Copolymerization mechanisms, differentiation

In by far the largest number of cases of free radical copolymerization, the reactivity ratios are practically independent of the nature of the starting reaction (thermal, photochemical, radical-forming type) and the site of propagation (bulk, solution, emulsion). Ionic copolymerization, by contrast, leads to quite different parameters (Table 22-13). Thus copolymerizations can be employed as a diagnostic tool for initiators whose mode of action is unknown, differentiating between free radical copolymerization and the nonradical mechanism (Table 22-14). On such evidence, boron alkyls appear to be free radical initiators in the copolymerization of methyl methacrylate with acrylonitrile, whereas lithium alkyls are anionic initiators. 

This mechanism has been extended to the spontaneous copolymerizations of styrene with maleic anhydride and other electron-acceptor monomers (see Scheme 1, Several authors have studied the spontaneous polymerization of st5rene with acrylonitrile focusing on isolated trimers that are produced presumably as a result of the initiation step however, the trimer structures do not suffice to differentiate between the Mayo mechanism and the Flory diradical mechanism. 

An ingenious method has been described for differentiating between a metallacycle mechanism (path a in Scheme 13) and the classical Cossee-Arlman mechanism (path b) for the [Ti(OBu")4] + Et3Al)-catalyzed copolymerization of H C= CH + H C= CH (4 96). The proton-coupled C mutation NMR spectrum shows that the polymer contains the C atoms linked by a double bond as predicted by path b and not a single bond as predicted by path... 

Thus, the differential polymer eomposition, F], is related to the monomer eomposition, /i, in the reaction mass at that instant. This relation has been obtained through the use of the mechanism written in Eqs. (6.3.1). The parameters rj and 2 measure the relative preference that the polymer radicals P] and 2 have for the two monomers M] and M2. If kp22 and kp are zero, the resultant polymer has an alternating sequenee of M] and M2 on the chain. If kp2 and kpX2 are zero, then there is no copolymerization, and the resulting polymer consists of two homopolymers. If, however, monomers M] and M2 display equal relative preferences for Pj and P2—that is, if... 

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