Copolymers, graft

A graft copolymer has a backbone consisting of one type of monomer and branches of another (Fig. 3.9). For example, high impact polystyrene is formed from a polystyrene backbone with grafted polybutadiene branches. 

Graft copolymers are commonly produced by building reactive sites into a iinear polymer. Then in a subsequent reaction, polymerization by the comonomer is carried out at these reactive sites. For example, the incorporation of random vinyi bromide units in poiystyrene provides sites for subsequent production of a graft poiystyrene-poiymethyi methacrylate copolymer. 

Graft copolymers are also formed by radical and ionic processes [262, 263, 284-286], They are usually difficult to characterize, and their synthesis is poorly reproducible. 

As with block copolymers, to describe their synthesis would require an extensive treatise. In principle they can be formed by some variant of the following processes 

Preparation of graft copolymers by chain biting reactions 

Carbanions can react with Cl in PVC macromolecules [295] and with the ester group of PMMA [284]. The rates of the two reactions are probably not very different by the addition of a-methylstyrene tetramer dianion to a PVC + PMMA solution, the copolymer poly(vinyl chloride)-gro/ir-poly-(methyl methacrylate) was obtained [296]. Macrocations formed by the reaction of strong acids with polyalkenes (see Chap. 3, Sect. 3.2) react with polyethers (polysiloxanes) yielding graft and block copolymers, e. g. poly(propylene)-0ra/ir-poly (oxyethylene) [297], poly(propylene)- /ocA - 

These reactions are just an example of many other possibilities since a backbone substituent, or the chain backbone directly is attacked, I propose to call these processes chain biting reactions. 

Graft copolymers should in principle exhibit similar structure/properties relationships as block copolymers. The problem is that pure graft copolymers which are not accompanied by large amounts of homopolymers are - with few exceptions - very difficult to synthesize. This is the reason why reliable property 

Although copolymers with equivalent eom-positions but different molecular architec- 

As shown in Eq. (35), graft copolymers containing a mesogenic monomer have been synthesized by free radical copolymer- 

Entry (Number average) block length PMMA LC Wt. ratio PMMA/LC Thermotropic behavior ( ) Weighted average s Tg (°C) 

Comparison of the data in Tables 14 and 19 of graft and block copolymers, respectively, based on methyl methacrylate and a mesogenic methacrylate confirm that block copolymers phase separate more easily than graft copolymers. Although not exactly comparable due to the different mesogenic methacrylates, the block copolymers phase separate at shorter block lengths than the graft copolymers. In addition, the distribu- 

III Molecular Engineering of Side Chain Liquid Crystalline Polymers 

Synthesis of graft copolymers resembles that of block copolymers with an increased number of reactive sites per macromolecule. There are two techniques similar to those used in block copolymer preparation, namely grafting from and grafting onto preformed chains. Another method, grafting through, is similar to that used in conventional random copolymerization, where in-chain units function as comonomers. 

Block copolymers are linear, but graft copolymers are branched, with the main chain generally consisting of a homopolymer or a random copolymer, while the grafted side chains are composed of either the same or another monomer or several monomers  

The numerous ways for the synthesis of graft copolymers can be divided into three categories. 

To the first category belong the homo- and copolymerization of macromonomers. For this purpose, macromolecules with only one polymerizable end group are needed. Such macromonomers are made, for example, by anionic polymerization where the reactive chain end is modified with a reactive vinyl monomer. Also methacrylic acid esters of long-chain aliphatic alcohols or monofunctional polyethylene oxides or polytetrahydrofurane belong to the class of macromonomers. 

The second possible route is called grafting from . This means that active sites are generated at the polymer backbone A which initiate the polymerization of monomer B, thus leading to long-chain branches  

CHjCl or nitroxy group (controlled radical polymerization), 

Three methods exist for synthesizing graft copolymers. Grafting onto involves the reaction between functional groups on two different polymers  

Grafting from involves a polymer with functional groups that initiate polymerization of 

Grafting through involves the polymerization (or copolymerization) of a macromonomer, usually a vinyl macromonomer  

SCCO2 from P(HEMA-co-PMMA) did not occur from all of the hydroxyl groups on the polymeric initiator. In all cases, steric effects were suggested to be responsible for the incomplete grafting reaction [116]. 

A surface-initiated enzymatic ROP has also been reported, whereby CL and DXO were grafted from hydroxyl-terminated self-assembled monolayers (SAMs) on gold, using Novozym 435 [117], while polycaprolactone-modified hydroxyethylcel-lulose films were prepared by the enzymatic ROP of CL [118]. 

