally high bending moduli can be expected, despite
the fact that the polymers are still single stranded (Fig. 2a). Dendritic rods could
therefore potentially be used for constructions on the nanometer scale very much
like an architect builds frames for houses, bridges, etc from plywood (Fig. 2b).
The first steps in this direction would be to isolate individual rods, deter-
mine their dimensions, and move them about. The high modulus results in a
strong tendency of these rods to form large ordered two-dimensional arrays in
which the individual rods are packed parallely to each other like tree trunks of a
raft, the only difference being that the dendritic trunks have a length distribution
(Fig. 2c). Please note that the designs in this figure do not account for this. Rub-
bing or one dimensional flow may be used to orient the rods over large areas
(Fig. 2d). Such large parallel-ordered arrays of nanometer scale objects on surfaces
are interesting for a number of applications, eg, as polarizers, polarized emitters,
or orienting surfaces in liquid crystal displays. Shielding comes into play when the
backbones are electrically conducting or when they fluoresce (Fig. 3a). In the for-
mer case dendritic cylinders can be used to contribute to the important question
of charge transport along an individual molecular wire. Because of their rigid-
ity and the mere size, single cylinders could be placed between nano electrodes,
the current be measured and correlated with one and only one molecular wire
(Fig. 3a). The dendritic layer would just act as an insulating shell which, at the
same time, provides some mechanical stiffness, a relevant issue for device fabrica-
tion. Insulation by dendritic layers can also help prevent fluorescence quenching
in bulk phase, since conjugated backbones have a strong tendency to aggregate,
which almost automatically results in quenching. Indeed, this spoils the potential
utility of conjugated polymers as light emitters. Finally, the dendritic layer could
possibly be used to harvest and channel energy to the backbone.

Apropos tree trunks and cylindrical surface: This may also be a starting point

to think about dendritic rods as supports with defined curvatures to which cat-
alytically active components could be attached (Fig. 3b). In a way, cylinders of
the size discussed here bridge the gap between homogeneous and heterogeneous
catalysis. They may combine the respective advantages of both. Attachment of cat-
alytically active groups to the surface at more or less constant distances may well
provide candidates, eg, for applications in flow reactors. Spherically shaped den-
drimers equipped with transition-metal complexes have already been successfully
employed for such purposes (47–50).

The fact that the tobacco mosaic virus also has a cylindrical shape at

first glance may appear as a somewhat artificial link to the biosciences, but
this striking similarity may nevertheless help initiate thought about the poten-
tial biological importance of dendritic rods and derivatives thereof. Spherically
shaped dendrimers have already been used as gene vectors. Positively charged
poly(amido amine) (PAMAM) dendrimers (51), for example, render DNA more
compact through coulombic interactions, which facilitates cell membrane trans-
fection (52,53). First experiments indicate a similar aggregation behavior between
the cylinders’ positive surface charges and the negatively charged phosphates of
DNA (Fig. 3c). This could influence endocytosis and endosome formation, which
are important matters during transfection.

Another important application of dendritic cylinders is as amphiphiles. Com-

mon amphiphiles like dodecylsulfonate can form micelles, bilayers, and other

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interesting and sometimes even commercially relevant supramolecular aggre-
gates (54). These amphiphiles normally are a few nanometers long and have di-
ameters of a few tenths of a nanometer. If it were possible to decorate the surface
of a dendronized polymer with one hydrophilic and one hydrophobic block seg-
regated perpendicularly to the backbone, giant amphiphiles would result, which
may lead to a big jump in the size of the corresponding aggregates. Alternatively,
if it were possible to decorate the cylinders not cross- but lengthwise, the result-
ing amphiphiles (Fig. 3d) would have the unique polarity distribution of some
ion channel membrane proteins and could be used for coatings to switch surface
polarities to their opposite, or for various applications as colloids. These latter
amphiphiles should also exhibit a rather unique aggregation behavior in which
a defined number of linear sequences of the individual amphiphilic constituents
(Fig. 3d, left), regardless of their respective chain length, form an indefinitely long
super cylinder with monodisperse diameter (Fig. 3d, right). Such aggregates would
exhibit an unprecedented mechanical stiffness when compared with the many
known cylindrical self-aggregates from small (tenth of nanometer) amphiphiles
(55).

Last but not least, the issues of synthesis and structural analysis ought

to be mentioned. Before entering into all the exciting options mentioned above,
dendronized polymers need to be synthesized and sufficiently characterized on
both the organic chemistry and polymer level. Without going into details the main
synthetic problems include (1) steric repulsion and thus incomplete reactions, (2)
autocatalytic decomposition of sensitive dendrons because of their exceptionally
high functional group density, (3) high molar mass and sometimes high molar mass
difference of reactants which makes it difficult to meet correct stoichiometry, and
(4) purification of high molar mass reactants. These factors may appear almost
trivial but can brew together to a seething mixture which renders a controlled
synthesis quite an endeavor and challenge. Additionally, the size of the molecules
involved makes the tools of organic chemistry absolutely insufficient. Additional
methods like small-angle neutron scattering (sans), scanning force microscopy
(sfm), x-ray diffraction (qv), matrix-assisted laser desorption ionization time of
flight (MALDI-TOF) mass spectrometry need to be employed to really prove what
one proposes to have.

Synthesis

There are two principally different synthetic routes to dendronized polymers
(Fig. 4). In the first, the polymer which becomes the core in the final product
serves as starting material. Its anchor groups are used to convergently attach a
dense sequence of dendrons (attach-to route, route A). In the second, monomers
already carrying dendrons are subjected to polymerization or polycondensation
(macromonomer route, route B). The intrinsic problems of both routes become
more serious and limiting, the more sterically demanding the dendrons are. An
obviously critical issue associated with route A is achieving complete coverage of
the backbone anchor groups with dendrons. Even if a very efficient coupling chem-
istry is available, a large dendron excess may be required to drive the reaction
to completion. This excess may, in turn, make it difficult to purify the product. If

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141

Fig. 4.

Routes A and B (a) and C (b) to dendronized polymers. They may be referred to

as attach-to route (A), macromonomer route (B), and mixed route (C). The dendrons shown
are of generation 3 (routes A and B) and generation 2 and 4 respectively (route C).

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large dendrons are to be attached, steric hindrance also comes into play for two
reasons: (1) the shielding of unreacted anchor groups on the backbone by dendrons
already attached in close proximity and (2) the dendron’s conformation, which may
lead to a self-shielding of the functional group at the focal point through which
the attachment ought to take place. Both factors lead to a decrease in the rate of
attachment, if they do not suppress the process entirely. This decrease of rate can
favor side reactions which may not just involve themselves or the solvent but also
the already attached dendrons.

