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GENE-DELIVERY POLYMERS

Vol. 6

GRAFT COPOLYMERS

Introduction

Graft copolymers are composed of a main polymer chain (the backbone) to which
one or more side chains (the branches) are chemically connected through cova-
lent bonds. A graft copolymer with one branch can be considered as a miktoarm
star copolymer. The backbone and the branches may be homopolymers or copoly-
mers but they differ in chemical nature or composition (1). The branches are
usually equal in length and randomly distributed along the backbone because of
the specific synthetic techniques used for their preparation. However, more elab-
orate recent methods have allowed the synthesis of regular graft copolymers with
equally spaced and identical branches and of exact graft copolymers, where all
the molecular and structural parameters can be accurately controlled (Fig. 1).

A simple graft copolymer can be represented as A

k

-graft-B

m

or polyA-graft-

polyB or poly(A-g-B), where A

k

or polyA is the backbone to which the B

m

or polyB

branches are grafted. The nomenclature of graft copolymers follows the rules rec-
ommended by the IUPAC Commission on Macromolecular Nomenclature (2).

Graft copolymers have been mainly used to modify polymer properties be-

cause of their unique mechanical, thermal, dilute solution, and melt properties
(3–7). The availability of such materials has permitted the researchers to explore
the correlation between the structure of the graft copolymers and their properties.

Synthesis of Graft Copolymers

Three general methods have been developed for the synthesis of randomly
branched graft copolymers: (1) the “grafting onto”, (2) the “grafting from”, and
(3) the macromonomer method (or “grafting through” method) (8) (Fig. 2).

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

Vol. 6

GRAFT COPOLYMERS

349

Fig. 1.

Graft Copolymers (1) random graft copolymer (identical branches randomly dis-

tributed along the backbone); (2) regular graft copolymer (identical branches equally spaced
along the backbone); (3) simple graft copolymer (3-miktoarm star copolymer); and (4) graft
copolymer with two trifunctional branch points. Exact graft copolymers.

The “grafting onto” method involves the use of a backbone chain containing

functional groups X randomly distributed along the chain and branches having
reactive chain ends Y. The coupling reaction between the functional backbone and
the end-reactive branches lead to the formation of graft copolymers.

In the “grafting from” method active sites are generated randomly along

the backbone. These sites are capable of initiating the polymerization of a second
monomer, leading to graft copolymers.

The most commonly used method for the synthesis of graft copolymers

is the macromonomer method. Macromonomers are oligomeric or polymeric
chains bearing a polymerizable end group. Macromonomers having two polymer-
izable end groups have also been reported (8). Copolymerization of preformed
macromonomers with another monomer yields graft copolymers.

“Grafting onto” Methods.

In the “grafting onto” method, reaction of pre-

formed polymeric chains having functional groups, with other polymeric chains
having active chain ends, takes place. In most cases the incorporation of func-
tional groups is performed by chemical modification of the backbone (9–14). A
common procedure is the chloro(bromo) methylation of polystyrene (eq. 1), and
the subsequent reaction with living polymeric chains.

(1)

350

GRAFT COPOLYMERS

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Fig. 2.

Three general methods of synthesis of randomly branched graft copolymers.

Using this method polystyrene-g-poly(ethylene oxide) (PS-g-PEO) graft

copolymers were prepared (9–12). The chloromethylation of PS was performed
using a CCl

4

solution of PS with chloromethyl methyl ether, with SnCl

4

as the

catalyst. The reaction conditions were controlled in such a way so as to give low
chloromethyl content (

<10 wt%). A similar synthetic approach was adopted for

the synthesis of PS-g-polyisoprene (PS-g-PI) graft copolymers (10,11). In order to
avoid the side reactions involving lithium–chlorine exchange, the

CH

2

Cl groups

were transformed to the

Si(CH

3

)

2

Cl group before reaction with the living poly-

mers. PS-g-poly(2-vinylpyridine) (PS-g-P2VP) and PS-g-P4VP graft copolymers
were prepared by partial chloromethylation of PS polymeric chains (13,14). Graft-
ing efficiency, ie, the ratio of the number of

CH

2

Cl groups used for the grafting

reaction and the number of the incorporated groups on the backbone, was as low
as 40% in this case. Chloromethylation of poly(N-vinyl carbazole) (PVCz) was also

Vol. 6

GRAFT COPOLYMERS

351

used in order to synthesize poly(N-vinyl carbazole)-g-PI graft copolymers (15). The
chloromethyl ether method was used in the presence of ZnCl

2

. The grafting effi-

ciency of PI chains was also found to be limited. Bromomethylation of anionically
prepared PS backbone was used for the synthesis of PS-g-poly(methyl methacry-
late) (PS-g-PMMA) copolymers (16). The polydispersity index of the resulted grafts
was higher than the polymeric precursors, indicating the presence of undesirable
reactions.

By taking advantage of living PS and PI anions with the phenyl rings

of P2VP, P2VP-g-PS and P2VP-g-PI graft copolymers were synthesized (17)
(eq. 2)

(2)

An excess of living chains compared to the 2VP groups were added. The

polydispersity index of the final products was higher than the polydisper-
sity of the backbone and the grafted chains, especially in the case of P2VP-
g-PI samples. Graft copolymers having as backbone random terpolymers of
styrene, maleic anhydride, and ethylhexyl methacrylate, or diethylfumarate,
were synthesized by radical polymerization (18). In all cases, polyethylene ox-
ides (PEO) were the grafted chains. Polyethylene glycol monomethyl ether was
grafted to the backbones after reaction with the succinic anhydride groups.
When the reaction was allowed to proceed to high conversions, gelation oc-
curred. This was attributed to the presence of difunctional polyethylene glycol,
which connects many backbones forming high molecular weight and cross-linked
products.

The “grafting onto” methodology was followed also for the preparation of

graft copolymers having poly(glycidyl methacrylate) as backbone and a mixture
of PI and PS grafted chains (19). The epoxy groups were used as grafting sites
for living PS and PI anionic chains. Graft terpolymers having poly[styrene-co-4-
(vinylphenyl)-1-butene] as backbone and either PS, PI, or grafted chains were also
prepared (20,21). The synthesis of the backbone involved the anionic copolymer-
ization of styrene with 4-(vinylphenyl)-1-butene. The backbone was partly hydrosi-
lylated, and the incorporated chlorodimethylbutylsilyl active groups were reacted
with living PS or PI chains. The graft terpolymers exhibited low polydispersity

352

GRAFT COPOLYMERS

Vol. 6

Fig. 3.

