Table 7.3 shows the extent of H–H placements of fluoro polymers. The only exceptions occur when the substituents on the double bond are small (and do not offer appreciable steric hindrance to the approaching radical) and do not have a significant resonance stabilizing effect, specifically when fluorine is the substituent. For instance, the synthesis of styrene, there is no detectable H–H placement of polystyrene. The H–T propagation is indeed the predominant (>98–99 %) mode of propagation in chain polymerization. Other polymers with head-to-head arrangement have been determined by 19F, 13C, and 1HNMR analysis. The presence of no more than 1–2 % head-to-head placement in various polymers of vinyl esters such as poly (vinyl acetate) has been determined by hydrolysis of the polymer to poly(vinyl alcohol) and the periodate oxidation of the 1,2-glycol units (see Sect. These theoretical predictions have been experimentally verified for a number of polymers. The terms syndioregicĪnd aregic are used for polymer structures with alternating and random arrangements, respectively, of H–T and H–H placements. Has been used to indicate a polymer structure with exclusive head-to-tail placements. A propagation proceeding with predominantly H–T placement is a regioselective process, that is, one orientation (H–T) is favored over another (H–H). Furthermore, the attachment of a propagating radical at the unsubstituted carbon 2 of a monomer molecule is much less sterically hindered compared with the attachment at the substituted carbon 1. The substituents cannot stabilize radical I, since they are not attached to the carbon bearing the unpaired electron. The radical II can be stabilized by the resonance effects of the X and Y substituents. The propagating radical (radical II) formed by attachment of a radical at carbon 2 is the more stable one. 7.4 are favored on both steric and resonance grounds. The head-to-tail placement is predominant, since successive propagations by Eq.
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