The problems associated with route B also have something to do with steric

hindrance. Here the critical point is the steric demand of both monomer and chain
end. Incoming monomer will only be connected to the chain end if steric hindrance
is not too high. Otherwise this process will be slowed or even rendered impossible.
Depending on the kind of polyreaction applied, this may lead to termination of
the reactive chain end and/or to side reactions of the monomer, like loss of cou-
pling functionality as in some polycondensations or auto-initiation specifically in
radical polymerizations. From this discussion it can be extracted that the basic
problems for both routes are incomplete coverage (route A) and low molecular
weight dendronized polymer (route B). Of course, intermediate solutions may
also be applicable and have actually been developed to some ripeness during
the last 3–4 years. If for some reason, eg, a fourth generation macromonomer
does not polymerize (which in fact is normally the case) and the alternative at-
tempt to attach a G4 dendron to a polymer also does not work (which is so in all
reported cases), route C may offer the solution. Here a polymer which already
carries second or third generation dendrons is converted to a higher generation
one by attaching a second or first generation dendron. This of course requires the
existence of functional groups at the surface of the starting polymer and the avail-
ability of a very efficient attachment chemistry whose efficiency must be quanti
fiable.

Besides synthetic hurdles there are also analytical ones. Dendronized poly-

mers tend to have repeat units with considerable molecular weight. Repeat units
with 1, 2, or even 3 kDa are no exception. Such high molar masses sometimes
render structural characterization difficult because the proportion of backbone to
dendron atoms becomes so unfavorable that nmr spectroscopy reaches its limits.
For example, sometimes the degree of attachment (coverage) simply cannot be
determined with sufficient accuracy because the signal intensity of the unreacted
anchor groups is too weak for comparison with reference signals in the spectrum.
The nmr characterization may occasionally be further complicated by large differ-
ences in the relaxation times of backbone and dendron nuclei. Thus, nmr signal
integrals are rendered unreliable if a sufficient pulse delay time is not applied.
Another problem with dendronized polymers is their molar mass determination.
Gel permeation chromatography (gpc) is a quick and easy method to roughly es-
timate the molar mass of a polymer (56). This estimation can only be reason-
ably used as long as the hydrodynamic volumina of the polymer under consid-
eration and the polymer used for calibration purposes, typically polystyrene, are
in the same range. The hydrodynamic volume of dendronized polymers strongly
deviates, however, from parent polystyrene, and gpc results should be treated
with care. Additionally, facile aggregation of these dendrimers is sometimes en-
countered, which leads to further complications. Other methods of molar mass

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143

determination like light scattering have to be used and measures taken to avoid
aggregation.

Looking back over what has happened regarding synthesis of dendronized

polymers since 1992, when work (14,15) began in this field, one has to arrive
at the conclusion that the attach-to route (route A) is inferior to the alternative
macromonomer approach (route B). There is no reported route A case in which a
complete coverage of a polymeric core with dendrons of higher generation than
G2 was achieved. Additionally the polymeric cores used [mostly of the poly(para-
phenylene) (PPP)-type] did not have very high molar masses. For route B, however,
many high molar mass G2 cases, a few G3 ones, and a low molar mass G4 case
are known. Furthermore, a high molar mass G4 polymer was reported recently.
Since the mixed solution (route C) also has proven to be a successful alternative
in certain situations, this article concentrates on route B and gives some insight
into what has been accomplished along the lines of route C. For a treatment of
route A representatives, however, the reader is referred to a pertinent review
(30).

Overview of Macromonomer Approach.

Synthesis of dendronized

polymers from macromonomers (route B) has been developed broadly in recent
years. A reason for this is the advantage that polymers, if they can be obtained
at all by this method, necessarily carry the predetermined number of dendrons
at the backbone. The above questions of dendron perfection and coverage degree
in divergent and convergent syntheses do not play a role. This only holds true,
of course, if the dendrons are compatible with the polymerization conditions and
do not degrade, a prerequisite which is fulfilled in many of the cases reported.
The polymerization procedures used may be divided into (a) radically initiated
and transition-metal catalyzed chain-growth and (b) step-growth polymerizations
(polycondensations). Figure 5 gives an overview of the macromonomers’ structures
reported to date and groups them into the subclasses a and b depending on the
respective type of polyreaction. Fr´echet-type dendrons (57) are drawn in an abbre-
viated form, which is explained in Figure 6. Monomers are polymerized by radical
polymerization [2 (15), 3 (58), 4 (59), 5 (60), 6 (61), 7 (62), 8 (63), 9 (64), 10 (61),
11 (55), and 12 (65)], insertion polymerization [13 (66)], ring-opening metathesis
polymerization (ROMP) (67) [14 (68,69)], Suzuki polycondensation (SPC) (70,71)
with 17 [15 (72) and 16 (73)] and 24 [20 (74), 21 (75), 22 (76), and 23 (76)], polyad-
dition with 19 under polyurethane formation [18 (77)], Heck coupling of 25 with
26 (78,79), Sonogashira/Hagihara-type coupling of 27 with 28 (80), and, finally,
Yamamoto reaction of 29 (81). Dendronized oligomers with enediine repeat units
were also prepared (82).

Some

features

of

the

dendronized

polymers

obtained

from

the

macromonomers of Table 1 will become important later in this article and
are therefore emphasized in the following: (1) Most of the macromonomers carry
G1 or G2 dendrons. Exceptions are only (12) (G3), (16) (G3), (18) (G3 and G4),
(23) (G4), (25) (G3), and (27) (G3 and G4); (2) Many dendrons are of the benzyl
ether type whose terminal phenyl rings are either unsubstituted or have one
to three long alkoxy or fluoroalkoxy chains. The corresponding polymers are
important for a variety of reasons but are unreasonable candidates, of course,
whenever chemical modification becomes an issue; (3) This is where dendrons
(7), (8), (10), (11), and potentially also (13) come into play which carry protected

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DENDRONIZED POLYMERS

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Fig. 5.

Structures of macromonomers to obtain dendronized polymers by chain-growth

(a) and step-growth procedures (b).

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DENDRONIZED POLYMERS

145

Fig. 5.

(Continued )

functional groups [hydroxy, amino, trimethylsilyl (TMS)] in the periphery. The
corresponding polymers are the starting point for the important aspect of surface
modification; and finally (4) dendron (8) (G2) has one nonpolar and two polar
branching units, and dendron (11) (G2) is completely nonpolar except for its
terminal protected amine groups. These structural features stay in context with
considerations regarding dendronized polymers with specific polarity patterns
which will become an important matter in future but will not be discussed in this
article.