Butadiene–styrene graft and double graft copolymers.

index comparable to the one of the precursors. Poly(carboxymethylcellulose)-
g-poly(N-isopropylacrylamide)

(CMC-g-PNIPAM)

graft

copolymers

were

synthesized (22) through formation of amide bonds. Amino-terminated NIPAM
polymeric chains were obtained by radical polymerization in water. The coupling
reaction between the carboxyl groups of the backbone and the terminal amine
group of the grafts, performed in water, resulted in the formation of the desired
products.

Well-defined butadiene–styrene graft copolymers (PBd-g-PS) were synthe-

sized using hydrosilylation reactions (23). Catalytic hydrosilylation of the 1,2-Bd
units (

∼10%) of PBd introduced chlorosilane groups (Fig. 3). Linking reactions be-

tween living PS anions and the

Si Cl groups of the backbone (1) gave PBd-g-PS

graft copolymers with randomly placed single PS branches. When HSiCl

2

CH

3

was

used in the hydrosilylation step, difunctional branching sites were introduced in
the backbone, resulting in the formation of P(Bd-g-S)

2

double grafts.

Block graft copolymers are copolymers having a backbone composed of a

diblock copolymer. Grafted chains can be attached to one or both of the backbone
blocks (Fig. 4). Block grafts with a triblock as the backbone are also possible (see
B

LOCK

C

OPOLYMERS

).

Vol. 6

GRAFT COPOLYMERS

353

Fig. 4.

Block-graft copolymer. In this case only one of the backbone blocks is grafted.

Using anionic polymerization techniques poly

{styrene-b-[(4-vinylphenyl-

dimethylsiloxane)-g-isoprene]

}, P[S-b-(VS-g-I)], block graft copolymer was syn-

thesized (24,25). The block copolymer P(S-b-VS) was prepared by the sequential
addition of styrene and 4-vinyl phenyl dimethyl vinylsiloxane. Living PI chains
were reacted with the vinylsilane groups of the block copolymer to form the
block graft copolymers. In a similar fashion poly

{4-methylstyrene-b-[(styrene-co-

3- methylstyrene)-g-2-vinyl pyridine]

} block graft terpolymers were synthesized

(26). The backbone was prepared by coordination polymerization and sequential
addition of 4-methylstyrene and a mixture of styrene and 3-methylstyrene. Chlori-
nation of the methyl groups of the substituted styrenic monomeric units generated
the grafting sites. The final coupling was accomplished by adding a THF solution
of the chlorinated terpolymer to living P2VP chains at low temperature.

PS-g-PI, (PS-g-PI)-b-PS, and (PS-g-PS)-b-(PS-g-PI) copolymers have been

synthesized by using a combination of TEMPO (2,2,6,6-tetramethylpiperidinyl-
1-oxy) living free-radical and anionic polymerization (27) (Fig. 5).

The PS backbone of the PS-g-PI was synthesized by living free-radical batch

copolymerization of styrene and p-chloromethylstyrene. PI living chains prepared
by anionic polymerization, end capped with 1,1-diphenylethylene, reacted with the
chloromethyl groups to give the grafts. In a similar way the synthesis of the (PS-
g-PI)-b-PS block graft copolymers was performed. The synthetic approach of the
(PS-g-PS)-b-(PS-g-PI) graft-block-graft copolymer involved first the preparation of
the PS-g-PS grafts, followed by the copolymerization of styrene and p-chloromethyl
styrene by living radical polymerization using the nitroxide end group of the PS
backbone as the initiating site. Finally, living diphenylethylene-capped PI chains,
prepared by anionic polymerization, were reacted with the chloromethyl groups
of the extended PS backbone.

Model block-double graft copolymers and terpolymers of styrene, butadiene,

and isoprene of the type poly[S-b-(PBd-1,2-g-X)] were recently synthesized (28),
where X is either S, Bd, I, or S-b-I by a combination of anionic polymerization,
hydrosilylation, and chlorosilane linking chemistry. The backbone P(S-b-1,2Bd)
was synthesized by sequential addition of the monomers. The vinyl groups of the
PBd-1,2 block were hydrosilylated with HSiCH

3

Cl

2

and used for the attachment

354

GRAFT COPOLYMERS

Vol. 6

Fig. 5.

Polystyrene–polyisoprene graft and block copolymers.

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GRAFT COPOLYMERS

355

of living chains X, resulting in the formation of the corresponding block double
graft copolymers.

“Grafting from” Methods.

In the “grafting from” method, the backbone

is chemically modified in order to introduce active sites capable of initiating the
polymerization of a second monomer. The number of grafted chains can be con-
trolled by the number of active sites generated along the backbone assuming that
each one of them participate to the formation of one branch. However, mainly
because of kinetic and steric hindrance effects there may be a difference in the
lengths of the produced grafts.

Anionic grafting techniques were used for the synthesis of PI-g-PS and PBd-

g-PS copolymers (29–33). The active sites were generated by metallation of the
allylic double bonds of the polydiene backbone, by organometallic compounds such
as n-C

4

H

9

Li, in the presence of strong chelating agents that facilitate the reaction

(eq. 3). The most common metallation procedure involves the use of n-C

4

H

9

Li in

the presence of N,N,N

,N

-tetramethylethylenediamine (TMEDA). The activated

polydiene then reacts in the styrene. The graft copolymers exhibited well-defined
molecular characteristics.

(3)

By using the “grafting from” technique, PMMA-g-(

β-butyrolactone) (34)

copolymers were synthesized. Anionically polymerized PMMA was treated with
18-crown-6 complex of potassium hydroxide resulting in a macromolecular initia-
tor (2) (Fig. 6).

The carboxylate active groups were used as initiating sites of

β-

butyrolactone. It was found that the grafting efficiency was high, and the density

Fig. 6.

PMMA backbone with

β-butyrolactone grafts.

356

GRAFT COPOLYMERS

Vol. 6

of grafting chains could be easily controlled. Poly(ethylene-g-styrene) copolymers
were also prepared (35). The approach for the synthesis of the backbone involved
the copolymerization of ethylene and p-methylstyrene by metallocene catalysts,
such as [C

5

(CH

3

)

4

(Si(CH

3

)

2

N-t-C

4

H

9

)]TiCl

2

/C

2

H

5

(Ind)

2

ZrCl

2

. The methyl groups

of the p-methylstyrene units were then metallated with s-C

4

H

9

Li/TMEDA com-

plex. The active sites produced were used to initiate the anionic polymerization of
styrene. The efficiency of the metallation was found to be 65%.

