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DIACETYLENE AND
TRIACETYLENE POLYMERS

Introduction

Polydiacetylene (1–5) and polytriacetylene along with polyacetylene are the only
known linear conjugated polymers (6). Conjugated polymers are distinguished
from traditional polymers in that they contain alternating

π-bonds along the poly-

mer backbone. This structural feature imparts useful electronic and optical prop-
erties for the development of advanced materials. The linearly conjugated poly-
mers are distinguished from the other conjugated polymers [eg, poly(phenylene
vinylene) (1)] because the linearly conjugated polymers do not have branch points
along the network of

π-bonds. Polydiacetylene (2), polytriacetylene (3), and poly-

acetylene (4) are also the first three members of a series of linearly conjugated
polymers with an increasing number of alkyne groups that terminates with poly-
carbyne.

Encyclopedia of Polymer Science and Technology. Copyright John Wiley & Sons, Inc. All rights reserved.

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The polydiacetylenes and polytriacetylenes differ from polyacetylene because

preorganization of the diacetylene and triacetylene is required for a successful
polymerization (7). This remarkable observation was first recognized (8,9) in 1969
and marks the beginning of modern polydiacetylene and polytriacetylene chem-
istry. In a few cases, this topochemically controlled polymerization occurs from a
crystal of the monomer to a crystal of the polymer, giving rare examples of macro-
scopic single polymer crystals (9).

The study of polydiacetylenes has been an active area of research. From 1969

there have been 3301 publications concerned with the preparation and study of
polydiacetylenes, and 2713 of these have been published since 1986.

Polydiacetylenes

Preparation of the Diacetylene Monomers.

Although a few inter-

esting methods have appeared (10), the oxidative coupling of monosubstituted
acetylenes, using copper salts (11,12) (Hay or Eglinton reaction conditions), con-
tinues to be the most common route to symmetrical diacetylenes (eq. 1). For
unsymmetrical and symmetrical diacetylenes the Cadiot–Chodkiewicz coupling
reaction, along with its modifications, has proven useful. When the traditional
copper coupling methods fail, some success has been had, mediating this reaction
with transition metals such as palladium (13).

(1)

Preparation of Polydiacetylene.

The preorganization for the 1,4-

polymerization of diacetylenes has been discussed previously (7,14,15). Successful
polymerization occurs when the diacetylenes have a translational repeat distance
(d) of about 0.49 nm and an angle (

π) of about 45

◦

with respect to the translational

direction and van der Waals contact (R

v

) of the polydiacetylene functionalities

(Fig. 1). If these structural parameters are met then the C1 and C4 carbon atoms
of adjacent diacetylenes will be in a position for a topochemically controlled poly-
merization. Because the 0.49 nm translational repeat distance (d) of the monomer
is about the same as the repeat distance in the polymer, the polymerization process
can occur with little disruption of the reactant packing.

A number of diacetylenes have been reported to undergo a solid-state poly-

merization to give polydiacetylenes. However, one of the more exciting aspects of
the diacetylene polymerization, shown in Figure 1, is the ability of some diacety-
lene single crystals to polymerize to give single crystals of the polymer. Although
there is a difference in the quality of the x-ray data, 16 examples of polymer sin-
gle crystals have been reported (16,17). Eleven of these were reported before 1986
whereas only five have been reported after this date. Given the large number (508)
of diacetylenes that have been reported in the Cambridge Structural Database
(CSD), only a very small percentage possess the structural parameters to undergo

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205

Fig. 1.

Topochemical requirements for the 1,4-polymerization of diacetylenes.

a crystal-to-crystal polymerization. Clearly, diacetylenes, by themselves, show
no tendency to self-assemble with the required structural parameters shown in
Figure 1. The organization of diacetylenes in the solid state for a topochemically
controlled polymerization would appear to be an excellent candidate for crystal
engineering (18) or supramolecular synthesis. However, there has been little ef-
fort devoted to the application of crystal design methods (19) to the preparation
of diacetylene crystals suitable for a topochemical polymerization.

The most critical structural parameter shown in Figure 1 is the transitional

repeat distance d of the diacetylene. The ideal value for d is about 0.49 nm. This is
a necessary structural parameter for a topochemically controlled polymerization.
If this structural condition is achieved and the diacetylene functionalities close
pack then a simple calculation demonstrates that the angle will be about 45

◦

and

the 1–4 carbons of the diyne will be in close contact.

