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发表时间：2008-01-01

发表刊物：高分子学报

所属单位：化工学院

期号：10

页面范围：947-954

ISSN号：1000-3304

摘要：Poly(L-lactide-co-epsilon-caprolactone) P(LLA-co-CL) copolymers with various L-lactide/epsilon-caprolactone(L-LA/epsilon-CL) mole ratios were prepared by the ring opening copolymerization of L-LA and epsilon-CL using nontoxic magnesium octoate as a catalyst in bulk. The time-conversion curves of L-LA and E-CL copolymerization were recorded by H-1-NMR. The results showed significantly more rapid polymerization of L-LA relative to E-CL. The carbonyl carbon signals in C-13-NMR are most sensitive to the sequence distributions of the lactidyl and caproyl units and used for calculating their average block length (L-LL(e) and L-C(e)). Two modes of transesterification occurred during the copolymerization process and played an important role in the redistribution of comonomer sequences. With polymerization progress, L-LL(e) decreased sharply and L-C(e) increased at first and then decreased gradually, as a result, the run number (R) increased gradually and the extent of randomness enhanced. The CLC sequence formed by the second mode of transesterification was observed at the end of reaction. Then, the reactivity ratios of L-LA and E-CL were measured to be r(LA) = 23 and r(CL) = 0.22, respectively, using the Fineman-Ross method. The results(r(LA) > 1 and r(CL) < 1) imply that L-lactide monomer added preferentially into the copolymer chain end in the first step of the reaction, resulting in the formation of long blocks of lactidyl unit in the polymer chain. The copolymer composition has a profound influence on the values of L-LL(e) and L-C(e). Both L-LL(e) and L-C(e) increased as the relative proportions of their respective monomers increased. The transesterification coefficient of the second mode of lactidyl units (T-II [CLC]) increased as the feed mole fraction of epsilon-CL increased. In terms of the overall feed compositions, the L-LL(r) values calculated from the reactivity ratio exceeded the L-LL(e) values determined from the product; however, the L-C(r) values were identical or shorter than the L-C(e) values. Therefore, the repeat unit sequence distribution of the copolymers was varied more or less toward a random sequence distribution by variation in the comonomer feed ratio and reaction time. The thermal properties and crystallization of the copolymers were investigated by DSC and XRD. The results showed that a close relationship between copolymer crystallinity and the length of the copolymer sequences. All copolymers of L-LA and epsilon-CL exhibited a single T-gs determined from the second heating scan. The experimental T-gs were in agreement with the calculated T-gs values from Fox equation. It can be concluded that the copolymers are random, or comprise a mixture of compatible block copolymers.

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