Monomer (23) (G4) requires a special comment: The attempt to subject this

G4 monomer to SPC met with a trivial but nonetheless serious synthetic diffi-
culty (76) which is typical for related polycondensations in which two monomers
with grossly different molar masses are to be reacted with one another in the
strictly required exact 1:1 stoichiometry. The molar mass difference between the
two components (23) and (24) is so considerable (23: 3540 g/mol; 24: 246 g/mol)
that it is difficult to meet in practice this requirement. For that purpose, monomer

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DENDRONIZED POLYMERS

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Fig. 5.

(Continued )

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DENDRONIZED POLYMERS

147

Fig. 6.

Fr´echet-type dendrons G1-G3 and their cartoon-like representations.

(23) was prepared on the 20-g scale as analytically pure material. This enabled
one to do SPC on a relatively large scale ensuring sufficient stoichiometry control.
In a series of optimization experiments, conditions were finally found which gave
the corresponding polymer P23/24 (G4) with P

n

= 25 and P

w

= 125 according to

gpc on the gram scale (76). Presently, fractions of this material with P

n

= 10, 75,

150, and 300 (gpc) are being investigated by sans in order to determine their true
molar mass. Though it is still unclear as to what the actual molar mass of this
polymer is, this data clearly shows that SPC works even for sterically enormously
loaded G4 monomers. This is in contrast to vinyl-type macromonomers. No such

Table 1. Molar Masses of Dendronized Polymers
P7 and P12 Determined by gpc and sans

M

w

× 10

− 3

Polymer

(gpc)

a

(sans)

M

w

(sans)/M

w

(gpc)

P7 (G1)

277

437

b

1.6

P7 (G2)

84

275

b

3.3

P12 (G1)

178

276

c

1.6

(P12 (G3))

710

2530

c

3.6

(P12 (G3))

59

233

c

3.9

a

In THF.

b

In CD

3

OD.

c

In C

6

D

6

.

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DENDRONIZED POLYMERS

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monomer carrying a G4 dendron has yet been found to undergo radically initiated
polymerization.

Finally, it should be pointed out that apart from common organic reactions,

the concentration of the reaction medium plays an absolutely important role
in the polymerization of vinyl macromonomers. This may go as far as that be-
low certain (very high) concentrations polymerization is not only slowed down
but rather rendered impossible. This matter has been treated in some detail
(33,34,55).

Route C and Cylinder “Surface” Modification.

Both the “mixed” route

(route C) and controlled surface modifications of dendritic nanorods are challeng-
ing goals for synthesis and many application related issues. The former allows
to increase the dendritic layer around a backbone whenever needed and other
synthetic routes (A and B) fail. The latter is an important option to engineer prop-
erties in various directions by attaching to one and the same polymer the different
groups considered relevant for a certain application. For both cases it is obviously
necessary to have accessible surface functions to which either another dendritic
fragment or the modifying groups can be attached. Some experiments along these
lines have been undertaken. The essential steps are

(1) Synthesis of monomers with protected functional groups in the periphery.

The protection is required for compatibility with the polymerization condi-
tions.

(2) Polymerization of these monomers and complete deprotection of the periph-

eral functional groups of the resulting polymer. The resulting deprotected
polymer needs to be soluble.

(3) Derivatization of the deprotected polymer with the desired dendritic or

other functional units.

Step 1 has been solved at least for up to generation 2. Monomers (7), (8),

(10), and (11) in Figure 5 are good examples. These monomers have all been poly-
merized to the corresponding high molar mass materials P7, P8, P10, and P11
(step 2) and their functional groups deprotected. For the use of the somewhat
uncommon trimethylsilyl(ethyloxy)carbonyl (Teoc) protecting group, see Refer-
ence 62. Figure 7 shows

1

H nmr spectra of monomer (7) (G2), the corresponding

polymer P7 (G2), and its deprotected counterpart. The deprotection can obvi-
ously be driven to completion. This aspect has also been quantified for many G3
and even G4 polymers, and deprotection was found to be virtually 100% in all
cases. This is important because the remaining protected functional groups are
an integral part of the polymer and cannot be removed by purification as in com-
mon organic chemistry. Step 3 has mainly been developed for the “mixed” route
even though the knowledge gained here should be fully transferable to all kinds
of surface modifications. Figure 8 shows an example in which high molar mass
polymer P11 (G2) was first converted into its G3 analog (polymer A) by depro-
tection of its Teoc protected amine groups with trifluoro acetic acid and subse-
quent reaction of the amines with the active ester G1 dendron 30. Polymer A was
then subjected to the same sequence of deprotection (to give B) and dendroniza-
tion with (30) to give polymer C. The degree of deprotection was determined by

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149

10

0

5

∗

∗

∗

∗

∗

∗

∗

δ

Fig. 7.

1

H nmr spectra of monomer (7) (G2), the corresponding polymer P7 (G2), and

its deprotected counterpart (from top to bottom) to illustrate the structure control during
polymerization and the degree to which deprotection of the polymer can be achieved. Signals
of the Teoc protecting group (full dots) and the solvent are marked (asterix).

rigorous application of 500-MHz nmr spectroscopy and that of the dendronization
by applying Sanger’s reagent, 2,4-dinitrofluorobenzene, and quantitative uv ab-
sorbance measurements. This reagent upon reaction with amines gives intensely
yellow derivatives whose formation can be accurately quantified when its uv ab-
sorbance is compared with model compounds. Under broadly varied conditions
and with the help of control experiments it was made sure that each amine group
of polymer B which had not reacted with the G1 dendron (30) had in fact done so
with the Sanger reagent (83). With a series of experiments it was proven that the

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DENDRONIZED POLYMERS

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Fig. 8.

Route C used to convert a third generation dendronized polymer (A) with terminal

amine groups into the corresponding fourth generation derivative C. The sequence involves
complete deprotection of A’s protected amine termini and reaction of the resulting polyelec-
trolyte B with the first generation dendron (30). Compound (30) has an active ester focal
point functionality which proved to react extremely efficiently with polymer B’s amines.

conversions of the dendronization of B furnishing polymer C, whose structure is
given in detail, can easily be driven beyond 99.3% (84).

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DENDRONIZED POLYMERS

151

Fig. 9.

Tapping mode sfm height image of a G4 polystyrene (C) on mica to visualize the

length of individual chains.