Inspection of the known crystal-to-crystal polymerizations reveals two pack-

ing themes for establishing a d value of about 0.49 nm. The diacetylene monomers
contain either an aromatic ring or amide functionality. Parallel translated aro-
matic rings is a common self-assembly for aromatic compounds. This assembly
produces a translational repeat distance between 0.46 and 0.54 nm (20), which in-
cludes the required d of 0.49 nm. The difficulty of using the

π-stacking to establish

a desirable d for polymerization, in a directed supramolecular synthetic scheme, is
that there are three other common assemblies of aromatic rings that are not suit-
able for polydiacetylene polymerizations. The

π-stacking of aromatic rings is not a

reliable supramolecular interaction. Nevertheless,

π-stacking has been observed

for the organization of diacetylenes for a topochemically controlled polymerization.
One notable example that has often been studied is PTS (polyhexadiyne-1,6-diol-
bis-p-toluene sulfonate; Fig. 2) (21).

A variation of this theme, the use of parallel translated aromatic rings

as a supramolecular synthon (19), has been exploited as a possible route
to the cis-polydiacetylene, an isomer of the trans-polydiacetylene shown in
Figure 1. The required supramolecular structure was achieved using the benzene–
hexafluorobenzene interaction (22), which stacks the aromatic rings almost on
top of each other. This arrangement places the terminal carbon atoms of adjacent
diynes into close contact for a polymerization to the cis-polydiacetylene.

Hydrogen bonded amides are commonly used by nature for supramolecu-

lar assembly. The translational repeat of amides (23) is within the range of the

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Fig. 2.

Crystal structures of PTS showing the

π-stacking of aromatic rings to establish

the required translational distance for a topochemically controlled polymerization.

0.49 nm shown in Figure 1. This hydrogen bond motif is persistent and has often
been used to organize diacetylenes for a topochemical polymerization. Because
of its solubility in common organic solvents one of the better known examples
of a polydiacetylene is derived from the symmetrical 4-BCMU [5,7-dodecadiyne-
1,12-bis(butylcarboxymethylurethane); Fig. 3] (3). Because of the ability of the
urethane functionality to self-assemble according to Figure 1 and the relative
ease of synthesis (24) (Fig. 4), the poly n-BCMUs have been a popular class of
compounds to study (25). By varying the nature of the alkyl ester, this class of
potentially polymerizable diynes can be further extended (26).

It has been observed that with N-phenyl urethanes the conformation of the

alkyl link between the diacetylene and urethanes can have an effect on the poly-
merization. When the alkyl group contained an even number of carbon atoms (four
or six) the polymerization occurred as anticipated but failed with an odd number
of carbon atoms (five). Although the association of all of these urethanes was the
same, the low energy molecular conformations of this latter compound do not allow
for close contact of the 1,4 atoms of adjacent diacetylenes shown in Figure 1. This
observation demonstrates the importance to consider both intermolecular and in-
tramolecular interactions in the design of a supramolecular synthetic strategy.

Polydiacetylenes with aromatic rings attached to the polymer backbone has

been a challenge to synthetic chemistry. These compounds are of interest be-
cause of the known abilities of aromatic systems to interact with the delocalized
π-system to produce new materials with interesting properties. For example, it
has been argued that aromatic rings extend the

π-system to give materials with

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207

Fig. 3.

Crystal structures of BCMU showing the hydrogen bonding of the carbamate

functionality to establish the required translational distance for a topochemically controlled
polymerization.

Fig. 4.

Preparation of n-BCMUs. R

= n-butyl.

enhanced nonlinear properties. Because of the tendency of aromatic rings to stack
in parallel planes, diacetylenes with aromatic rings attached to the dialkyne func-
tionality sometimes have favorable parameters for a topochemical polymerization
(27). However, the reluctance of diaryl-substituted diacetylenes to polymerize is
well known and is believed to be due to the development of unfavorable nonbonded
interactions between the aromatic rings along the reaction trajectory.

In order to prepare polydiacetylenes with aromatic rings directly attached to

the polymer backbone an interesting strategy has been employed. The aromatic
ring is attached to one position of the diyne and a functionality for proper self-
assembly (Fig. 1) is attached to the other position of the diyne. A carbamate func-
tionality has commonly been used to provide the proper supramolecular structure
(Fig. 5). Advantages of this scheme are that the reduced nonbonded interactions

Fig. 5.

The use of carbamates and host–guest chemistry to organize diacetylenes for a topochemically controlled polymerization.

208

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DIACETYLENE AND TRIACETYLENE POLYMERS

209

between the aromatic rings will facilitate polymerization and a choice of carba-
mates analogous to the n-BCMUs may impart favorable solubility properties to
the polydiacetylenes. A disadvantage of this scheme is that the molecular synthe-
sis of the monomers becomes more tedious. This strategy has been applied to the
synthesis of a number of interesting polydiacetylenes (28) and will undoubtedly
continue to be important in the future.

