Chain copolymerization block

It is highly unlikely that the reactivities of the various monomers would be such as to yield either block or alternating copolymes. The quantitative dependence of copolymer composition on monomer reactivities has been described [Korshak et al., 1976 Mackey et al., 1978 Russell et al., 1981]. The treatment is the same as that described in Chap. 6 for chain copolymerization (Secs. 6-2 and 6-5). The overall composition of the copolymer obtained in a step polymerization will almost always be the same as the composition of the monomer mixture since these reactions are carried out to essentially 100% conversion (a necessity for obtaining high-molecular-weight polymer). Further, for step copolymerizations of monomer mixtures such as in Eq. 2-192 one often observes the formation of random copolymers. This occurs either because there are no differences in the reactivities of the various monomers or the polymerization proceeds under reaction conditions where there is extensive interchange (Sec. 2-7c). The use of only one diacid or one diamine would produce a variation on the copolymer structure with either R = R" or R = R " [Jackson and Morris, 1988]. 

Although homopolymerization of cyclopentene results in 1,3 enchainment of the monomer units in copolymerization, blocks of cyclic monomer units are rarely observed as a consequence of the unfavorable copolymerization parameters. The isolated cyclopentene units maybe incorporated in a cis-1,2 or cis-1,3 fashion, with their ratio dependent on the catalyst used (238-240). Thus, ethylene compensates for the steric hindrance at the a carbon atom of the growing chain after insertion of the cyclopentene. 

So far, our discussion has been restricted to chain block and graft copolymerization. This is largely because the practical utility of copolymerization is more elaborate in chain polymerization than step polymerization. Also, in step copolymerization, block copolymers are generally preferred to the other types of copolymers. Therefore only block step-polymerization copolymers are discussed here and only in a very limited scope to illustrate the principles involved in their preparation. 

Here, a third monomer can be included to interpolymerize with the complex that acts as a unit. The product is a terpolymer. A diradical intermediate was also postulated in sulfur dioxide copolymerizations and terpolymerizations with bicycloheptene and other third monomers. These third monomers enter the copolymer chain as block segments, while the donor-acceptor pairs enter the chains in a one-to-one molar ratio. This one-to-one molar ratio of the pairs is maintained, regardless of the overall nature of the monomer mixtures. 

In the international nomenclature, -co-, -alt-, -b-, -g- are often inserted between two monomers to represent random copolymerization, alternating copolymerization, block copolymerization, and graft copolymerization, respectively. In random copolymer names, the former is the main monomer, and the latter is the secondary monomer. In block copolymer names, the order of monomers represents the order of polymerization, whereas in graft copolymer names, the former is the main chain and the latter is the branched chain. 

A special class of copolymers are functionalized thermoplastics (FTPs) that contain functional groups. FTPS typically have Unear backbones, and functionalization is introduced at the chain-ends or on the polymer chain via block and graft copolymerizations. The functional groups allow the polymer to be more interactive and have the ability to form inter-molecular and intramolecular bonds [69,70]. 

All of these initiating systems enhance the livingness of the (meth)acrylate polymerization to varying degrees to suppress secondary reactions and achieve living polymerization that enables manipulations of active chain-ends such as chain extension, block copolymerization, and functionalization. In addition, they moderate the position and the dynamics of the association equilibrium. Some details of the polymerization of (meth)acrylates using these new initiating systems are outlined below. More detailed results were reviewed by Baskaran. ... 

Several types of polymers have been developed for use as thermoplastic elastomers and the materials are best described by considering specific examples. Block copolymers are often employed and thermoplastic elastomers can be produced by both step and chain copolymerization. 

Similarly, the random introduction by copolymerization of stericaHy incompatible repeating unit B into chains of crystalline A reduces the crystalline melting point and degree of crystallinity. If is reduced to T, crystals cannot form. Isotactic polypropylene and linear polyethylene homopolymers are each highly crystalline plastics. However, a random 65% ethylene—35% propylene copolymer of the two, poly(ethylene- (9-prop5lene) is a completely amorphous ethylene—propylene mbber (EPR). On the other hand, block copolymers of the two, poly(ethylene- -prop5iene) of the same overall composition, are highly crystalline. X-ray studies of these materials reveal both the polyethylene lattice and the isotactic polypropylene lattice, as the different blocks crystallize in thek own lattices. 

VEs do not readily enter into copolymerization by simple cationic polymerization techniques instead, they can be mixed randomly or in blocks with the aid of living polymerization methods. This is on account of the differences in reactivity, resulting in significant rate differentials. Consequendy, reactivity ratios must be taken into account if random copolymers, instead of mixtures of homopolymers, are to be obtained by standard cationic polymeriza tion (50,51). Table 5 illustrates this situation for butyl vinyl ether (BVE) copolymerized with other VEs. The rate constants of polymerization (kp) can differ by one or two orders of magnitude, resulting in homopolymerization of each monomer or incorporation of the faster monomer, followed by the slower (assuming no chain transfer). 

Copolymerizations of benzvalene with norhornene have been used to prepare block copolymers that are more stable and more soluble than the polybenzvalene (32). Upon conversion to (CH), some phase separation of nonconverted polynorhornene occurs. Other copolymerizations of acetylene with a variety of monomers and carrier polymers have been employed in the preparation of soluble polyacetylenes. Direct copolymeriza tion of acetylene with other monomers (33—39), and various techniques for grafting polyacetylene side chains onto solubilized carrier polymers (40—43), have been studied. In most cases, the resulting copolymers exhibit poorer electrical properties as solubiUty increases. 

By means of a ring-opening polymerization of the condensation type Vlasov et al. [50] synthesized polypeptide based MAIs with azo groups in the polymeric backbone. The method is based on the reaction of a hydracide derivative of AIBN and a N-carboxy anhydride. Containing one central azo group in the polymer main chain, the polymeric azo initiator was used for initiating block copolymerizations of styrene and various methacrylamides. 

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... 

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