To obtain more insight in the ring-opening polymerization of lactones, complex formation of the initiators tetraphenyltin, stannous octoate, tin tetrachloride, aluminium bromide and triisobutylaluminium (TIBA), and the monomers L(-)-lactide, D,e-lactide and glycolide was studied by i.r.-spectroscopy. When equimolar benzene or toluene solutions of initiators and monomers were combined, only complexes of aluminium bromide and D,L-lactide or glycolide, and of TIBA and D,L-lactide or glycolide were observed. The complex formation was studied in detail by varying the initiator and monomer concentrations. From these results and theoretical considerations, it is concluded that complexes are formed by the coordination of a carbonyl oxygen of monomer and the aluminium atom of initiator. The corresponding polymers were formed when TIBA was used as an initiator. When AIBr 3 was used, polymers were obtained only when traces of water were added. It is concluded that in the case of AIBr~ the actual initiating species is HBr and that the polymerization initiated with TIBA proceeds very probably through a coordinated insertion of the lactone monomer into the aluminium-carbon bond.
A number of samples of poly(D,L‐lactic acid) (PLA) with weight‐average molecular weights M̄w in the range 15,000–350,000 were prepared by a ring‐opening polymerization. The molecular weight distributions (MWDs) of these samples were determined by gel permeation chromatography (GPC). The method involves a universal calibration of the columns on the basis of polystyrene standards and a rapid iteration algorithm leading to the establishment of the Mark–Houwink relationship. In addition, osmometry and viscometry data are presented. The effect of hydrolytic degradation on the MWD of two PLA samples was studied by GPC.
SynopsisMelt polymerization conditions for D,Llactide initiated with tetraphenyltin were studied with regard to polymer molecular weight and weight distributions. "Single" polymerization, "multiple" polymerization (four or eight reactions a t the same time), and timedependent studies are described. Single polymerizations using constant initiator concentrations resulted in a broad scattering of nonreproducible molecular weight values. Multiple polymerizations at constant initiator concentrations, however, resulted in nearly identical molecular weight profiles. Multiple polymerizations at different initiator concentrations did not show an inverse dependency of initiator concentration on polymer molecular weight. Both the single and multiple melt polymerizations resulted in rather broad molecular weight distributions. The presence of hydrolysis products of lactide during the melt polymerization most likely has a detrimental effect on molecular weight. After a short induction period the rather slow polymerization of D,L-lactide resulted in a maximal molecular weight followed by a slight decrease in molecular weight to a constant value. It is concluded that the polymerization of D,L-lactide in the melt initiated with tetraphenyltin does not proceed through a "living" mechanism.
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