Propagating radicals, effect

Certain monomers may act as inhibitors in some circumstances. Reactivity ratios for VAc-S copolymerization (r< 0.02, rVu -2.3) and rates of cross propagation are such that small amounts of S are an effective inhibitor of VAc polymerization. The propagating chain with a terminal VAc is very active towards S and adds even when S is present in small amounts. The propagating radical with S adds to VAc only slowly. Other vinyl aromatics also inhibit VAc polymerization.174... 

Experimental studies on models of the propagating radicals in S-AN copolymerization32,33 and a few other systems34 provide support for an explicit penultimate unit effect. Of particular interest is the data of Tirrcll and coworkers. They investigated the relative reactivity of S and AN towards various /-substituted propyl radicals (Scheme 7.3 and Table 7.2). They found that ... 

It is known that the penultimate unit influences the conformation of both model radicals and propagating radicals.32 3 Since addition requires a particular geometric arrangement of the reactants, there are enthalpic barriers to overcome for addition to take place and also potentially significant effects on the entropy of activation. Comparisons of the rate constants and activation parameters for homopropagation with those for addition of simple model radicals to the same monomers also provide evidence for significant penultimate unit effects (Section 4.5.4). 

If it is assumed that penultimate unit effects on the reaction entropy are insignificant, the terms in eqs. 18 and 19 corresponding to the stabilization energy of the reactant propagating radical will cancel and rVli=ryly There should be no explicit penultimate unit effect on copolymer composition. On the other hand, the radical reactivity ratio j (eq. 20) compares two different propagating radicals so... 

Given the important role that stcric and polar factors play in determining the rate and regiospecifieity of radical additions (see 2.3), it might be anticipated that reagents which coordinate with the propagating radical and/or the monomer and thereby modify the effective size, polarity, or inherent stability of that species, could alter the outcome of propagation. 

Generally, radical chain reactions are carried out in nonpolar solvents although strong solvent effects on propagation steps are rare. Apart from the polarity, a much more important criterion for the solvent choice is the solvent s inertness towards the chain propagating radicals involved. 

The addition of certain substances suppresses the polymerization of monomers. These substances act by reacting with the initiating and propagating radicals and converting them to either nonradical species or radicals of reactivity too low to undergo propagation. Such polymerization suppressors are classified according to their effectiveness. Inhibitors stop every radical, and polymerization is completely halted until they are consumed. Retarders are less efficient and stop only a portion of the radicals. In this case, polymerization occurs,... 

Polar effects appear to be of prime importance in determining the effect of quinones. p-Benzoquinone and chloranil (which are electron-poor) act as inhibitors toward electron-rich propagating radicals (vinyl acetate and styrene) but only as retarders toward the electron-poor acrylonitrile and methyl methacrylate propagating radicals. A further observation is that the inhibiting ability of a quinone toward electron-poor monomers can be increased by the addition of an electron-rich third component such as an amine. Thus the presence of triethylamine converts chloranil from a very weak retarder to an inhibitor toward methyl methacrylate. 

Reductants also terminate propagating radicals, but much less effectively. 

An especially interesting case of inhibition is the internal or autoinhibition of allylic monomers (CH2=CH—CH2Y). Allylic monomers such as allyl acetate polymerize at abnormally low rates with the unexpected dependence of the rate on the first power of the initiator concentration. Further, the degree of polymerization, which is independent of the polymerization rate, is very low—only 14 for allyl acetate. These effects are the consequence of degradative chain transfer (case 4 in Table 3-3). The propagating radical in such a polymerization is very reactive, while the allylic C—H (the C—H bond alpha to the double bond) in the monomer is quite weak—resulting in facile chain transfer to monomer... 

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