8-Propiolactones, 6-substituted mechanisms

Questions about the gross mechanism that remain to be answered are whether the proton in the reactive protonated ester is on the carbonyl oxygen or on the ether oxygen, whether the attacking water molecule first adds or substitutes directly, and whether or not the transition state is highly polar. These questions are similar to those discussed earlier in connection with the hydrolysis of 8-propiolactone, and the possible transition states are similar to those in Table 7. Again, of course, the representations are of extreme transition states, and the true one may be intermediate. A decision in favor of one of these representations is to be taken as an abbreviated statement, the fuller description being that the particular transition state chosen is nearer the true one than any other. 

The configuration of the asymmetric carbon atom -(-CHR- in a substituted monomer is retained in the polymer. An exception to this general mechanism is the polymerization of -propiolactone (jf = l, R = H) by weak bases (e.g. CHsCOO Na ) in which nucleophilic attack takes place at the O—CH2 carbon atom causing scission of this bond and formation of carboxylate ion active species (i.e. CHR-(-CH2-)xCOO ). 

When weak bases or ammonium carboxylates are used as initiators, a similar mechanism has been observed which involves the carboxylate anions responsible for propagation. Subsequently, it was confirmed that, in the case of a,a -dialkyl-P-propiolactone, no chain transfer could occur as a-proton abstraction could no longer take place [29]. Interestingly, a similar result was recently highlighted by Guerin et al. who showed that, compared to poly(malolactonate) (as obtained from a,a, P-trisubstituted P-lactones), polymers prepared by the ROP of P-lactones without substitution in the a-position were characterized by major discrepancies between the experimental molecular weights and the theoretical values expected for a controUed/ Uving polymerization (see below) [30]. 

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