Ketene acetals chain transfer agents

New Chain Transfer Agents from Ketene Acetals... 

Since the diethyl ketene acetal (XII) appears to be a moderately effective chain transfer agent as well as a comonomer, a search... 

Although there are other unsaturated compounds that will undergo addition-elimination with free radicals, the benzyl ketene acetal XIV appears to be the most active double bond as far as rate of addition is concerned and the most efficient as far as regards to the extent of elimination is concerned. A comparison with the list of chain transfer agents listed in the Polymer Handbook (23) indicated that only the sulfur compounds appear to be more effective than XIV. Hydrolysis of the end-capped oligomer gives a macromer that is terminated with a carboxylic acid group. 

Termination may also occur by chain transfer with the initiator (e.g., water or alcohol) or a deliberately added chain-transfer agent. Deliberate termination of growth is carried out to produce polymers with specific molecular weights or, more often, telechelic polymers with specific end groups. Hydroxyl and amine end groups are obtained by using water and ammonia as chain-transfer agents. Carboxyl-ended telechelics can be obtained by termination with ketene silyl acetal followed by hydrolysis with base [Kobayashi et al., 1989]. 

Since the 4-phenyl-2-methylene-l,3-dioxolane (XXl) underwent much more extensive ring opening than did the unsubstituted 2-methylene-l,3-dioxolane (I), it was reeisoned that a benzyl ketene acetal would be a more effective chain transfer agent than diethyl ketene acetal. Thus vhen methyl benzyl ketene acetal was used with styrene, a complete addition-elimination occurred to produce an oligomer of styrene. 

Thos the ketene acetals are a new class of chain transfer agents that do not have a sulfur or a halogen present. The introduction of functional groups, such as hydroxyl groups, into the benzyl and the alkyl group makes possible the direct synthesis of functionally capped oligomers without the necessity of hydrolysis. 

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