Synthesis of hydroxyl‐terminated polycarbonates of controlled number‐average molecular weight

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Abstract
Hydroxyl‐terminated polycarbonates are important starting materials for the synthesis of multiblock copolymers. Earlier papers from our laboratory and elsewhere have demonstrated their utility in siloxane, aryl ether, and ester systems. One synthesis problem that must be addressed is the control of the number‐average molecular weight and hence block size of the polycarbonate oligomeric precursor. The facile phosgene‐hydroxyl reaction is often difficult to monitor precisely. The present article describes a novel, simple, and convenient technique for the synthesis of hydroxyl‐terminated polycarbonates of well‐controlled number‐average molecular weight. The approach involves an in situ blocking of some of the phenolic groups either prior to or during phosgenation. The protecting groups are easily removed after the polymerization is complete. In a practical laboratory experiment the technique does not require any additional step beyond that necessary for the preparation of nonfunctional polycarbonates of controlled molecular weight. The method is demonstrated in this article with the polycarbonate of bisphenol‐A via the use of trimethylchlorosilane, trifluoroacetic anhydride, and trifluoroacetic acid as blocking groups. Ultraviolet, 19F‐NMR, and 1H‐NMR measurements as well as vapor‐pressure osmometry were used to characterize the oligomers.