Chain copolymerization random

Hexafluoiopiopylene and tetiafluoioethylene aie copolymerized, with trichloiacetyl peroxide as the catalyst, at low temperature (43). Newer catalytic methods, including irradiation, achieve copolymerization at different temperatures (44,45). Aqueous and nonaqueous dispersion polymerizations appear to be the most convenient routes to commercial production (1,46—50). The polymerization conditions are similar to those of TFE homopolymer dispersion polymerization. The copolymer of HFP—TFE is a random copolymer that is, HFP units add to the growing chains at random intervals. The optimal composition of the copolymer requires that the mechanical properties are retained in the usable range and that the melt viscosity is low enough for easy melt processing. 

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]. 

A pair of vinyl or other unsaturated groups can also be linked by their direct reactions with free radicals. Similar end groups can be placed on siloxane chains by the use of an end blocker during polymerization,73,74 as mentioned earlier. Reactive groups such as vinyl units can, of course, be introduced as side chains by random copolymerizations involving, for example, methylvinylsiloxane trimers or tetramers.11... 

According to Orr [210] the residual entropy is negligible as is, according to Temperley [211], the configurational entropy. Sawada [212] described the entropy of copolymerization randomness (unit placement in the chain) by the relation... 

There are several examples of random copolymers of methacrylates (R-l to R-3). MMA/nBMA copolymerization was carried out with a copper catalyst, but the products were of low molecular weight because this study was directed to mechanistic studies.263 Random copolymers of MMA and nBMA (R-l) were also obtained in emulsion (MJMn = 1.2—1.3).254 Two monomers were consumed almost simultaneously to give a random or statistical distribution of repeat units along the chains. Copolymerization of MMA... 

In many copolymerizations, though, the four reaction rates are not very different from each other, i.e., neither of the two radicals Mi and Ml has any particular preference for either monomer Mi or Mi. In this case the two monomers will enter the copolymerization chain at random, as is illustrated below ... 

Chemical crosslinks may be obtained by randomly joining segments in already formed chains, by random copolymerization, or by end-linking functionally-terminated chains. Sulfur cures, peroxide cures, and high-energy irradiations are familiar methods of random crosslinking. Copolymerization monomers where at least one type has three or more reactive sites also lead to randomly crosslinked networks. Formation of networks by end-linking... 

Ethylene and vinyl acetate are copolymerized to a poly (ethylene-co-vinyl acetate). The reaction can be carried out in every ratio of the comonomers as the reactivity ratios are both close to 1 [3,4]. Consequently, the distribution of the comonomers along the chain is random. 

Park et al. [5] presented the first report on blends of potato starch with commonly used plastics such as LDPE and the aliphatic polyester APES (starch/ahphatic polyester, chemical name poly-(butylene succinate-co-adipate)) with addition of an ionomer. The ionomer used in this study was essentially polyethylene (PE) with a small amount of methacryhc acid copolymerized randomly into the ethylene chain. This was neutralized with Na cation. For each blend, a different level of starch (from 10 to 90% by weight) was used. Measurements of tensile properties... 

Random copolymerization is rather unusual. Sometimes a monomer which does not easily form a homopolymer will readily add to a reactive group at the end of a growing polymer chain. In turn, that monomer tends to make the other monomer much more reactive. 

Copolymer. Acetal copolymers are prepared by copolymerization of 1,3,5-trioxane with small amounts of a comonomer. Carbon-carbon bonds are distributed randomly in the polymer chain. These carbon-carbon bonds help to stabilize the polymer against thermal, oxidative, and acidic attack. 

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). 

A living cationic polymeriza tion of isobutylene and copolymeriza tion of isobutylene and isoprene has been demonstrated (22,23). Living copolymerizations, which proceed in the absence of chain transfer and termination reactions, yield the random copolymer with narrow mol wt distribution and well-defined stmcture, and possibly at a higher polymerization temperature than the current commercial process. The isobutylene—isoprene copolymers are prepared by using cumyl acetate BCl complex in CH Cl or CH2CI2 at —30 C. The copolymer contains 1 8 mol % trans 1,4-isoprene... 

Several different types of copolymers can be defined, depending on the distribution of monomer units in the chain. If monomer A is copolymerized with monomer B, for instance, the resultant product might have a random... 

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