Cyclic polyglycolides via ring-expansion polymerization with cyclic tin catalysts

Glycolide was polymerized in bulk with two cyclic catalysts - 2,2-dibutyl-2-stanna-1,3-dithiolane (DSTL) and 2stanna-1,3-dioxa-4,5,6,7-dibenzepane (SnBiph). The monomer/initiator ratio, temperature (140 - 180 degrees C) and time (1---4 days) were varied. The MALDI TOF mass spectra exclusively displayed peaks of cyclic polyglycolide (PGA) and revealed an unusual "saw-tooth pattern" in the mass range below m/z 2 500 suggesting formation of extended ring crystallites. The DSC measurements indicated increasing crystallinity with higher temperature and longer time, and after annealing for 4 d at 160 degrees C a hitherto unknown and unexpected glass transition was found in the temperature range of 170-185 degrees C. Linear PGAs prepared by means of metal alkoxides under identical conditions did not show the afore-mentioned features of the cyclic PGAs, neither in the mass spectra nor in the DSC measurements. All PGAs were also characterized by SAXS measurements, which revealed relatively small Lvalues suggesting formation of thin crystallites in all cases with little influence of the reaction conditions.