These very high conversions were found for high molar mass polymers A

which had degrees of polymerization between approximately 400 and 800.
Figure 9 illustrates this point by showing a tapping mode sfm image of polymer
C on mica. Even though a statistical evaluation of the length is not yet available,
it can be seen that some chains reach considerable lengths of up to 250 nm. This
equals 1000 r.u.’s assuming the backbone attains an all-trans zigzag conforma-
tion, which may not even be the case. This astounding result not only underlines
the effectiveness of the route C dendronization protocol but also shows the power
of amide formation between surface amines and activated ester components like
(30). This latter aspect opens an avenue to all kinds of surface modifications based
on this chemistry. All what is required is to use active esters which carry sugars,
amino acids, catalytically active sites, etc. An alternative way to get to modified
dendronized polymers is to polymerize modified dendronized monomers, for ex-
ample, monomer (9) (G1 and G2) which gives sugar-coated polymer P9 (64).

Molar Mass Determination.

In practically all cases the molar masses

of dendronized polymers were obtained from gpc versus polystyrene standard. As
pointed out above, this is not an appropriate method for such polymers. The values
reported (32,40,85) should therefore be treated at best as rough guesses (see below)
and a direct comparison of different polymers is impossible. This refers also to
polymers with the same backbone but different generation dendrons. Despite the

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DENDRONIZED POLYMERS

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importance of an accurate molar mass determination of dendronized polymers, a
systematic study on this matter is not yet available. The most extended one is one
where gpc molar mass of five polymers obtained from monomers (7) (G1), (7) (G2),
(12) (G1), (12) (G2), and (12) (G3) are compared with those obtained from sans
(86). Both fractions and representative samples of these polymers were employed.
Table 1 summarizes the results. In all cases the sans data are higher than those
from gpc and the factor by which gpc underestimates the real molar masses varies
between 1.6 and 3.9. A qualitative explanation for this finding is to be seen in the
much larger mass per unit length of dendronized polymers than the calibration
standard polystyrene. The true molar mass should therefore be higher than the gpc
molar mass. There is, however, an opposing effect which has to do with the chain
stiffening of the polymer chain imposed by the spatially demanding substituents.
This leads to an increased hydrodynamic volume, causing the gpc molar mass
to become larger than the actual one. For the cases of Table 1, the latter effect
obviously does not overcompensate the former. Whether this will still be the case
even for polymers with G4 substituents awaits to be seen. The sans measurements
also revealed that polymer (12) (G3) is a rigid rod whose persistence length is of
the order of the contour length.

The degree of polymerization (P

n

) of polymer P16/17 (G3), a representative

of a step-growth polymer (Fig. 5b) obtained from monomers (16) (G3) and (17), was
also investigated carefully in order to learn about the effectiveness of step-growth
procedures [here SPC (70,71)] in the case of sterically demanding monomers. Its
P

n

could not be directly determined because of unsurmountable problems with

aggregation. Through some chemical modification, however, an average P

n

= 110

was finally obtained. According to Carother’s equation (87), this P

n

results when

each coupling step proceeds with a conversion of 99.1%. From a synthetic point
of view, this result is truly remarkable. It shows the enormous potential of SPC
even for cases where steric congestion is considerable.

Some Aspects of the Molecular Structure.

As discussed above, one

objective for research of dendronized polymers was to use the decoration with
dendrons as a means to stiffen the backbone to the point that it is fully stretched
linear. This should be reached when the appendent individual dendrons are evenly
distributed around the backbone and tightly packed at van der Waals distance.
This fully stretched conformation will also be reached, of course, at a somewhat
less tight situation, provided solvent molecules are sucked into the dendritic layer
through osmotic effects. Both cases lead to a rigid rod with a cylindrical envelope;
however, only in the first case this shape would be practically independent from the
surrounding medium, whereas in the second, the dendritic layer would collapse
under conditions where the solvent diffuses out (eg, in vacuum). The question of
tightness of packing is an essential one for synthesis. Generally speaking, the
synthesis becomes more difficult, the less spatial mobility the reaction partners
have to attain the mechanistically required relative conformation in the tran-
sition state of the reaction (here, growth step). To evaluate the feasibility of a
certain polymerization on grounds of sterics, it would be ideal to have a detailed
picture of the spatial changes around the reaction center on going along the reac-
tion coordinate. Since it is practically impossible to obtain this with a reasonable
amount of computational effort, only some molecular dynamics (MD) calculations
of final products were performed. Additionally, some polymers were visualized by

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DENDRONIZED POLYMERS

153

Fig. 10.

The lengths of repeat units of polymers P12 (G3) (0.25 nm) and P16/17 (G3)

(0.825 nm) to qualitatively assess the average distance between attached dendrons.

computer-generated 3-D images in order to help the synthetic chemist to assess
whether or not a macromonomer undergoes polymerization.

MD simulations were performed in vacuo with polymers P12 (G3) (88) and

P16/17 (G3) (89), which carry both of the same G3 dendrons but differ by the av-
erage distance between the dendrons’ anchor groups [P12 (G3): 0.25 nm, P16/17
(G3): 0.825 nm] and the flexibility of the backbone (Fig. 10). Unfortunately, sol-
vent molecules could not be considered because of the enormous number of atoms
already contained in the polymers. Figure 11 shows the minimum conformations
obtained. While the dendritic layer of P12 (G3) is quite compact, that of P16/17
(G3) is much more open and loose. The diameters of P12 (G3) and P16/17 (G3)
are on average approximately 4.4 nm and 2–4 nm, respectively. This difference
in compactness is certainly to a large extent due to the difference of the average
distance of the dendrons’ anchor groups at the backbones, but the backbones’ dif-
ferent stiffnesses also play a role here. While the contour length of P16/17 (G3) of
the starting conformation upon equilibration remains practically constant, that
of P12 (G3) is reduced by some 30% when the minimum conformation is reached.
This shows that the dense appearance of the latter polymer can be partially at-
tributed to some backbone coiling. It is reasonable to assume that it is exactly this
small space still available which rendered the polymerizations feasible. Consid-
erations regarding preaggregation of polymerizable dendrons into ordered cylin-
drical arrays prior to polymerization through the aggregate (85) do not seem to be
applicable here.

Figure 12 shows computer-generated structures of P16/17 (G1) through

P16/17 (G4). Though these structures are not fully energy minimized, it is quite

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DENDRONIZED POLYMERS

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(a)

(b)

Fig. 11.

