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.
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.
Herein we describe the obtention of polylactide stereocomplexes using a novel initiator system comprising Mg(II) and Ti(IV) centers. The stereocomplexes were successfully prepared using two different routes: by PDLA and PLLA co-precipitation from chloroform solutions and two-step diblock copolymerization. For the PDLA/PLLA mixing procedure, individual homochiral polymers were prepared in solution at different monomer/initiator ratios; whereas, the PLA stereoblock was prepared employing one-pot sequential polymerization of both chiral monomers. The products analyzed by WAXD showed the characteristic stereocrystal reflections while the high melting temperatures found in the DSC runs confirmed the formation of PLA stereocomplexes in both cases. Surprisingly, one of the stereocomplexes showed a second endothermic event at 250.1 C and to our knowledge this is the highest melting temperature reported for PLA stereocomplexes till date. These results point to the discovery of an effective initiator based on low toxicity metals for the preparation of biodegradable materials with interesting thermal properties.
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