Cationic grafting techniques have been used for the synthesis on poly(ethyl

vinyl ether-g-ethyloxazoline) graft copolymers (36). A random copolymer of ethyl
vinyl ether with a small quantity of 2-chloroethyl vinyl ether was synthesized
by cationic polymerization. The pendant alkyl groups were the initiating sites of
the cationic polymerization of ethyloxazoline grafts. In a similar way, acid chlo-
ride groups were incorporated along the polymer chain by partial hydrolysis of
poly(vinyl acetate) (3) and reaction of the produced hydroxyl groups with phos-
gene or diphosgene (37) (eq. 4). These groups were the initiating sites for the
polymerization of 2-phenyl or 2-methyl oxazoline in the presence of potassium
iodide (eq. 5).

(4)

(5)

Poly(isobutylene-g-indene) and poly(isobutylene-g-styrene) copolymers were

synthesized by cationic polymerization (38). The backbone was prepared by
cationic copolymerization of isobutylene and p-chloromethylstyrene, using TiCl

4

/t-

C

4

H

9

Cl as the initiator. The chloroalkyl groups were used, in the presence

of Al(C

2

H

5

)

2

Cl, as the initiation sites of the cationic polymerization of in-

dene or styrene. The grafting efficiency was as low as 40–50 %. [MMA-co-
(p-chloromethylstyrene)]-g-[2-methyl-2-oxazoline] amphiphilic copolymers were
prepared by combining radical and cationic polymerization techniques (39).
The synthesis of the backbone was performed by radical copolymerization of
MMA and p-chloromethylstyrene in the bulk. The cationic macroinitiator was
dissolved in benzonitrile along with 2-methyl-2-oxazoline (MeOXz) and the

Vol. 6

GRAFT COPOLYMERS

357

production of PmeOXz grafts took place at 110

◦

C. A convenient route for the

preparation of poly(methylphenylsilane)-g-PS was presented (40). The synthetic
approach involved the alkali metal mediated reductive-coupling reaction of
dichloro methylphenylsilane with sodium in toluene at 100

◦

C (eq. 6). The phenyl

groups were then bromomethylated (eq. 7), and the incorporated bromomethyl
groups were used as the initiators of the polymerization of styrene by atom trans-
fer radical polymerization (ATRP) (eq. 8).

(6)

(7)

(8)

Polypropylene-g-PS copolymers were synthesized by combination of met-

allocene and TEMPO living free-radical polymerization techniques (41). The
backbone was synthesized by copolymerization of propylene and a TEMPO-
functionalized derivative containing a

α-double bond. The TEMPO groups were

then used for the polymerization of styrene by living free-radical polymerization
(Fig. 7).

By using ATRP, poly(2-hydroxyethyl methacrylate)-g-PS (PHEMA-g-PS) and

PHEMA-g-poly(n-butyl acrylate) (42) were synthesized. The backbone, comprised

358

GRAFT COPOLYMERS

Vol. 6

Fig. 7.

Synthesis of Polypropylene-g-polystyrene.

of trimethylsilyl-protected 2-hydroxyethyl methacrylate, was synthesized by us-
ing ATRP methodology, followed by deprotection of the hydroxyl group. Subse-
quent esterification (eq. 9) with 2-bromoisobutyryl bromide resulted in poly[2-(2-
bromoisobutyryloxy)ethyl methacrylate]. (7).

PS or poly(n-butyl methacrylate) were then grafted from the macromolec-

ular initiator (eq. 10). Subsequent analysis revealed that all the initiation sites
participated in the polymerization of the second monomer.

Vol. 6

GRAFT COPOLYMERS

359

(9)

(10)

In a later work, copolymers of PVC with various grafted chains like poly(butyl

acrylate), PMMA, PS, and poly(methyl acrylate) were synthesized (43). The back-
bone was a random copolymer of vinyl chloride and vinyl acetate. The carbonyl
substituted alkyl halide groups were used as the initiator for the polymerization
of these monomers by ATRP.

Metallocene and ATRP techniques were combined for the synthesis of graft

ter- and quaterpolymers poly(ethylene-co-styrene) as backbone and one of PS,
PMMA, PMMA-b-PS, PMMA-b-polymethylacrylate, and PMMA-b-PHEMA as
grafted chain (44). The aromatic rings of styrene were partially brominated and
the C–Br groups that were formed were subsequently used as initiating sites for
ATRP polymerization.

Block-graft copolymers have also been prepared by the “grafting from”

method. Reaction of the block copolymers P(S-b-V) (25) with n-C

4

H

9

Li in THF

at

−30

◦

C for 1 h leads to metallation of the vinyl groups of the vinyl segments.

Then D

3

was added for the formation of P[(S-b(VS-g-DMS)] block graft copoly-

mers. The molecular characterization revealed that the number of the introduced
PDMS branches was between 170 and 325.

The synthesis of the poly[styrene-b-(hydroxystyrene-g-ethylene oxide)-b-

styrene] (25), P[S-b-(HS-g-EO)-b-PS], has been reported. The backbone was a tri-
block copolymer, poly(styrene-b-t-butoxystyrene-b-styrene) (8), prepared by an-
ionic polymerization by sequential addition of monomers. The protected t-butyl
group was removed by treatment with HBr leading to the formation of P(S-b-HS-
b-S) triblocks (9) (eq. 11). The metallation of the hydroxyl groups was performed
in THF using either cumyl potassium or diphenylethylene potassium (eq. 12). The
addition of EO generated the block graft copolymers (eq. 13).

360

GRAFT COPOLYMERS

Vol. 6

(11)

(12)

(13)

P(S-g-t-BMA) and PS-b-P(S-g-t-BMA) block graft copolymers were prepared

using TEMPO living and atom transfer radical polymerization techniques (45).
The backbone of the P(S-g-t-BMA) copolymer was synthesized by TEMPO living
radical copolymerization of styrene and p-chloromethylstyrene. Subsequently, the
chloromethyl groups in the presence of CuBr and bipyridine were used as initiation
sites of ATRP of t-BMA. In the case of PS-b-P(S-g-t-BMA) copolymer, the synthesis
was performed in a similar way.

Macromonomer Method.

The synthesis of graft copolymers by the

macromonomer method is characterized by its own specific features (46–49).
The number of branches is determined by the ratio of the macromonomer and
comonomer molar concentrations and their copolymerization behavior, described
by the reactivity ratios r

1

and r

2

. These parameters determine how random the

placement of the branches along the backbone will be. It is evident that during
the copolymerization the relative concentrations of the macromonomer and the
comonomer change with time, leading to the formation of graft copolymers with
subsequently different number of branches. In addition, the copolymerization is
not homogeneous throughout the course of the reaction since phase separation
may occur. For the above reasons it can be concluded that the graft copolymers
prepared by this method are generally characterized by increased compositional
and chemical heterogeneity.

Vol. 6

GRAFT COPOLYMERS

361

The synthesis of macromonomers can be accomplished by almost all the avail-

able polymerization techniques. Among them, living polymerization methods offer
unique control over the molecular weight, the molecular weight distribution, and
chain-end functionalization.

