Copolymers block and graft

Functionalised PO as block and graft copolymers used as compatibilisers or to increase interactions with other materials are prepared by free radical grafting (the simplest method), metallocene-catalysed copolymerisation of olefins with functional monomers, or anionic polymerisation (silane-containing PO). They are also produced by controlled/living polymerisation techniques such as nitroxide-mediated controlled radical polymerisation, atom transfer radical polymerisation (ATRP), and reversible addition-fragmentation chain transfer (RAFT). 

Acres and Dalton (1963b) studied the graft and/or block copolymeri-zation of methyl methacrylate onto irradiated polystyrene latex and styrene onto irradiated poly(methyl methacrylate) latex. A number of experimental variables and conditions that give good yields were investigated. It was difficult to separate the copolymers from the homopolymers but it was clear that good yields of nonrandom copolymers were indeed obtained. 

The direct radiation grafting of vinyl monomers to natural rubber latex was studied by two groups of workers. Cockbain et a . (1958,1959) grafted methyl methaciylaie using both y radiation and a chemical redox system. 

Radiatitm-Induced Emulsion Polymerization l ing Electron Accelerators 

Kamiyama and Saito (1975) extended these studies to methyl and ethyl acrylates and styrene. The conversions leveled off at about 90%. The latiees were less turbid than those obtained by y radiation. Again, those obtained at the highest dose rates were almost transparent, with particle sizes as low as 23 nm in dianaeter. in the case of styrene, the rates were almost independent of the dose rate but were found to be to the 0.3, 0.5, and 0.7 powers for methyl and ethyl acrylates and vinyl acetate, respectively. At dose rates higher than 2000rad/ssc the d ndency dropped to 0.2. The acrylates were so reactive that limiting conversions were reacted in a few seconds. 

Kamiyama and Shimizu (1975) also studied vinyl propionate and vinyl it-butyrate. The dose rate dependencies were 0.55 and 0.46, respectively, even at the maximum dose rates. The rate-determining process at the high dose rates associated with electron accelerat( s was suggested as being the rate of diffusion of the monomer from the droplets to the aqueous phase in the case of styrene, whereas with vinyl acetate the competition between polymer nuclention and radical recombination of radicals in the aqueous phase could be important. A further brief study of the vinyl acetate system was presented by Kamiyama (1975). 

The complete hydrogenation of block copolymers of styrene with diene monomers has been reported. Poly(styrene- /oc -diene) based block copolymers in which the polydiene block has been hydrogenated have been commercially available since the 1960s. The most common polymeric structures of this type are based on poly(styrene-bl-butadiene-bl-styrene) and poly(styrene-bl- 

Further exploration [57] into the variation in properties available in the fully saturated poly(styrene-bl-butadiene-bl-styrene) materials focused on modifying the vinyl content in the polybutadiene block. Exploring practical elastomeric properties such as rebound and modulus, this work showed plainly that the level of vinyl in the polybutadiene block dominated the room temperature elastomeric properties of the block copolymer with a preferred level of about 40mol% percent 1,2-microstructure. 

Despite the body of patent literature describing fully hydrogenated block copolymers and their properties, it has been suggested that complete saturation of styrenic block copolymers with butadiene would result in materials that were incapable of microphase separation. This argument was based on the supposition that the difference in solubility parameters of the fully saturated block copolymer would be so slight that they would not have useful mechanical properties at achievable molecular weights [58]. This assumption has since 

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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.]...

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

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. 

Polypropylene block and graft copolymers are efficient blend compatibilizers. These materials allow the formation of alloys, for example, isotactic polypropylene with styrene-acrylonitrile polymer or polyamides, by enhancing the dispersion of incompatible polymers and improving their interfacial adhesion. Polyolefinic materials of such types afford property synergisms such as improved stiffness combined with greater toughness. 

Block and graft copolymers have many similar characteristics. Thus, both graft and block copolymers behave in various respects as two immiscible polymers... 

In contrast to two-phase physical blends, the two-phase block and graft copolymer systems have covalent bonds between the phases, which considerably improve their mechanical strengths. If the domains of the dispersed phase are small enough, such products can be transparent. The thermal behavior of both block and graft two-phase systems is similar to that of physical blends. They can act as emulsifiers for mixtures of the two polymers from which they have been formed. 

Interfacial polycondensation between a diacid chloride and hexamethylenediamine in the presence of small amounts of ACPC also yield polymeric azoamid, which is a macroazo initiator.[27] In this manner, azodicarbox-ylate-functional polystyrene [28], macroazonitriles from 4,4 -azobis(4-cyano-n-pentanoyl) with diisocyanate of polyalkylene oxide [29], polymeric azo initiators with pendent azo groups [3] and polybutadiene macroazoinitiator [30] are macroazoinitiators that prepare block and graft copolymers. 

The use of initiators such as 68 has been promoted for achieving higher molecular weights or higher conversions in conventional polymerization and for the production of block and graft copolymers. The use and applications of multifunctional initiators in the synthesis of block and graft copolymers is briefly described in Section 7.6.1. 

Functional and end-functional polymers are precursors to block and graft copolymers and, in some cases, polymer networks. Copolymers with in-chain functionality may be simply prepared in copolymerizations by using a functional monomer. However, obtaining a desired distribution requires consideration of the chain statistics and, for low molecular weight polymers, the specificity of the initiation and termination processes, l hese issues are discussed in Section 7.5.6... 

Many block and graft copolymer syntheses involve radical polymerization at some stage of the overall preparation. This section deals with direct syntheses of... 

The success of the multifunctional initiators in the preparation of block and graft copolymers depends critically on the kinetics and mechanism of radical production. In particular, the initiator efficiency, the susceptibility to and mechanism of transfer to initiator, and the relative stability of the various radical generating functions. Each of these factors has a substantial influence on the nature and homogeneity of the polymer formed. Features of the kinetics of polymerizations initiated by multifunctional initiators have been modeled by O Driscoll and Bevington 64 and Choi and Lei.265... 

Many block and graft copolymer syntheses involving transformation reactions have been described. These involve preparation of polymeric species by a mechanism that leaves a terminal functionality that allows polymerization to be continued by another mechanism. Such processes are discussed in Section 7.6.2 for cases where one of the steps involves conventional radical polymerization. In this section, we consider cases where at least one of the steps involves living radical polymerization. Numerous examples of converting a preformed end-functional polymer to a macroinitiator for NMP or ATRP or a macro-RAFT agent have been reported.554 The overall process, when it involves RAFT polymerization, is shown in Scheme 9.60. 

For ihe case of NMP and RAFT, there exist two basic ways of growing star copolymers (this discussion also applies to block and graft copolymer synthesis). 

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