Summary: Bulk ring opening polymerization (ROP) of L-lactide (LA) initiated by calcium methoxide (Ca(OMe) 2 ) was investigated in LA/Ca molar ratio from 100 to 5000 and reaction times up to 3h at 180 8C. Ca(OMe) 2 showed good activity as ROP catalyst, particularly at high catalyst concentration. The polymerization shows a linear relationship between M n and reaction time, typical of a controlled process. The increase of LA/Ca molar ratio decreases the conversion which was maximum (66%) when the polymerization was carried out at 180 8C and LA/Ca ¼ 100. Semicrystalline poly(L-lactic acid) (PLLA) with moderate M n varying from 8,500 to 19,500 and polydispersities from 1.20 to 1.44 were obtained. The increase of LA/Ca molar ratio decreased M n , increased the degree of crystallinity and decreased racemization processes which can be controlled by the polymerization condition. Racemization could be reduced by using low catalyst concentration.
The title compounds are salts of the general form (Q(+))(2)[Zn(dmit)(2)](2-), where dmit corresponds to the ligand (C(3)S(5))(-) present in both and Q(+) to the counter-cations ((n)Bu(4)N)(+) [or C(16)H(36)N(+)] and (Ph(4)As)(+) [or C(24)H(20)As(+)], respectively. In the first case, Zn is in the 4e special positions of space group C2/c and hence the [Zn(dmit)(2)](2-) dianion possesses twofold axial crystallographic symmetry. Including these, there are now 11 known examples of [Zn(dmit)(2)](2-) or its analogues, with O replacing the exocyclic thione S, and [Zn(dmio)(2)](2-) dianions in nine structures with various Q. Comparison of these reveals a remarkable variation in details of the conformation which the dianion may adopt in terms of Zn coordination, equivalence of the Zn-S bond lengths, displacement of Zn from the plane of the ligand and overall dianion shape.
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