Poly(L-lactide-co-ε-caprolactone) P(LLA-co-CL) copolymers with various L-lactide/ε-caprolactone(L-LA/ε-CL) mole ratios were prepared by the ring opening copolymerization of L-LA andε-CL using nontoxic magnesium octoate as a catalyst in bulk.The time-conversion curves of L-LA and ε-CL copolymerization were recorded by1HNMR.The results showed significantly more rapid polymerization of LLA relative to ε-CL.The carbonyl carbon signals in 13C-NMR are most sensitive to the sequence distributions of the lactidyl and caproyl units and used for calculating their average block length (LLLe and LCe).Two modes of transesterification occurred during the copolymerization process and played an important role in the redistribution of comonomer sequences.With polymerization progress

LLLe decreased sharply andLCe 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 ε-CL were measured to berLA=23 and rCL=0.22

respectively

using the FinemanRoss method.The results(rLA1 and rCL 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 ofLLLe and LCe.Both LLLe and LCe increased as the relative proportions of their respective monomers increased.The transesterification coefficient of the second mode of lactidyl units(TⅡ［CLC］) increased as the feed mole fraction of ε-CL increased.In terms of the overall feed compositions

the LLLr values calculated from the reactivity ratio exceeded the LLLe values determined from the product; however

the LCr values were identical or shorter than the LCe 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 ε-CL exhibited a single Tgsdetermined from the second heating scan.The experimental Tgswere in agreement with the calculated Tgsvalues from Fox equation.It can be concluded that the copolymers are random

or comprise a mixture of compatible block copolymers.