Ckinventional free-radical polymerization, either by so-called graffing-ffom or grafting-onto techniques are the oldest and were the most widely used procedures for the synthesis of graft copolymers because they are very simple [2 ]. In fact, graft copolymers can easily be obtained by polymerization of a monomer A in presence of a preformed polymer B acting, either as a chain-transfer agent or as a macroinitiator. However, these procedures usually 

PEC methacrylic macromonomer PLA methacrylic macromonomer POE methacrylic macromonomer 

Adapted and completed from Ref. ]15], Copyright 2003, with permission from Elsevier. 

A first improvement, as shown by Rempp and Merrill [109] was the grafting-onto by ionic polymerization techniques as for instance for the preparation of amphiphilic PS-g-PEO graft copolymers. Such copolymers were obtained by deactivation of a living PEO, of known molecular weight, on a partially chloromethylated PS backbone. 

In the case of composites, the surface modification leads to a good dispersion of the inorganic material in polymers matrices and, depending on the nature of the dispersed phase, imparts improved chemical and physical properties such as mechanical properties, UV attenuation, flame retardancy, thermal stability, thermal and electrical conductivity, gas barrier properties, superhydrophobicity, antimicrobial properties, etc. 

The functionalization or modification of the particle surface with chemical groups or polymer grafts is the key to achieve excellent dispersion in polymer matrices and 

The modification of polymers can be readily conducted by chemical coupling reactions when the chain to be modified possesses groups such as vinyl, hydroxyl, or azide [23], etc. The Diels-Alder reaction between a diene and a dienophile, discovered by Otto Diels and Kurt Alder in 1928 [24], is the most important example of click chemistry. These robust and efficient click coupling reactions have been widely exploited in the construction of tailor-made functional polymeric materials with complex molecular architectures 

The production of thermoplastics by polymer grafting synthesis techniques is widely used in the industry today. Large amounts of commercial thermoplastics, especially styrenic polymers, are nowadays produced by diverse grafting techniques, but other graft polymers are also produced commercially. Some of the most relevant examples are discussed below. 

1 High Impact Polystyrene (HIPS) HIPS is a heterogeneous material produced by continuous bulk or bulk-suspension processes, in which a butadiene-based elastomer (polybutadiene (PB), or a block copolymer of styrene-butadiene) is first dissolved in styrene monomer (St) and the resulting mixture is then heated so that the polymerization proceeds either thermally or with the aid of a chemical initiator. At the molecular level, the product is a mixture of free polystyrene (PSt) chains and elastomer chains grafted with PSt side chains. The process yields a continuous (free) PSt matrix containing 

The extent of the cross-linking, as shown above, is not clear. It is known, however, that cleavage reactions that are followed by free-radical recombinations can take place [274]  

Polymeric chains bearing free radicals combine with each other to give branched structures. Additions of chains with freeradical to double bonds result in formations of cross-links [274], 

Many other miscellaneous cross-Unkings of pol5mieric materials are reported in the literature. For instance, poly(acryloyl chloride) can be cross-linked with diamines [275]  

In a similar manner, polymers with pendant chlorosulfonate groups cross-link when reacted with diamines or with glycols [275]. 

This is an important part of polymer syntheses that is used in many industrial processes. In 1967, Battaerd and Tregear [282] published a book on the subject that contains 1,000 references to journal publications and 1,200 references to patents. In addition, there are several monographs and many review papers [283]. The synthetic methods developed to date range from using free radical attacks on polymeric backbones to highly refined ionic reactions. There are examples where these ionic reactions attach the side-chains at well-designated locations. 

---

In graft copolymers the chain backbone is composed of one kind of monomer and the branches are made up of another kind of monomer. 

Remember from Sec. 1.3 that graft copolymers have polymeric side chains which differ in the nature of the repeat unit from the backbone. These can be prepared by introducing a prepolymerized sample of the backbone polymer into a reactive mixture—i.e., one containing a source of free radicals—of the side-chain monomer. As an example, consider introducing polybutadiene into a reactive mixture of styrene ... 

Figure 9.17 Plot of log [i ]M versus retention volume for various polymers, showing how different systems are represented by a single calibration curve when data are represented in this manner. The polymers used include linear and branched polystyrene, poly(methyl methacrylate), poly(vinyl chloride), poly(phenyl siloxane), polybutadiene, and branched, block, and graft copolymers of styrene and methyl methacrylate. [From Z. Grubisec, P. Rempp, and H. Benoit, Polym. Lett. 5 753 (1967), used with permission of Wiley.]...

Solution polymers are the second most important use for acryflc monomers, accounting for about 12% of the monomer consumption. The major end use for these polymers is in coatings, primarily industrial finishes. Other uses of acryflc monomers include graft copolymers, suspension polymers, and radiation curable inks and coatings. 

Monomer compositional drifts may also occur due to preferential solution of the styrene in the mbber phase or solution of the acrylonitrile in the aqueous phase (72). In emulsion systems, mbber particle size may also influence graft stmcture so that the number of graft chains per unit of mbber particle surface area tends to remain constant (73). Factors affecting the distribution (eg, core-sheU vs "wart-like" morphologies) of the grafted copolymer on the mbber particle surface have been studied in emulsion systems (74). Effects due to preferential solvation of the initiator by the polybutadiene have been described (75,76). 