Molecular dynamics (MD) simulations of dendronized polymers P12 (G3) (a) and

P16/17 (G3) (b). P12 (G3) has 50 r.u’s and the structure shown was obtained after 300 ps.
End-to-end distance 9.1 nm, average diameter 4.4 (

±0.2) nm. The backbone atoms are kept

in yellow, the terminal benzene rings in red, all other atoms in green. P16/17 (G3) has
40 r.u.’s and a contour length of 33 nm. The backbone and the hexyl chain atoms are in
yellow, the terminal benzene rings in red, and all other atoms in green. The diameter
fluctuates between approximately 2 and 4 nm.

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DENDRONIZED POLYMERS

155

Fig. 12.

Computer-generated images of dendronized polymers P16/17 (G1) to P16/17 (G4)

(a–d).

obvious that there is space between consecutive dendrons even for the G4 case.
This space, together with the surprisingly successful polymerization of monomer
(16) (G3) supported the idea to also try SPC with monomer (16) (G4).

Behavior in the Bulk

The bulk properties have been investigated for polymers P2 (G1 and G2) (59), P4
(G2) (59,90) (P2, P4: R

= OC

12

H

25

), P18/19 (G4) (77), and P25/26 (78). All four

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DENDRONIZED POLYMERS

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R

= 2.99 nm

(a)

(b)

Fig. 13.

Schematic representation of a supramolecular cylinder of dendronized polymer

P2 (G1) (R

= OC

12

H

25

, n

= 3) in the 

h

mesophase: (a) top view of a cylinder containing 6

repeat units in a stratum with the alkyl tails melted to match the average column radius
determined by x-ray scattering experiments, (b) side view of a cylinder containing 30 repeat
units of the polymer assembled with melted alkyl tails. Reproduced with permission from
Ref. 15.

structures have flexible backbones with a noncompact sequence of dendrons. In the
first two cases and in the last one, the dendrons have liquid crystalline properties,
whereas in the third they do not. Polymer P2 (G1) self-assembles into a tubu-
lar supramolecular structure exhibiting enantiotropic columnar hexagonal (

h)

phases. These phases are characterized by differential scanning calorimetry, wide-
angle and small-angle x-ray scattering, thermal optical polarized microscopy, and
molecular modeling (15). The structure model proposes the stratum of the column
to be formed by the backbone and the linking segments melted and segregated
in the center of the column and their melted dendrons radiating toward the col-
umn periphery (Figs. 13 and 14). When a second generation dendron is used as
in P2 (G2) and P4 (G2), an additional interesting feature is observed (59). As the
degree of polymerization increases, the dendrimer’s shape shifts from spherical
to cylindrical. This is accompanied by the backbone going from random-coil to
the extended conformation. This phenomenon provides the unique possibility for
molecular engineering of polymer shapes, backbone conformation, and properties
and is a way to show the interplay and impact of the above-mentioned interactions.
Spherical and extended architectures of these dendrimers are basically proven by
x-ray diffraction and can also be visualized by an sfm investigation using bilayers
on a mica substrate. For a G1 polymer with closely related structure (not shown), a
second columnar liquid crystalline phase was observed (90). For a polymer coated

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DENDRONIZED POLYMERS

157

3.7 nm

6 nm

2.8 nm

DP

= 59

0.374 nm

(a)

(b)

Fig. 14.

An idealized representation of the supramolecular cylinder self-assembled from

a single chain of dendronized polymer P2 (G1) (R

= OC

12

H

25

, n

= 3) in the hexagonal

columnar phase, assuming one single backbone per cylinder, drawn to proportion with
existing data: (a) a tilted side view, (b) top view. Reproduced with permission from Ref. 15.

with two tapered units per side chain (not shown), a hexagonal columnar liquid
crystalline superlattice was observed (91).

Though polymer P18/19 (G4) does not show any liquid crystallinity, it nev-

ertheless forms supramolecular structures in the bulk (77). This was concluded
from x-ray diffraction patterns obtained from Si plates coated with this polymer
and monomer (18), respectively. A series of Bragg peaks for the polymer showed
the existence of an ordered structure. Based on MD calculations and additional
small-angle x-ray scattering experiments, this structure was proposed to be body-
centered cubic.

Polymer P25/26 self-orders in solvent-cast films, with the backbones parallel

to the substrate and a strong solvent dependence of the degree of ordering (78).
Spacings of 2.2–2.6 nm are observed by x-ray diffraction, indicating interdigita-
tion of the dendritic side chains. P25/26 also forms thermotropic nematic liquid
crystalline phases. With optical microscopy, Schlieren textures are observed for
thin films cast from solution.

Assembly and Manipulation at Surfaces

Dendronized polymers can self-assemble on surfaces into highly ordered layers,
which renders them into a material attractive for various applications. The assem-
bly process can be investigated on the molecular scale using the sfm. In addition,
the sfm may be used to manipulate single macromolecules on solid substrates in
order to generate assemblies, which would not form spontaneously.

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DENDRONIZED POLYMERS

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The behavior of the dendronized polymers at interfaces is determined not

only by intra- and intermolecular forces but also by interfacial forces. The latter
(92) include the van der Waals force, which is important on the nanometer scale
and which is due to the difference in the dielectric properties of the two adja-
cent phases. This force is always attractive, and therefore favors a high molecular
segment density at the interface, which in turn favors 2-D crystallization, since
crystals exhibit usually higher densities than less ordered phases. Counteract-
ing is the influence of entropy: Immobilizing flexible molecules in 2-D crystals
at interfaces reduces entropy, since the molecules lose both translational as well
as conformational entropy. For the adsorption from a liquid mixture, this favors
the adsorption of the largest molecules, since the loss of translational entropy is
counted per particle, wherear the van der Waals energy gained is proportional to
the mass. Entropy effects also favor the adsorption of the most rigid molecules,
since they lose the least conformational entropy. As a result, the large and rigid
rods will be preferably adsorbed from a liquid molecular mixture. In addition,
there are short-range chemical forces, which are specific for the particular chem-
ical species at the interface and may be attractive or repulsive. Well-known con-
sequences of these forces are epitaxial layering on crystalline substrates and the
ordering of amphiphilic molecules at the water–air interface. Moreover, there are
electrostatic forces for charged species.

In the following, the behavior of two classes of dendronized polymers at in-

terfaces will be discussed: those with one type of dendrons (homophilic systems)
and others with a hydrophobic and a hydrophilic dendron at each repeat unit
(amphiphilic systems).

Homophilic Systems.

Dendronized polymers can be observed by sfm ei-

ther in ordered ultrathin films or as individual molecules on solid supports. SFM
(93,94) is a powerful tool for investigating morphology, molecular packing, and
molecular dynamics at surfaces with a resolution on the molecular scale. Both,
synthetic and biological polymers (87,95,96) as well as spherical dendrimers have
been studied (97–99). Of particular interest for rather soft organic materials is the
operation in the “intermittent contact” or “tapping” mode (94), which minimizes
sample wear during imaging.

