Chain polymerization ideal copolymerization

Indeed, cumyl carbocations are known to be effective initiators of IB polymerization, while the p-substituted benzyl cation is expected to react effectively with IB (p-methylstyrene and IB form a nearly ideal copolymerization system ). Severe disparity between the reactivities of the vinyl and cumyl ether groups of the inimer would result in either linear polymers or branched polymers with much lower MW than predicted for an in/mcr-mediated living polymerization. Styrene was subsequently blocked from the tert-chloride chain ends of high-MW DIB, activated by excess TiCU (Scheme 7.2). 

It should be noted, too, that the r values for this system do not permit an azeotropic polymerization, as predicted by Eq. (2.39). With respect to the distribution of styrene monomer units in the copolymer, the monomer reactivity ratio product, rers = 0.8, is close to a value of 1.0, which would correspond to an ideal copolymerization (Odian, 2004b) which would correspond to a random distribution of styrene units along the chain. For an ideal copolymerization, the relative rates of incorporation of the two monomers are independent of the chain end unit as predicted by Eq. (2.42). 

The variability and potential of the graft polymerization technique is best discussed in terms of the various parameters involved. The graft reaction is, to a large extent, controlled by the structure of the backbone prepolymer. The temperature at which grafting can take place and the number of grafted chains can be controlled via the type and concentration of the azo functions. Additionally, the molar mass of the backbone prepolymer has an influence on the number of azo groups per polymer chain and thus on the number of side chains. The comonomer for the backbone can be freely chosen unless quantitative conversions are required. In this case a comonomer should be used which copolymerizes ideally with the azo monomer. 

These monomers copolymerize ideally. Halogen substituents reduce the rate of polymerization while methyl or ethyl groups in the para position have a slight activating effect. This is in conformity with the view that monomer entry into the polymer chain is determined only by individual reaction rates (Table 24) [200]. 

It is amply evident that statistical copolymerization, both free radical and ionic, cannot produce an ideal network because of the unequal reactive ability of the comonomers in their competition for interaction with the active functional end of the growing polymer chain. However, using the so-caUed living anionic polymerization, it is possible to eliminate the competition between the comonomers by separating the stages of the formation of chain precursors and the formation of network per se, that is, chain crosslinking. Such an approach may be realized in two subsequent stages via anionic stepwise block polymerization of first styrene and then DVB. 

In the early 1940s when the polymerization theory was developed, tiie ideal, terminal, and penultimate models for fhe copolymerization were established also the possible distribution laws for the monomer sequence along the copolymer chains were defined Bemoullian, firsf- and second-order Markoffian. ... 

The product of the polymerization constants, riT2, is very frequently used as an index for evaluating the alternating tendency in binary copolymeriz-ation/" In fact, the reciprocal of the product rir2 is often called the alternation tendency index. The ideal (random) copolymerization condition exists for the case / i/ 2 = Where ri and r2 are very low and the rir2 product tends to zero the alternating tendency increases.The product r r2 can be zero in two cases—where one or both reactivity ratios are zero. Where r — 0, the copolymer chain is built of isolated Mi units separated by sequences of M2 monomer units. Strictly alternating copolymer would be obtained when both / i and V2 are zero. The second condition is often found for MA copolymerizations, as described in Chapter 10. Where ri > 1, the polymer is richer in Ml monomers than the monomer feed. For ri < 1, the opposite holds. 

However, computation of the equilibrium concentrations of copolymer chains of a given sequence of copolymer units is more complex and can be found in the dted paper of Szymanski. It stems from the fact that the probabilities of finding a unit of a given kind in a copolymer chain depends, in a general case, on X (degree of polymerization), considered position, and a type of active center. Only for an ideal equilibrium copolymerization KxxKyy=KxyKyx)i the probabilities of finding a copolymer unit at any chain position (but the first) are dependent only on the unit type and thus computation of aU copolymer equilibrium concenttations is relatively simple. ... 

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