A different but related strategy to the polymerization of diacetylenes has

been to use host–guest chemistry. One molecule, the host, provides the required
molecular scaffold and the other, the guest, contains the diacetylene functionality.
Reliable functionalities for producing the molecular repeat distance d (Fig. 1) are
ureas or, better, oxalamides (29). The carboxylic acid–pyridine interaction has
been used to assemble the host and the guest.

The practical goal of the above studies is to prepare different polydiacetylenes

with different properties. This is done by using synthetic chemistry to alter the
nature of the substituents on the diacetylene monomer that can produce a poly-
diacetylene with different properties. A major problem with this approach is that
a change in the substituent can change the solid-state packing and can produce
a diacetylene without the proper structural parameters for a topochemically con-
trolled polymerization. Rather than modifying the monomer an alternate strategy
is to modify the polymer. This elegant approach has led to some new azo and tri-
cyanovinylated polydiacetylenes with interesting properties. Postmodification of
polydiacetylene polymers will undoubtedly prove to be an effective method for the
production of advanced polymers.

The structural parameters required for a topochemically controlled diacety-

lene polymerization can be achieved by organizing the diacetylenes into layers.
Advantages of layered materials are that they are anisotropic and can more read-
ily be adapted to traditional fabrication methods than single crystals. Layered
diacetylene structures can be constructed using a number of methods such as
Langmuir–Blodgett films (30), membranes (31), liquid crystals (32), and SAMs
(33). Layers meet the translational spacing requirement shown in Figure 1. If the
head groups are spaced at about 0.49 nm and the chains containing the diacetylene
functionality associate within the van der Waals radii, then the diacetylenes will
be properly oriented for polymerization. Topochemically controlled diacetylene
polymerization has been observed in a number of layered structures. Amphiphilic
molecules frequently contain a polar, hydrogen bonding head group, such as a
carboxylic acid or amide, to establish the required 0.49 nm translational spacing.
If the hydrocarbon chains close pack then carbon atoms 1 and 4 of the diacetylene
functionality will be in close contact for a 1,4-diacetylene polymerization.

Related to the above approach for the preparation of polydiacetylenes

is the recent report of forming single polydiacetylene nanowires of 10,12-
nonacosadiynoic acid on a graphite surface. The polymerization of single arrays
of this diacetylene were initiated and terminated using the probe of a scanning
tunneling microscope (STM). This technique has the interesting possibility of in-
terconnecting nanostructures by using polydiacetylene nanowires (34).

The use of monolayers to achieve the supramolecular structural parameters

for a topochemical polymerization is a powerful strategy that will undoubtedly
prove to be useful for the preparation of sensors (35) and devices (36) that exploit
the diacetylene-conjugated polymeric backbone (Fig. 6).

Fig. 6.

A lipid monolayer with the diacetylene functionalities properly oriented for a topochemically controlled polymerization.

210

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211

Properties of Polydiacetylenes.

Among the interesting properties dis-

played by conjugated polymers such as polydiacetylenes, nonlinear optical
response and electrical conductivity have attracted the most attention (37). In
addition, polydiacetylenes display chromism that is induced by environmental
conditions. Interestingly, the chromism (blue to red to yellow) of polydiacetylenes
has been known for a long time but its orgins are still controversial (25). The color
changes have generally been attributed to a decrease of the effective conjugation
length of the polymer backbone. The changes in the effective conjugation length
can be affected by changes in the conformation of the polymer backbone. Polymer
conformational changes can be the result of a number of different actions involv-
ing polymer associations or the interactions of the substituents on the polymer
backbone.

Whatever its origin, the chromism of polydiacetylenes has been the ba-

sis of interesting sensor development. Amphiphilic diacetylenes are known to
form vesicles that can be polymerized to colored polydiacetylenes. If a spe-
cific ligand is attached to the vesicle then binding to receptor can induce a
color change due to a conformational change of the polymer backbone (38).
For example, sialic acid binds to the influenza virus lectin, hemagglutinin. A
polydiacetylene liposome with sialic acid attached undergoes a color change
from blue to red when exposed to the influenza virus. These impressive sen-
sors are acting in an analogous, although primitive, manner to a biological
membrane.

A robust polydiacetylene/silica composite has recently been prepared that

undergoes a color change in response to thermal, chemical, and mechanical stimuli
(39). This new hybrid material holds considerable promise for the fabrication of
new devices.