Ultrathin layers of homophilic dendrimers can be prepared by solution cast-

ing (slow layer formation) or spin coating (fast layer formation) from organic sol-
vents. SFM images of P12 (G3) were obtained for thin (in the range of 20–30 nm)
solution casted layers on highly oriented pyrolytic graphite (HOPG) (Fig. 15) (88).
They reveal a remarkably high degree of order: One observes domains which con-
sist of periodic arrays of rows (Fig. 2c) with a periodicity of D

= 5.0 ± 0.5 nm. For

comparison, MD simulations (Fig. 5a) and sans measurements reveal rod diam-
eters of D

= 4.4 ± 0.2 nm and D = 5.1 ± 0.5 nm, respectively. These numbers

are very similar to the distances between the rows, suggesting that the rows can
be attributed to molecules, which are predominantly oriented parallel to the sur-
face. They are grouped in anisotropic domains, whose sizes vary between 20 and
200 nm (parallel to the director axis) and between 20 and 100 nm (perpendicular
to the director axis). However, a reliable correlation of individual chain lengths
with domain size is not possible, since, despite the high resolution of the image,
only the chain ends at the grain boundaries can be clearly identified, while those
in the interior may be obscured if two dendrimers with the same director axis

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DENDRONIZED POLYMERS

159

Fig. 15.

Tapping mode phase contrast sfm image of an ultrathin layer of P12 (G3) on

highly oriented pyrolytic graphite (HOPG) (50). The rows are attributed to molecules, which
are predominantly oriented parallel to the surface as well as to each other. Their orientation
within the surface plane reflects also the threefold symmetry of the substrate, as indicated
by the Fourier-Transform in the inset.

are in tight contact and oscillate somewhat along this axis (88). Large-scale sfm
images show that the films exhibit terraces with height differences

δh between ad-

jacent terraces of 4.2

± 0.2 nm. This is close to 5.0 nm × cos 30

◦

= 4.3 nm, which

one would expect for a model of closely packed cylinders of a diameter of 5 nm
(Fig. 16).

Interestingly, the different domains in Figure 13 exhibit three molecular ori-

entations at 60

◦

± 8

◦

relative to each other, indicating that even in a layer of

a thickness of the order of five monolayers, the top layer reflects the threefold
symmetry of the graphite substrate. While this symmetry is stable in time, re-
orientations by 120

◦

do occur. The case of a similar dendronized polymer, P16/17

(G3), is demonstrated in Figure 17 (89).

The results described above show that the ultrathin layers of the described

dendronized polymers offer the possibility to orient the polymeric backbones along
substrate axes, and to access molecular dimensions in arrays by sfm imaging.
In order to follow the assembly process on the molecular scale, it is desirable to
prepare submonolayers with isolated single molecules on the surface. A convenient
preparation for this is spin-coating HOPG from a dilute solution. SFM images
of P14 (2

×G1) (100–102) (Fig. 18a) reveal single molecules stretched out on the

surface with sharp kinks of typically 120

◦

, which reflect again the symmetry of the

substrate. The sharpness of the kinks indicates that this polymer is quite flexible.
Upon annealing, compact monolayer islands can form (Fig. 18b), revealing that
the flexibility of the chains allows the formation of very sharp bends. SFM images

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DENDRONIZED POLYMERS

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1.00

2.00

−10.0

x – x

⬘, ␮m

10.0

(a)

(b)

(c)

h

⬘, nm

HOPG substrate

⌬h

⌬h

x

⬘

x

⬘

x

x

(100) surface

5.0 nm

Fig. 16.

Tapping mode sfm of ultrathin films of P12 (G3) on HOPG and a packing model

(97). (a) Cross-sectional profile along x-x



as indicated in (b). The height difference between

adjacent terraces has the dimension of a monolayer (

h = 4.2 ± 0.2 nm), (b) Large-scale

(2.4

× 2.4 µm

2

) sfm image of molecular terraces, (c) Schematic model of close-packed molec-

ularly defined cylinders in ultrathin films of P12 (G3) on HOPG. From Ref. 88.

may also be used to determine the distributions of contour lengths. They revealed
that P14 (2

×G1), ie, a polymer with a spatially not very demanding side group,

appeared to be at least two times shorter than the contour length as expected on
the basis of size exclusion chromatography and static light scattering. This was
attributed to a disordered helix-like conformation of the polymeric backbone, and
can be contrasted to the case of P4 (G1) with a spatially more demanding side
group, whose contour lengths agreed fairly well with the expectation (100–102).
On one hand, it shows that bulky dendrons force the backbone into extension.
On the other hand, there is a degree of freedom for less bulky dendrons, namely
the conformation of the flexible polymer backbone, which may be used to control
the contour length of the dendronized polymer (eg, by contraction through helix
formation).

Polymers at surfaces may also be manipulated with the sfm (Fig. 2). An ex-

ample for the manipulation of dendronized polymers by rubbing (Fig. 2d) with
the sfm is shown in Figure 19, which has been obtained as follows: First a rather
low molecular weight fraction (of the order of 10 r.u.’s on average according to gpc
with polystyrene standard) of P16/17 (G4) was spin-coated to give a monolayer on
HOPG. The result is an apparently disordered surface. Scanning with the sfm tip,
however, orients the molecules almost prefectly uniformly in the whole scan win-
dow which can be 1

µm

2

or more (104), limited apparently only by the domain size

of the underlying HOPG, which is typically of the order of 10

µm, but can also vary.

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DENDRONIZED POLYMERS

161

Fig. 17.

Tapping mode phase contrast sfm images of an ultrathin film of P16/17 (G3) on

HOPG (89). The rows are again attributed to molecules, which are predominantly oriented
parallel to the surface as well as to each other. Their orientation within the surface plane
reflects also the threefold symmetry of the substrate (three molecular orientations denoted
A, B, and C in panel a). Images (a) and (b) have been recorded at the same position at an
interval of 10 min. Domain A reorients in the same direction as domain B, resulting in a
new domain D.

162

DENDRONIZED POLYMERS

Vol. 2

0

300

nm

nm

0

300

Fig. 18.

