Transesterification is normally a deleterious side reaction because it leads to a loss of control of molecular mass in a living polymerization and, because in a stereoselective ring-opening polymerization, transesterification scrambles the stereocenters along a chain. These reactions are collectively shown in Scheme 1. It is now generally accepted that metal-alkoxide species are the key reactive intermediates in the propagation steps and, furthermore, that these, too, play a role in the competing side reactions of transesterification and chain transfer ( 6). This is commonly brought about by the use of metal catalyst precursors, although organic catalysis is possible by employing a nucleophilic base such as 4-dimethyl aminopyridine or an N-heterocyclic carbene with alcohol initiators ( 2 – 5). The ring-opening polymerization of cyclic esters such as lactides (LA) is a well established procedure for the production of polyesters ( 1). The 18-membered ring ( R, R, R, S, S, S)- and meso-( R, S, R, S, R, S)-(MeCHC(O)O) 6 and the 24-membered ring (MeCHC(O)OCHPr iC(NH)O) 4 have been characterized by single-crystal x-ray diffraction studies, together with the complex Na 2BPh 4. Metallated supports derived from Me 2Mg and Et 2Zn are less reactive but do show some ability for lactide ring-enlarging. For lactide this is shown to form the basis for chemical amplification from a dynamic combinatorial library and lactide can be converted to (MeCHC(O)O) 6 in >80% yield. Addition of NaBPh 4 to a heated benzene solution containing these rings preferentially removes the 18-membered rings from solution.
These have been characterized by electrospray ionization MS, and some small rings have been separated by gel-permeation chromatography. With ≈10 eq of these monomeric six-membered rings and with heating, cyclic esters (MeCHC(O)O) n and n are reversibly released to the solution. The reaction between Bu nLi in benzene and the solid polystyrene support PS-C 6H 4CH 2NH 2 leads to a lithiated species that can be represented as PS-C 6H 4CH 2NHLi(LiBu) x, where x ∼ 4, which is active in the ring-opening of the cyclic esters L-lactide, rac-lactide, and 2,5-morpholinediones.
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