Anionic polymerization is one of the best methods for the synthesis of well-

defined macromonomers. Functional initiation or termination by a suitable elec-
trophilic reagents are the best ways for the incorporation of the reactive end groups
(50). According to this methodology living polystyryllithium initially reacts with
ethylene oxide to form the less reactive alkoxide, followed by the reaction with
methacryloyl chloride for the synthesis of the macromonomer (eq. 14) (51).

(14)

These macromonomers were then copolymerized with vinyl monomers mainly
by free-radical techniques (52) and also other methods as by metallocene cata-
lysts (53) to provide graft copolymers. The reaction of polystyryllithium with p-
chlorovinylbenzene at low temperatures affords macromonomers with styryl end
groups (eq. 15) (54).

(15)

Allyllithium was used to polymerize hexamethylcyclotrisiloxane, D

3

(eq. 16).

Subsequent termination (eq. 17) with chlorotrimethylsilane yielded polydimethyl-
siloxane (PDMS) macromonomers carrying allyl end groups (55).

(16)

(17)

Examples of graft copolymers synthesized using anionically prepared

macromonomers are given in Table 1.

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GRAFT COPOLYMERS

Vol. 6

Table 1. Graft Copolymers Prepared by Macromonomers Synthesized Anionically

Macromonomer

End group

Comonomer

Copolymerization

Reference

PS

Methacrylate

MMA

Radical

56,57

PS

p-Vinylbenzene

MMA

Radical

54

PS

Norbornene

Norbornene

ROMP

58

PS

p-Vinylbenzene

Butadiene

Anionic

59

PS

Methacrylate

Ethyl acrylate

Radical

60

PDMS

Methacrylate

MMA

GTP

61

PMMA

p-Vinylbenzene

Styrene

Radical

62

PI

Methacrylate

MMA

GTP

61

PS-b-PI

Methacrylate

Styrene

Radical

63

PDMS

Allyl

Ethylene

Ziegler–Natta

55

PDMS

Methacrylate

Styrene

Radical

64

PS-b-PDMS

Diol

Diphenylmethyl

Polycondensation

65

diisocyanate

+

butanediol

P

t

C

4

H

9

Ma

a

p-Vinylbenzene

4-Vinylpyridine

Radical

66

PI

p-Vinylbenzene

Styrene

Metallocenes

67

PBd

Norbornene

Norbornene

ROMP

68

PEO

Norbornene

Norbornene

ROMP

69

PS

Allyl

Propylene

Metallocene

70

PEO

Methacrylate

Styrene

ATRP

71

PDMS

Methacrylate

MMA

ATRP, radical

72

PS, PI, PDMS

Methacrylate

MMA

Metallocene

53

a

Poly(t-butyl methacrylate).

Cationic polymerization has also been used for the synthesis of

macromonomers, especially after the development of living cationic polymer-
ization techniques (73). Macromonomers were prepared by the cationic ring-
opening polymerization of tetrahydrofuran (THF) using methyltrifluoromethane
sulfonate, followed by termination with 3-sodio-propyloxydimethylvinylsilane to
give a macromonomer with vinyl silane end groups (74) (eq. 18).

(18)

These macromonomers were copolymerized with vinyl acetate (Vac) using azo-
bisisobutyronitrile (AIBN) as radical initiator to produce PVAc-g-PTHF graft
copolymers. Subsequent saponification using NaOH provided poly(vinyl alcohol)-
g-PTHF graft copolymers.

Termination of living PTHF (13) with 3-(dimethylamino propyl) isocyanide

leads to the formation of macromonomers having end isocyanide groups (75)
(eq. 19).

Vol. 6

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363

(19)

p-Iodomethylstyrene was used as a functional initiator to promote the poly-

merization of 2-phenyl-2-oxazoline leading to the formation of macromonomers
having styryl end groups (76) (eq. 20).

(20)

Radical copolymerization with styrene produced PS-g-poly(2-phenyl-2-oxazoline)
graft copolymers. Other examples of graft copolymers prepared using macromono-
mers, which were synthesized by cationic polymerization, are given in Table 2.

Free-radical polymerization has been the most common technique for

the synthesis of macromonomers because of its less demanding experimental
conditions, the absence of special purification procedures of reagents used, and its

Table 2. Graft Copolymers Prepared by Macromonomers Synthesized by Cationic
Polymerization

Macromonomer

End group

Comonomer

Copolymerization Reference

Poly(2-alkyl-2-

Diethanolamine

ε-Caprolactone +

Polycondensation

77

oxazoline)

4,4

-methylene

di(phenylisocyanate)

Poly(2-alkyl-2-

(Meth)acrylate

78

oxazoline)

PPO, poly

(Meth)acrylate

Styrene, MMA

Radical

79

(epichlorohydrin)

Polyisobutylene

Methacrylate

MMA

GTP

80

364

GRAFT COPOLYMERS

Vol. 6

applicability to a wide variety of monomers. However, the method suffers many
disadvantages such as the poor control over the molecular weight, the molecu-
lar weight distribution, and the degree of functionalization. The basic method for
the incorporation of the functional end groups involves the use of chain-transfer
agents (81).

Poly(methyl methacrylate) (PMMA) macromonomers have been prepared us-

ing thioglycolic acid as a chain-transfer agent, followed by reaction with glycidyl
methacrylate (82) (eq. 21).

(21)

α-(Bromoethyl) acrylate has been also used as a chain-transfer agent for the

synthesis of PMMA macromonomers carrying acrylic end groups (83) (eq. 22).

(22)

Table 3 summarizes examples of graft copolymers prepared by macromonomers
that were synthesized by free-radical polymerization methods.

The evolution of the living free-radical polymerization techniques (mainly

TEMPO (2,2,6,6-tetramethylpiperidinyl-1-oxy) and ATRP methods) very soon led
to the synthesis of macromonomers. These methods combine the advantages of
the free-radical polymerization with those of the living polymerization techniques,
despite the fact that control over the functionalization reaction is not always com-
parable to the anionic polymerization methods (91,92).

A multifunctional reagent (14) containing reactive sites [TEMPO, oxazoline,

and t-butyl dimethyl silyl protected hydroxyl (TBDMS) group] able to initiate an-
ionic, cationic, and living free-radical polymerization was used to prepare a variety
of macromonomers (93). Styrene was polymerized at high temperatures through
the alkoxyamine functional group to produce macromonomers with oxazoline end
groups (15) (eq. 23). Cationic copolymerization of these macromonomers with ox-
azoline using methyl triflate as initiator leads to the formation of polyoxazoline-
g-PS graft copolymers.