Tapping mode sfm height images of P14 (2

×G1) with a theoretical P

n

= 100

on HOPG prepared by spin casting from a solution in THF (102): (a) as prepared, (b)
after annealing at 100

◦

C for 24 h. The molecular orientations reflect the symmetry of the

substrate; the molecules are flexible enough to let them be frequently bent by 120

◦

. The

strong tendency for the single molecule to align along lattice axes of the substrate has been
attributed to the orientation of the pendent alkyl chains (103). Upon annealing, compact,
domains are formed.

But not only layers can be manipulated by the sfm tip, also single macro-

molecules can be moved across the surface (Fig. 2b). Figure 20 (left) shows an
sfm image of P7 (G2) with two C12 chains per terminal amine group (not shown)
on HOPG. The alkyl chains cause the backbones again to orient preferentially
according to the threefold symmetry of the substrate, and the flexibility of the
backbone allows sharp kinks to form. Figure 18b shows the same area after one
molecule has been moved to the left by sliding the sfm tip at stronger interaction
with the substrate along a path indicated by the red arrow. The image reveals
that the macromolecule also bent because of its flexibility.

Such manipulations should be better defined for polymers with higher rigid-

ity. Since Coulomb-charges along the backbone should stiffen the macromolecule,
polyelectrolytes, similar to the polymers with bulky dendrons P12 (G3) and
P16/17 (G4), should be promising candidates to manipulate them one by one with
the sfm tip, and to determine single molecule properties, like persistence lengths
and Young’s or bending moduli (Fig. 2a).

Amphiphilic Systems.

Amphiphilic dendronized polymers may lead to

amphiphilic cylinders which, depending on the surrounding medium, could segre-
gate lengthwise into two different halves (Figs. 3d, Fig. 21). This structural motif
is rather unique. In nature it can be found in some ion channel membrane proteins,
which means that amphiphilically dendronized polymers are of interest as models
for such proteins (105–108). They may also serve as novel and giant constituents
of self-aggregated assemblies and should show interesting behavior at interfaces.
A good candidate is a dendronized polymer, whose repeating units are equipped
with two sterically demanding substituents, one of which being hydrophobic and

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DENDRONIZED POLYMERS

163

0

100

200

nm

300

400

Fig. 19.

Tapping mode phase contrast sfm image of a monolayer of oligomeric P23/24 (G4)

after scanning three areas with the sfm (104). It reveals that upon scanning the molecules
are perfectly oriented along one of the three symmetry axes of the substrate.

Fig. 20.

Manipulation of individualized P7 (G2) with two C12 chains per terminal amine

group on HOPG by switching from an sfm tapping mode imaging modus to manipulation
by approaching the tip to the sample and moving along the path of the red arrow. (a) image
before and (b) image after manipulation. From Ref. 104.

the other hydrophilic (62). This area was entered with three differently equipped
PPPs P20/24, P21/24, and P22/24, which differ in the relative spatial demand of
their polar and nonpolar substituents.

There are different ways to prepare ultrathin layers from amphiphilic poly-

mers: One is to form Langmuir monolayers at the water–air interface followed by

164

DENDRONIZED POLYMERS

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Fig. 21.

Representations of amphiphilically equipped dendritic cylinders with varying

spatial demands of polar and nonpolar domains. The lengthwise segregation of these do-
mains is interesting for surface modifications and for attaining unprecedented aggregates
in solution.

transfer to solid substrates using the Langmuir–Blodgett technique. Another is
spin coating as for the homophilic systems.

Amphiphilic dendron-based molecules have been investigated at the air–

water interface and stable Langmuir monolayers were found for some of these
systems (109–114). In one study (115), the behavior of the hydrophobic Fr´echet-
type dendrons (Fig. 6) carrying hydrophilic ethyleneoxy (EO) chains at their focal
point was studied at this interface by systematically varying the dendron size (G3
to G5) and chain length. Longer hydrophilic chains increased the stability of the
Langmuir films for G3 and G4 by roughly 3.1 and 4.1 mN/m (

=dyn/cm), respec-

tively, per additional EO unit. On going from the lower to the higher generations,
the shape of the dendrons changes from vertically elongated to more flat. The col-
lapse pressures ranged from approximately 6 mN/m for G3 with one EO group to
25 mN/m for G3 with six EO groups.

Langmuir monolayers of monomer 20 and polymer P20/24 have been pre-

pared at the air–water interface (74), exhibiting surface pressure-area isotherms
at room temperature, which reveal stable monolayers. The monolayers of 20 ex-
hibit very good reversibility for compression, decompression, and repeating cycles,
with an area per molecule of about 0.73 nm

2

per molecule at 20 mN/m. This is con-

sistent with a monolayer structure, in which the four EO chains per water-soluble
dendron are close-packed and oriented perpendicularly to the monolayer, thereby
defining a minimum area per molecule. In comparison, polymer P20/24 exhibits a
10% larger area per repeat unit (0.82 nm

2

/r.u.) upon the first compression with a

hysteresis in the first decompression and a shift to a more reversible isotherm,
and a smaller area per repeat unit in the second cycle (0.77 nm

2

/r.u.). The good

agreement between the areas per molecule of monomer (20) and per repeat unit
of polymer P20/24 indicates a structure of the polymer monolayer, in which the
rod-like polymer molecules are oriented with their long axes within the mono-
layer plane and close-packed. Moreover, the hydrophylic ethyleneoxide chains are
on that side of the polymer which faces the water subphase, while the more hy-
drophobic dendron faces the air side of the polymer. This picture is supported by
a control experiment using a closely related polymer in which the hydrophilic

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DENDRONIZED POLYMERS

165

dendron of P20/24 is replaced by another G2 Fr´echet dendron (structure not
shown). Under the same conditions this polymer does not form a stable mono-
layer at the air–water interface.

Also, polymers P21/24 and P22/24 with their corresponding monomers (21)

and (22) were investigated at the air–water interface (75). Except P22/24 they
all form stable monolayers on the Langmuir trough, which can be transferred
by the Langmuir–Blodgett (LB) technique to mica. While the transferred films of
(21) and (22) are unstable, P21/24 forms stable, rather smooth monolayers, whose
morphology remains unchanged for at least a week. The fact that both monomers
are stable on water but not on mica reflects the discrepancy of the space demand
of the hydrophilic and hydrophobic parts. Since the polymer P21/24 is stable on
mica, the covalent attachment of the amphiphilic repeat units can stabilize an LB-
layer. On the other hand, the fact that P22/24 does not form a stable monolayer,
not even on water, indicates that this polymer with its amphiphilic misbalance
does not behave like an amphiphile under these conditions.

