Geminate chain termination

The mechanism can be best understood within the framework of the conventional theory of radical chain kinetics, provided that certain of the usual simplifying assumptions are omitted. A solution is given to the problem of steady-state polymerization rate as a function of monomer and initiator concentration, taking into account termination reactions of primary radicals and recombination of geminate chains arising from the same initiation event. This model is shown to account for the kinetic data reported herein. With appropriate rate constants it should be generally applicable to radical polymerizations. 

Chain end analysis of the polypentene by H NMR shows the presence of geminally disubstituted olefin resonances, indicating chain termination by /6-H elimination. NMR analysis of the polymer at the pentad level shows a remarkably high degree of isotacticity. This catalyst also initiates the polymerization of 1-hexene and afford polymers (M = 24,000) with M jM =1.75. A similar C NMR analysis showed the tacticity of more than 95%. 

Substituted cycloalkenes usually react in the ring and not in the side chain. Internal alkenes with CH2 groups in both allylic positions yield a mixture of isomers, whereas terminal alkenes give primary alcohols as a result of allylic rearrangement. Later studies revealed, however, that the reactivity depends on both the structure of the alkene and reaction conditions.674 675 In alcoholic solutions, for example, racemic products are formed. Geminally disubstituted alkenes may exhibit a reactivity sequence CH > CH2 > CH3.675 676... 

An alternative, and more recent, approach is shown in reaction (42).9091 Here, a cyclophosphazene is synthesized with two non-geminal alkoxy chains that bear terminal olefinic groups. Treatment of these compounds with an organometallic ADMET catalyst causes loss of ethylene and formation of a cyclolinear polymer. The chain lengths achieved by this method are generally longer than those produced by the dehydrohalo-genation technique. 

Lastly, the NMR spectrum of B13 obtained on an -60% enriched sample (23. Figure 6, clearly shows the presence of the two geminal ring methyls and one terminal chain methyl. On this basis the molecular structure of compound B13 has been identified as la(H), 6B(H)-2,2-dimethyl-9-ethylbicyclo-[4,4,0]-8-thiadecane ... 

Phthalazine-l(2 T)-thione reacts in a similar manner with alkenes by [2 + 2] addition, but the intermediate diradical either cyclizes on to the /jen-position to give, after oxidation, the fused tricyclic system (105), or undergoes hydrogen transfer to give open chain sulfides (Scheme 68). The formation of the fused products is favoured by stabilization of the terminal alkyl radical by geminal methyl substitution <90CPB1205>. 

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