Probably the most studied and exciting property of polydiacetylenes is their

nonlinear optical properties (40). Polydiacetylenes are one-dimensional molecular
quantum wires. Along the polymer backbone of the polymer the polydiacetylenes
are effectively centrosymmetric and have only odd order nonlinearities such as 3.
PTS has the largest known off-resonant nonlinear refractive index (41,42). This
property makes the polydiacetylenes exciting candidates for the preparation of
optical devices such as waveguides (43). The development of polydiacetylenes that
can be deposited as oriented noncrystalline thin films would be desirable. However,
an advantage of organic materials is that their properties can be fine-tuned using
organic synthesis.

One of the interesting aspect of conjugated polymers is their ability to be-

have as electrical conductors (44) (see E

LECTRICALLY

-C

ONDUCTING

P

OLYMERS

). The

metallic appearance of many polydiacetylene crystals suggests they would be good
candidates as electrical conductors. However, these organic materials are insula-
tors with conductivities less than 10

− 10

. Polyacetylene also shows low conductivity

in the solid state but it is dramatically altered by doping. Because of the similarity
of polydiacetylene to polyacetylene it was anticipated that doping would enhance
the conductivity of polydiacetylene. Although increases in conductivity have been
observed, it is not as large as has been observed for polyacetylene (4). One of the
problems that appears is the inability of the dopant to penetrate the polydiacety-
lene materials because of their high crystallinity.

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Polytriacetylenes

Preparation of the Triacetylene Monomers.

Triacetylenes are most

commonly prepared using the Cadiot–Chodkiewicz coupling reaction (11,12). Us-
ing this method each triple bond is added in a separate step.

Preparation of Polytriacetylene.

Soon after the early understanding of

the diacetylene polymerization was reported (8,9), attempts were made to poly-
merize a triacetylene to produce a polytriacetylene (45). However, these early at-
tempts as well as more recent efforts (7) were not successful. The difficulty of the
topochemically controlled polymerization is the organization of the triacetylene
monomer with a translational repeat distance of about 0.74 nm.

The first oligomers with the backbone structure of polytriacetylene were ac-

tually prepared by the oxidative coupling of ene diynes (Fig. 7) (46). An advantage
of oligomers is that they can usually be prepared with a defined length, making
them excellent models for conjugated polymers. The triacetylene oligomers pre-
pared by oxidative coupling have been proven to be remarkably stable and are
promising candidates for second hyperpolarizabilities (47).

Polytriacetylenes have recently been prepared by a topochemically controlled

polymerization of a triacetylene. This was accomplished using the host–guest
strategy (Fig. 8). A vinylogous amide was used to establish the required trans-
lational repeat distance and

γ -rays were necessary to induce the polymerization

(48).

The pyridine host can be removed from the polytriacetylene by washing with

acid and the polytriacetylene can be dissolved in base. The optical and Raman
spectra are in accord with those that have been observed for the oligomers.

Fig. 7.

Two strategies for the preparation of polytriacetylene.

Fig. 8.

The host–guest strategy for the preparation of polytriacetylenes.

213

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DIACETYLENE AND TRIACETYLENE POLYMERS

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Properties of Polytriacetylenes.

Most of what is known about the prop-

erties of polytriacetylene has been determined from studies on the oligomers pre-
pared by oxidative coupling of ene diynes (46). It has been determined that the
effective conjugation length of the polymer is about 10 monomer units; that is,
most properties of polytriacetylenes will be expressed in oligomers containing 10
monomer units. These oligomers display high environmental stability and excel-
lent solubilities in common solvents. These properties facilitate their study and
possible development into devices.

The solution optical gap for polytriacetylenes has been estimated to be E

g

=

2.3 eV. This is comparable to E

g

= 2.3 eV for polydiacetylenes (49).

Their second hyperpolarizabilities (47) have been studied using third-

harmonic generation and degenerate four-wave mixing. These studies suggest
interesting macroscopic hyperploarizability (7) for properly prepared bulk mate-
rials.

The polydiacetylenes and the newer polytriacetylenes are clearly exciting

families of conjugated polymers. Because these polymers are highly organized
and their properties can be controlled by organic synthesis they offer considerable
potential for the development of advanced materials. Their major disadvantage is
their preparation. The preparation of a given polymer requires two major steps.
Preparation of the monomers, followed by their organization in the solid state
for a topochemically controlled polymerization. This latter step, the control of
supramolecular structure, is particularly challenging. If the preparation of ad-
vanced materials is to ever be a rational process, it is important that the scientific
community meet this challenge.

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F

RANK

F

OWLER

SUNY

DNA.

See P

OLYNUCLEOTIDES

.