A quantitative comparison of the collapse pressures of the Curtis–Hawker

amphiphiles (114) with the ones described here cannot be made. There are
substantial structural differences even if this comparison is restricted to the
monomers (20), (21), (22). Additionally, the Curtis–Hawker amphiphiles have EO
chains with terminal (polar) hydroxy functions whereas the monomers have (non-
polar) methoxy groups instead. The collapse pressures are, however, in a similar
range.

Usually, no molecular scale resolution sfm images were obtained for the LB-

layers, contrary to the analogous homophilic systems prepared by spin coating.
Therefore, spin coating has been employed as an alternative preparation method
for the amphiphilic systems also (104). Figure 22 displays an sfm image of a

Fig. 22.

Molecular resolution tapping mode sfm height image of spin-coated amphiphiles

P20/24 on HOPG. From Ref. 104.

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DENDRONIZED POLYMERS

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spin-coated monolayer of P20/24 on HOPG, which reveals rather straight rods.
Again, their orientation within the plane is not random but it reflects the threefold
symmetry of the substrate. Within the experimental error, the length distribution
of the rods agrees with their determination by size exclusion chromatography.
However, the distance between the rods is 4.4

± 0.5 nm, which is larger than

what one would expect for close-packed molecules. A possible explanation is a
well-defined aggregation of the amphiphilic rods, eg, into dimers, which exhibit
hydrophobic surfaces in contact with both the rather hydrophobic substrate and
the ambient.

Optical Properties

Dendronized polymers may exhibit interesting optical and electrical properties
which should at least be briefly mentioned here. Poly(phenyleneethynylene)s are
among the few conjugated polymers which are interesting candidates for blue
light-emitting diodes (116,117). A problem which hampers the use of these poly-
mers in commercial devices is luminescence quenching of the excited state because
of intimate contact between the backbones. Wrapping of such a backbone by a den-
dritic layer, which itself does not quench the luminescence, can be a solution (80).
Over a wide concentration range, the quantum yield of the luminescence of P27/28
(G4) was nearly 100%, contrary to the case of P27/28 (G3 or G2) where it drops
below 40% (Fig. 23). Additionally, upon 278-nm excitation of the dendritic sub-
stituents of P27/28 (G4) in tetrahydrofuran (THF), the observed fluorescence was
11 times more intense than that of the lower generation analogs under identical
conditions. In this case, the dendritic layer harvests the uv photons, channels them
to the backbone which then emits with high efficiency in the blue Light-emitting
diode.

100

80

60

40

20

0.00

0.08

0.04

0.12

Absorbance

A

B

C

Fluorescence quantum yield (

FL

), %

Φ

Fig. 23.

Fluorescence quantum yields of P27/28 (G2) (A), P27/28 (G3) (B), P27/28 (G4)

(C) upon excitation of the conjugated backbone in THF with absorbances of 0.01–0.1 at the
excitation wavelength

λ = 425 nm under Ar at 20

◦

C. From Ref. 80.

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DENDRONIZED POLYMERS

167

The same idea of isolating active backbones from one another and, thus,

exploiting their intrinsic undisturbed properties, were also followed (78,79,81). In
Reference 81, polymers P29 (n

= 0,1) were prepared and it was shown that the

attachment of bulky substituents does not influence the electronic characteristics
of the backbone. No increase of the dihedral angle between consecutive repeat
units was observed. The emission maximum (

λ

exc

.

= 380 nm) of a drop-casted film

of P29 (n

= 1) was virtually unchanged and no sign of aggregation behavior was

found. P29 (n

= 1) was incorporated into a device which produces blue emission

with an onset voltage below 4 V. On very similar experiments with P25/26, an
enhancement in solid-state photoluminescence by a factor of 4 because of the
intermolecular separation of the chromophores by the attached dendrons was
found (78,79).

Summary and Outlook

High molecular weight dendronized polymers are synthetically accessible. If
certain prerequisites are taken into account, even the sterically demanding
macromonomers (12) (G3), (16) (G3), and (23) (G4) can be employed, which widens
the scope of both radically initiated polymerizations as well as Suzuki polycon-
densation. The successful synthesis of polymers like P12 (G3), P23/24 (G4), and
C (G4) allows investigation of the impact of the dendritic decoration on the back-
bones’ coiling behavior and addresses the important question of whether or not
steric congestion can be used as a tool to force molecules into a certain shape, which
is more or less independent of the surrounding medium (in solution, adsorbed on
surfaces, spread at solid–liquid interfaces, and in the solid state). SFM and sans
measurements as well as MD simulations reveal that, eg, polymer P12 (G3) is
quite stiff and can be viewed as a cylindrically shaped molecular object with a de-
fined diameter of approximately 5 nm and a persistence length on the length scale

Fig. 24.

Tapping mode sfm height images of (a) unprotected and quaternized (charged!)

polymer C (G4) on mica and (b) the same when complexed with plasmid DNA. The brighter
and wider features in (b) are attributed to the aggregate between the two oppositely charged
polyelectrolytes while the other features represent neat DNA. From Ref. 118.

168

DENDRONIZED POLYMERS

Vol. 2

of a few tenths of a nanometer. One may even be inclined to assign a “surface” to
this polymer. Driven by the idea of generating molecular nanoobjects with func-
tional surfaces just by polymerizing appropriately equipped monomers, first steps
were made to decorate these surfaces with both functional and polar/nonpolar
groups in order to engineer properties and explore possible fields of application.
Research on nano-objects of the described kind is a truly interdisciplinary enter-
prise, and progress depends upon tight cooperation between synthetic chemists,
experimental physicists, and theoreticians. Some future directions will be (1) to
develop synthesis to the point where several rigid objects with various surface
functionalities are available and their length distributions are somewhat better
under control, (2) to isolate individual very rigid cylinders on surfaces and deter-
mine quantitatively molecular properties like bending moduli, (3) to move them
about on surfaces for patterning and construction purposes, and (4) to start in-
vestigations into their properties along the lines described under “Dendronized
Polymers.” Figure 24 gives, as an example, a taste of what is presently being in-
vestigated in the directions indicated in Fig. 3c. Complex formation of the kind
described here is possible. Figure 24a shows an sfm image of unprotected and
quaternized (positively charged) polymer C (G4) spin-coated on mica. Figure 24b
shows the same polymer complexed with (negatively charged) DNA after complex-
ation in solution and subsequently spin-coated on mica. The image reveals both
neat DNA and complexes of the dendronized polymer with DNA, which are a little
thicker than the neat polymer (118).

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A. D

IETER

S

CHL

¨

UTER

Freie Universit ¨at Berlin
J ¨

URGEN

P. R

ABE

Humboldt-Universit ¨at zu Berlin