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365

Table 3. Graft Copolymers Prepared by Macromonomers Synthesized by Free-Radical
Polymerization

Macromonomer

End group

Comonomer

Copolymerization Reference

PMMA

Methacrylate

Acrylic acid

Radical

84

PMMA

Acrylate

Styrene, vinyl

chloride, vinyl
acetate

Radical

83

Polyvilylpyrrolidone p-Vinylbenzene

Styrene

Radical

85

Poly(4-

vinylpyridine)

p-Vinylbenzene

Styrene

Radical

85

PMMA

Aromatic

dicarboxy

Terephthalic acid

+ bisphenol

Polycondensation

86

PS

Carboxyl

Ethyl cellulose

UV irradiation

87

PMMA

Dihydroxyl

ε-Caprolactone

Polycondensation

88

Poly(ethyl

methacrylate),
poly(n-butyl
methacrylate)

Methacrylate

Methacrylic acid

Radical

89

PMMA

Acrylate

Styrene

ATRP

90

(23)

Poly(dimethylaminoethyl methacrylate) macromonomers were prepared by

ATRP using allyl 2-bromoisobutyrate (ABIB, (16))/CuBr/tris[2-di(butyl acrylate)
aminoethyl] amine, (BA6-TREN) or allyltrichloroacetamide/CuBr/BA6-TREN as
the initiation systems (94) (eq. 24).

(24)

Group transfer polymerization has also been used for the synthesis of

macromonomers. The functional end groups are incorporated mainly using the
suitable initiation system (95). Trimethylsilyloxy ethyltrimethylsilyl dimethyl
ketene was used as initiator for the synthesis of PMMA macromonomers (17)

366

GRAFT COPOLYMERS

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in the presence of tetrabutylammonium benzoate as the catalyst (eq. 25). Treat-
ment with dilute HCl (eq. 26) provided the

OH functional macromonomers (18),

followed by the reaction with acryloyl chloride to give the final macromonomer
structure (96) (eq. 27).

(25)

(26)

(27)

Polyacrylate macromonomers were prepared using triphenyl phosphine and

trimethylchlorosilane in the presence of zinc halide as the catalyst (97) (eq. 28).
The resulting polymers (19), having trimethylphosphinium end groups, were
transformed to the corresponding macromolecular ylide (20) by reaction with
sodium ethanolate (eq. 29). Subsequent Wittig reaction (eq. 30) of the ylide pro-
vides the macromonomer (21) where R

= methyl, ethyl, or n-butyl.

(28)

(29)

(30)

More examples concerning the synthesis of macromonomers by group trans-

fer polymerization and the subsequent formation of graft copolymers are given in
Table 4.

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367

Table 4. Graft Copolymers Prepared by Macromonomers Synthesized by Group
Transfer Polymerization

Macromonomer

End group

Comonomer

Copolymerization Reference

Poly(vinyl alcohol)

p-Vinylbenzene

Styrene

Radical

98,99

Poly(2-phenyl-1,3,2-

dioxaborole)

p-Vinylbenzene

Styrene

Radical

100

Methyl, hexyl,

n-butyl acrylate

Vinyl

Styrene

Radical

97

PMMA

p-Vinylbenzene

Styrene

Radical

101

PMMA

Methacrylate

n-Butyl acrylate

ATRP, radical

102

Polycondensation techniques have been employed for the synthesis of

macromonomers. Depending on the nature of the functional end group the copoly-
merization with comonomers can be performed with addition polymerization tech-
niques, giving rise to very interesting graft copolymer structures.

Poly(

γ -benzyl-

L

-glutamate) macromonomers were prepared by polymeriza-

tion of benzyl(S)-3-(2,5-dioxo-1,3-oxazolidin-4-yl)propionate (21) (or

γ -benzyl-

L

-

glutamate-N-carboxyanhydride) using the primary amino group of the N-methyl-
N-4-vinylphenethyl ethylene diamine (22)(103,104) (eq. 31).

(31)

Subsequent reaction with aminoalcohols provides another type of macromonomer
with side

OH groups (eq. 32).

(32)

368

GRAFT COPOLYMERS

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Table 5. Graft Copolymers Prepared by Macromonomers Synthesized by
Polycondensation Methods

Macromonomer

End group

Comonomer

Copolymerization Reference

Polyguanamine

Isopropenyl

Styrene, MMA

Radical

105

substituted triazine

Polyurethane

Methacrylate

MMA

Radical

106,107

Polyamine

p-Vinylbenzene

2-Hydroxyethyl

Radical

108

methacrylate

Characteristic examples concerning the synthesis of macromonomers pre-

pared by polycondensation reactions and the subsequent formation of graft copoly-
mers are given in Table 5.

Macromonomers have been also prepared by post-polymerization reactions.

Poly(ethylene oxide) (PEO) or poly(propylene oxide) (PPO) has one or two OH end
groups depending on the method of their preparation. These

OH groups may re-

act with (meth)acryloyl chloride, p-vinyl benzyl chloride, norbornenyl chloride, 4-
vinyl benzoyl chloride, etc. for the synthesis of the corresponding macromonomers
(109,110) (eq. 33).

(33)

Hydrosilyl-terminated PDMS chains may undergo hydrosilylation reactions

with comonomers having double bonds to produce macromonomers (111,112)
(eq. 34).

(34)

More examples of macromonomers prepared by post-polymerization reactions are
provided in Table 6.

Other methods have also been employed for the synthesis of macromonomers.

They do not have always general applicability but can be successfully
used in specific cases. For example, polypropylene macromonomers with

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GRAFT COPOLYMERS

369

Table 6. Graft Copolymers Prepared by Macromonomers Synthesized by
Post-Polymerization Reactions

Macromonomer

End group

Comonomer

Copolymerization Reference

PEO

Acrylate

Styrene

Radical

109

PDMS

Methacrylate

Styrene

Radical

112

PPO

Norbornene

Norbornene derivative ROMP

113

PEO

p-Vinylbenzene n-Butyl methacrylate

Radical

110

PEO

Methacrylate

Acrylamide

Radical

114

Oligo(

α-

Methacrylate

t-Butyl acrylate

Radical

115

hydroxyalkanoic
acid)

PEO

Allenyloxy

Coordination

116

polymerization

vinylidene end groups were prepared using metallocene-catalyzed polymerization.
Bis(cyclopentadienyl)zirconium dichloride/methylaluminoxane (Cp

2

ZrCl

2

/MAO)

and other catalytic systems were used. The olefinic end groups were obtained
through the

β-hydrogen elimination mechanism. These macromonomers were ei-

ther used directly to produce graft copolymers after copolymerization with olefins
using metallocene catalysts or were transformed to other structures by post-
polymerization reactions. Addition of thioacetic acid introduced thiol functional
groups, whereas hydroboration and subsequent reaction of the resulting hydroxyl
group with methacrylic acid produced methacryloyl-terminated polypropylenes.
Radical copolymerization with MMA provided PMMA-g-PP graft copolymers
(117).