In addition to graft copolymer attached to the mbber particle surface, the formation of styrene—acrylonitrile copolymer occluded within the mbber particle may occur. The mechanism and extent of occluded polymer formation depends on the manufacturing process. The factors affecting occlusion formation in bulk (77) and emulsion processes (78) have been described. The use of block copolymers of styrene and butadiene in bulk systems can control particle size and give rise to unusual particle morphologies (eg, coil, rod, capsule, cellular) (77). 

M. G. Huguet andT. R. Paxton, Colloidal andMorphological Behavior of Block and Graft Copolymers Plenum, New York, 1971,pp. 183—192. 

Material Protection. The graft copolymers of ethylene sulfide on polyethyleneimine can be used as an antifouHng anticorrosion substrate for iron (439). PEIs or their derivatives are also used in electrolysis baths as brighteners in the electrochemical deposition of metals (440,441). 

In addition to providing fully alkyl/aryl-substituted polyphosphasenes, the versatility of the process in Figure 2 has allowed the preparation of various functionalized polymers and copolymers. Thus the monomer (10) can be derivatized via deprotonation—substitution, when a P-methyl (or P—CH2—) group is present, to provide new phosphoranimines some of which, in turn, serve as precursors to new polymers (64). In the same vein, polymers containing a P—CH group, for example, poly(methylphenylphosphazene), can also be derivatized by deprotonation—substitution reactions without chain scission. This has produced a number of functionalized polymers (64,71—73), including water-soluble carboxylate salts (11), as well as graft copolymers with styrene (74) and with dimethylsiloxane (12) (75). 

H. A. J. Battaerd and G. W. Tregear, Polymer Reriem, Graft Copolymers, Vol. 16, Wiley-Interscience, New York, 1967. 

A hydrolyzed cereal soHd, predominately a hexasaccharide, is used in high pH lime muds for reducing the yield point and gel strength (67). This additive has been used in systems treated with both sodium hydroxide and potassium hydroxide in addition to other additives common to lime muds (68). A second viscosity-reducing additive used in lime muds is a graft copolymer of acryflc acid and calcium flgnosulfonate (69). Both of these materials are used at levels of 6—17 kg/m (2—6 lb /bbl). 

Acrylamide graft copolymers such as those with starch (qv)(131), dextran (132), and lignin (qv) (133), have been studied to try to reduce copolymer costs. A general disadvantage of acrylamide copolymers is greater cost compared to partially hydroly2ed polyacrylamides. 

Grafting can also occur in the amide nitrogen, either through an anionic-type mechanism which is beheved to operate when ethylene oxide [75-21 -8] and similar copolymers are grafted to polyamides, or through a polycondensation mechanism when secondary amides are formed as graft copolymers (70). 

Fig. 6. Illustration of (a) compatibiLization of immiscible blends of polymers and B by block or graft copolymers and (b) the subsequent modification of...

The additive approach to compatibilization is limited by the fact that there is a lack of economically viable routes for the synthesis of suitable block and graft copolymers for each system of interest. The compatihilizer market is often too specific and too small to justify a special synthetic effort. 

Moreover, commercially available triblock copolymers designed to be thermoplastic elastomers, not compatihilizers, are often used in Heu of the more appealing diblock materials. Since the mid-1980s, the generation of block or graft copolymers in situ during blend preparation (158,168—176), called reactive compatibilization, has emerged as an alternative approach and has received considerable commercial attention. 

Etherification and esterification of hydroxyl groups produce derivatives, some of which are produced commercially. Derivatives may also be obtained by graft polymerization wherein free radicals, initiated on the starch backbone by ceric ion or irradiation, react with monomers such as vinyl or acrylyl derivatives. A number of such copolymers have been prepared and evaluated in extmsion processing (49). A starch—acrylonitrile graft copolymer has been patented (50) which rapidly absorbs many hundred times its weight in water and has potential appHcations in disposable diapers and medical suppHes. 

Copolymers of VDC can also be prepared by methods other than conventional free-radical polymerization. Copolymers have been formed by irradiation and with various organometaHic and coordination complex catalysts (28,44,50—53). Graft copolymers have also been described (54—58). 

Although they lack commercial importance, many other poly(vinyl acetal)s have been synthesized. These include acetals made from vinyl acetate copolymerized with ethylene (43—46), propjiene (47), isobutjiene (47), acrylonitrile (48), acrolein (49), acrylates (50,47), aHyl ether (51), divinyl ether (52), maleates (53,54), vinyl chloride (55), diaHyl phthalate (56), and starch (graft copolymer) (47). 

---