Parallelism Between Model and Polymerization Reactions

As discussed in the Model Studies in Cationic Polymerization Section for every reaction in the Kennedy-Gillham scheme a parallel polymerization step exists. By disturbing the basic scheme, quantitative and qualitative effects of changing variables on product distributions can be examined. 

the ratio Cf2/C13 reflects the relative rate of proton elimination (transfer)/methylation(termination) in the first generation. By changing the coinitiator Me3Al, Me2AlCl, the effect of counter anions Me3AlCl , Me2AlCl2 on this ratio can be studied. 

Since steric hindrance prevents propagation (Cf2 + Cg — Cf0) the above ratio is a quantitative measure of the coinitiation efficiency (t-Bu + Cg — Cf2— first generation products) and can be studied under various conditions with various initiators. 

Results of a large number of experiments carried out to study the effect of temperature and the nature of the coinitiator in various solvents on conversion and product distributions are summarized in Table 3. The effect of temperature on overall conversions obtained in methyl chloride solvent is plotted separately in Fig. 2. These results will be discussed in terms of the overall coinitiator reactivities and coinitiator efficiencies. Further, the effect of temperature, solvent and the nature of the alkyl-aluminum coinitiator-alkylhalide initiator system on the fundamental reactions of polymerization will be examined 

The least reactive coinitiator is Et3Al. Reactions coinitiated with Et3Al are least affected by temperature. The increase in temperature from — 80° to — 65° slightly increases the conversion. Increased amounts of first generation products indicate that this is due to an increased rate of initiation. From — 65° to — 30° conversions remain fairly constant in any solvent. Et3Al, the weakest Lewis acid studied, probably gives the most nucleophilic counter anion, and consequently the least separated ion pair which would explain fast ethylation or hydridation (termination) of the f-Bu . 

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