Styryl amylose amide (VAA) was prepared from maltopentose-substituted

styrene (VM5A) by phosphorylase-catalyzed polymerization of glucose-1-
phosphate (Glu-1P) (118) (Fig. 8). Subsequent radical copolymerization with acry-
lamide gave the corresponding graft copolymers..

Ring-opening metathesis polymerization was employed for the synthesis

of polynorbornene macromonomers (24). A well-defined molybdenum initiator
of the type Mo(CHC(CH

3

)

2

C

6

H

5

)(N-2,6-(i-C

3

H

7

)

2

C

6

H

3

)(OR)

2

[R

= OC(CH

3

)

3

,

OCCH

3

(CF

3

)

2

] was used. The polymers produced were cleaved from the initia-

tor fragment in a Wittig-like reaction using p-(CH

3

)

3

SiC

6

H

4

CHO, according to

a well-established method (eq. 35). Deprotection under basic conditions gave
macromonomers having phenyl end groups (25) (eq. 36). Subsequent reaction with
norbornene carboxylic acid chloride yields macromonomers with norbornene end
groups (119) (eq. 37).

(35)

370

GRAFT COPOLYMERS

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Fig. 8.

Preparation of the macromonomer VAA.

(36)

(37)

Regular Graft Copolymers.

The term regular graft copolymers corre-

sponds to graft copolymers with identical equally spaced branches along the
backbone. Regular graft copolymers of PI-g-PS have been prepared. The syn-
thetic approach involves the selective replacement of one chlorine atom of methyl-
trichlorosilane by PS followed by step-growth polymerization of the produced
(PS)Si(CH

3

)Cl

2

with

α,ω-dilithium PI. The polydispersity of the reaction products

Vol. 6

GRAFT COPOLYMERS

371

Fig. 9.

Synthetic approach to regular PI–PS graft copolymers.

is high (M

w

/M

n

= 2.0–3.0) but it is reduced by fractionation (M

w

/M

n

= 1.2–1.5).

By using SiCl

4

instead of Si(CH

3

)Cl

3

and by the replacement of two chlorines by

PS, graft copolymers with tetrafunctional branching points are synthesized (120)
(Fig. 9).

Exact Graft Copolymers.

The term exact graft copolymers refers to

molecules where all the molecular and structural characteristics, such as the back-
bones’ and branches’ molecular weights and molecular weights distribution, the
number of branches, and the specific grafting points on the backbone, can be con-
trolled and varied at will. The parameters that are most difficult to control are
the number and the spacing distribution of branches along the backbone chain.

Exact graft copolymers having rather simple structures, ie one or two

branches, have been prepared so far. Among these are the A

2

B and AA

B sin-

gle graft copolymers (121–128) and the H- and

π-shaped copolymers (129–131)

(Fig. 10). Several techniques have been used for the synthesis of these structures
and mainly anionic polymerization and controlled chlorosilane chemistry.

A new promising methodology was reported for the synthesis of exact graft

copolymers with PI backbone and PS branches (132) (Fig. 11). The method is

Fig. 10.

Exact graft copolymers: AA

B, H-shaped, and

π-shaped.

372

GRAFT COPOLYMERS

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Fig. 11.

Synthesis of exact graft copolymers of styrene and isoprene.

based on the use of 1,4-bis(1-phenylethenyl)benzene (27), which upon reaction
with living polyisoprenyllithium leads to the formation of a PI macromonomer.
This macromonomer is subsequently linked with polystyryllithium, followed by
the anionic polymerization of isoprene. The living single graft copolymer (27) then
reacts again with (26) and the same reaction sequence is repeated to produce the
exact graft copolymer with two branches. In principle it is possible to continue the
synthesis of graft copolymers with more branches, using the same reaction series.

Purification and Molecular Characterization of Graft Copolymers

Molecular characterization is an essential step in the study of graft copolymers
since the knowledge that is gained is essential in deducing the structure–property
relationships. Graft copolymers, like all copolymers, may present molecular
weight, compositional, and architectural (distribution in the number of branches
and the position of the branching points) heterogeneity (133–135). Following the
synthesis of graft copolymers, the next step is to thoroughly characterize the

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GRAFT COPOLYMERS

373

obtained samples in order to determine their average molecular weight, average
composition, molecular and compositional homogeneity and obtain information
on the architectural characteristics of the material. Additionally, information
about the molecular size of the individual macromolecule may be desired. Because
of the imperfections present in the synthetic methodology used for the preparation
of nearly every synthetic macromolecule, the resulting material is a very complex
system whose identity can only be revealed by the employment of a combination
of analytical techniques (133,135).

Sometimes a number of purification procedures must be followed before the

desired materials can be isolated and characterized. Graft copolymers may con-
tain varying amounts of impurities, like homopolymers (ie, backbone or branches
resulting from incomplete coupling) or copolymers of different architecture, as a re-
sult of the synthetic strategy followed for their preparation. These heterogeneities
in all cases affect the final properties of the material. Fractionation is a powerful
tool usually employed to separate heterogeneous mixtures of polymers before the
final characterization of each fraction (134). Copolymers can be fractionated ac-
cording to their molecular weight or chemical composition (135). The mostly used
fractionation methods that result in the isolation of actual samples (fractions) are
batch fractionation and column-elution fractionation. In some cases extraction
with solvents selective for a specific impurity can be used, ie, in separating excess
homopolymer branches.

Determination of Molecular Weight, Constitution, and Composition.

Many analytical techniques are available for the determination of the primary
molecular structure of a block copolymer and its average chemical composition.
Among them spectroscopic techniques used for low molecular weight compounds
are the most powerful and most widely employed. NMR can provide both quali-
tative and quantitative information with respect to comonomer composition and
stereochemical configuration of polymeric molecules (136,137). The IR technique
provides information on chemical, structural, and conformational aspects of poly-
meric chains (138). Because of the inherently high sensitivity of UV spectroscopy
the technique is often utilized for the identification and quantitative determina-
tion of comonomers in block copolymers.

The average molecular weight of block copolymer is a very important param-

eter that characterizes a certain copolymer sample and it is strongly related with
the properties of the polymeric material. Therefore, its determination and knowl-
edge is imperative in the study of these materials. A variety of techniques are
available for the determination of different average molecular weights of graft
copolymers and polymers in general. These include membrane osmometry and
light scattering for the determination of the absolute values of M

n

and M

w

re-

spectively (133). In the case of light scattering it should be always kept in mind
that the M

w

obtained is apparent, unless the dn/dc is high for all parts of the

graft copolymer (backbone and branches) and the material is characterized by
high molecular weight and compositional homogeneity. Knowledge of the molecu-
lar weights of the individual components of the graft copolymer can be combined
with the determination of the total molecular weight in order to determine the
average number of branches per macromolecule. This determination can be aided
by the selective sampling of individual parts of the graft copolymer during synthe-
sis, when this is possible, or selective degradation of a part of the macromolecule

374

GRAFT COPOLYMERS

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after isolation of the product. Size exclusion chromatography (sec) has emerged
as a powerful analytical technique for the determination of molecular weight and
molecular weight and composition distribution of block copolymers (139). A num-
ber of different types of mass/concentration and molecular mass detectors have
been developed so far for simultaneous use in sec instruments, providing a large
amount of information in a single run.

In cases where the molecular size of the graft copolymer must be known in

solution, several methods can be applied. These include primarily static and dy-
namic light scattering, small-angle neutron and x-ray scattering, and viscometry.
Static light, small-angle neutron (sans) and x-ray (saxs) scattering provide infor-
mation for the radius of gyration of isolated chains (or parts of them using sans and
contrast variation techniques) whereas dynamic light scattering and viscometry
measure the hydrodynamic radius and properties of the molecules (133,140,141).

Properties and Uses

Solution Properties.

Graft copolymer properties in solutions have been

studied extensively for many decades. Because of the arrangement of branches
along the backbone certain changes in its conformation and effective stiffness
are observed. Studies on model graft copolymers have shown that the con-
formation of the whole molecule becomes more extended compared to linear
chains (32,122,142). The phenomenon is more pronounced in the case of poly-
macromonomers (graft copolymers that are derived from the homopolymerization
of macromonomers) where each repeating unit of the backbone carries a polymer
chain (143–147). Crowding effects are maximal in this case, resulting in an in-
creased induced stiffness on the backbone and a bottlebrush conformation. This
specific conformation can impart interesting rheological properties in the corre-
sponding solutions of polymacromonomers.

Detailed light scattering investigations were performed in the case of

poly(isoprene-g-styrene) block copolymers in thermodynamically good solvents,
isorefractive for different parts of the molecule (32). This technique enabled the
determination of the size of the backbone and the branches independently, lead-
ing to the conclusion that segregation exists between the backbone and the grafts
even in nonselective solvents and the molecule adopts a more or less core–shell
structure.

In solvents selective for one of the parts of the molecule, graft copolymers

show the ability for micelle formation (148–157). Micelles with cores comprised
from the insoluble part and coronas comprised from the soluble one are formed.
However, aggregation numbers and size of the micelles are lower than the cor-
responding ones for linear block copolymers. Critical micelle concentrations are
also higher in the case of graft copolymers. This was attributed to the additional
constraints imposed to micelle formation because of the presence of a large num-
ber of branching points along the backbone. Ideally these branching points should
be situated at the micelle core–corona interface, a requirement that increases the
free energy of the system as excluded volume effects between the branches and
conformational entropy due to looping of backbone are increased. As a result of the
molecular architecture, unimolecular micelles of graft copolymers are more easily

Vol. 6

GRAFT COPOLYMERS

375

formed especially in the case where the solvent is selective for the branches (151–
153). In this case the backbone can adopt a collapsed conformation protected from
unfavorable interactions with the solvent molecules by the well-solvated branches
forming the corona. The increased number of branches favors the stabilization of
the structure in solution.

Morphology.

The peculiar macromolecular architecture of graft copoly-

mers also influences their bulk properties. The arrangement of a relatively large
number of branching points along one of the constituent chains (the backbone)
imparts some additional constraints in the formation of microphases and the ge-
ometry of the final microstructure. Macrophase separation cannot be excluded in
graft copolymer systems where there is a large molecular, compositional, and ar-
chitectural heterogeneity. Nevertheless, the bulk morphology of a graft copolymer
substantially influences other properties of the material (mechanical, rheological,
optical, etc). Therefore, elucidation of equilibrium morphology and factors affect-
ing ordering are essential in the design of new materials for specific applications.

The effect of chain architecture on the phase separation, order–disorder tran-

sition, and final morphology of graft copolymer systems has been investigated both
experimentally (123,127,129–131,158–161) and theoretically (162–164). Studies
on model graft copolymers elucidated, using transmission electron microscopy
(TEM) and saxs or sans, many aspects of microphase separation of these complex
macromolecules. In general, microphase separation and long-range order decrease
as the number of branches increases. The morphology of graft copolymers can be
predicted from the four morphologies of the simple constituting units which is a
miktoarm star copolymer. The same is valid in the case of

π and H exact graft

copolymers (Fig. 12).

This is attributed to the presence of the branching points. Each one of them

can be considered as a point where interfacial curvature between the two phases
changes, and this causes severe frustration during the organization of macro-
molecules within the microphases. Consequently, microdomain sizes and grain
structure are altered.

Mechanical Properties.

The viscoelastic and mechanical properties of

bulk graft copolymers are directly connected to their morphology, which in turn
depends on the molecular characteristics of the copolymer. Chemical nature of
the backbone and grafts and their mutual solubility as well as the molecular
weight and the degree of branching are the primary parameters affecting the
mechanical properties of such systems (165–167). For example, Young’s modulus,
stress at break, toughness and yielding stress for poly(butyl acrylate-g-styrene)
graft copolymers were found to increase, whereas yield elongation decreased as
the number of branches increased (at the same branch molecular weight) (165)
(Fig. 13).

This behavior was identified as a result of the increase in the hard phase

(PS) content in the graft copolymer. Grafting of styrene and vinylacetate onto
EPDM (ethylene–propylene–diene terpolymer) improved the tensile strength of
the resulting material in comparison to ungrafted EPDM, whereas grafting of
N-substituted acrylamides on EPDM gave copolymers with decreased tensile
strength (166). Poly(isoprene-g-styrene) copolymers with tetrafunctional branch
points were found to have increased strain at break (167) than the commercial
thermoplastic elastomers (Fig. 14). The mechanical properties of polymeric blends

376

GRAFT COPOLYMERS

Vol. 6

Fig. 12.

(a) Transmission electron microscopy image of poly(butadiene-g-styrene) ran-

domly branched graft copolymers with two PS branches on every grafting point. (b) Cor-
responding image for a poly(isoprene-g-styrene) regular graft copolymer with two PS
branches on every grafting point. The copolymers contain approximately 61% and 67%
by volume PS respectively. Notice the poor long-range order and the grain structure char-
acteristics. From Refs. 158 and 159.

compatibilized with graft copolymers have received a relatively larger attention
(168–171).

Compatibilization of Polymer Blends.

Polymer blending is one of the

most general, flexible, and efficient methods for generating new polymeric high
performance materials (172). The resulting polymer mixture combines the prop-
erties of its components. These properties may in principle be varied by changing
the blend composition according to the desired final performance of the material.
However, because of entropic reasons, the majority of polymer pairs are incom-
patible. Graft copolymers are widely used as compatibilizing agents of immiscible
blends (172–182). The different parts of a graft copolymer can be chosen and syn-
thesized in such a way that interactions with the constituents of the blends are
maximized. The constituents of the graft copolymer may have the same chemical
structure with either the components of the blend or a chemical constitution (ie,
complementary functional groups) that favors the interactions with the functional
groups of the components. In this way graft copolymers are aiding in the adhesion
of the different phases by forming a bicompatible interface. The interface reduces
the interfacial energy between the phases in the blend and permits a more stable
and finer dispersion of the components. The resulting mechanical properties of the
formulation are improved since strong adhesion of phases helps in assimilating
stresses and stains without disruption of the blend’s morphology. The possibility
of morphology changes during processing conditions is also minimized. A more
recent compatibilization methodology, namely reactive blending, involves the for-
mation of block or graft copolymers in situ (183–193). The method relies on the use

Vol. 6

GRAFT COPOLYMERS

377

5

6

7

8

9

10

11

12

160

200

240

280

0

0

5

10

15

20

25

30

35

2

4

6

T

oughness, kJ/m

2

Stress at break, MPa

Y

oung

’s modulus, MPa

PS grafts incorporated

Fig. 13.

Variation of the Young’s modulus (E), tensile strength at break (

σ

B

), and

toughness as a function of the number of PS grafts on the backbone for a poly(n-butyl
methacrylate-g-styrene) graft copolymer prepared by the macromonomer technique. To
convert MPa to psi, multiply by 145, to convert kJ/m

2

to ft

·lbf/in.

2

, divide by 2.10. From

Ref. 165.

of reactive functionalities previously incorporated in one or more polymers of the
blend for the production of block or graft copolymers after mixing the individual
components and during melt-processing. This concept has been applied to a num-
ber of blend systems such as polyamide/polyolefins, polyamide/ABS, PS/olefins,
polyester/polyamide using different types of reactions that can occur rapidly at
elevated processing temperatures.

Other Present and Potential Applications.

Graft copolymers are

steadily assuming an increased importance because of their tremendous industrial

378

GRAFT COPOLYMERS

Vol. 6

Fig. 14.

Comparison of the mechanical properties of tetrafunctional multigraft

poly(isoprene-g-styrene) copolymers with commercial Styroflex and Kraton copolymers.
From Ref. 167.

application potential (194–205). Graft copolymers of commercial utility include
ABS, obtained by grafting acrylonitrile and styrene monomers onto polybuta-
diene, high impact polystyrene, a family of poly(butadiene-g-styrene) materials,
alkali-treated cellulose-g-acrylonitrile and starch-g-acrylonitrile which are used
as “superabsorbent” components in diapers, sanitary napkins, and the like. Graft
copolymers containing acrylic monomers are used as pressure-sensitive adhesives
(197). Other graft copolymers are essential materials in oil recovery operations.
A large number of commercial paint, printing, and coating formulations involve
graft copolymers as dispersion stabilizers, rheology modifiers, and final coating
properties improvers (eg, corrosion resistivity, environmental compatibility, etc.)
(198–205).

The potential of using graft copolymer micelles and nanoparticles derived

from graft copolymers as drug carriers for controlled drug delivery systems and for
environmental purification methodologies is high (206–211). These applications
rely on the ability of the micelles to solubilize low molecular weight compounds in
their cores. By controlling the molecular characteristics of the copolymers and the
chemical nature of the corona, drug containing vehicles of the desired size, loading
capacity, release kinetics, targeting capabilities, biocompatibility and biodegrad-
ability can be produced. Formulations in the gel and bulk state are also candidates
for this kind of application. Size and loading capabilities of the micellar microcon-
tainers are also important in water treatment systems. The application of graft

Vol. 6

GRAFT COPOLYMERS

379

copolymer membranes in separation processes, like removal of deleterious organic
compounds from contaminated drinking water, is also of interest (212–214). With
judicious choice of chemical components in conjunction with graft copolymer mor-
phology design, perselectivity and permeability of the resulting membranes can
be adjusted.

Graft Copolymer Production/Consumption

It is difficult to accurately measure the amount of graft copolymers produced com-
mercially. Here we refer to those polymers made either by the reaction of two
polymers with each or by the growth of a chain of one monomer from a polymer
made of a different one. In each case one has a nonlinear structure where the
various segments of the resultant polymer (ie, backbone, branches, etc.) are chem-
ically different. The difficulty in determining how much of this kind of polymer
is produced is that it is done in many ways, and it is not even clear whether the
producers are always cognizant of what they have made. The grafting often occurs
during a mixing process, and the graft polymer itself is not separated from the
other components of the blend. However, it is clear that graft copolymer technology
is crucial to the modern plastics industry and that graft copolymers are present
in a large fraction of the plastic materials made today.

The largest portion of this can be found in the styrenic polymers. Impact

polystyrene [often called high impact polystyrene (HIPS)] is produced by polymer-
izing styrene monomer in the presence of polybutadiene. This results in grafting
of some of the growing PS chains to the PB ones, giving good impact strength
to the final product. Of the 12.7 million tons (10

6

t) of PS made worldwide each

year (215), perhaps a quarter to a third is HIPS (216–218). ABS is made in a
similar fashion as HIPS, except that acrylonitrile is also added to the styrene as
a polymerizing monomer (219–222). About 4.9

× 10

6

t of this polymer is made

each year. The next most common use of grafting is probably in the “super-tough”
polyamides, made by a grafting reaction between the polyamide and a functional-
ized ethylene–propylene rubber (223–227). Perhaps 10–20% of the 1.8

× 10

6

t of

polyamide is made this way. There are many other uses of graft copolymers, mostly
for compatibilizing various polymer blends (228). As stated above, it is difficult to
capture the amount of these made. However, it is safe to say that between 5 and
10% of the 141

× 10

6

t of plastics made each year contain a significant fraction of

graft copolymer.

BIBLIOGRAPHY

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Union Carbide Corp., and J. E. McGrath, Virginia Polytechnic Institute and State Univer-
sity; “Graft Copolymers” in EPSE 2nd ed., Vol. 7, pp. 324–434, by P. Dreyfuss, Michigan
Molecular Institute, and R. P. Quirk, University of Akron.

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N. H

ADJICHRISTIDIS

S. P

ISPAS

M. P

ITSIKALIS

H. I

ATROU

University of Athens
D. J. L

OHSE

ExxonMobil Research and Engineering Company