Polymers of Carbonic Acid. 15. Polymerization of Cyclotrimethylene Carbonate with TiCl₄or SbCl₅as Initiator

Bulk polymerization of cyclotrimethylene carbonate (TMC) was conducted with either TiCl4 or SbCl5 as initiator. Reaction time, temperature, and monomer/initiator (M/I) ratio were varied. High yields (>90%) were obtained with both initiators, but the weight-average molecular weights (M w) never exceeded 75,000. In contrast to SbCl5, TiCl4 does not cause decarboxylation at temperatures below 150°C so that polycarbonates free of ether groups were formed. However, both initiators yield polycarbonates with more CH2Cl than CH2OH endgroups. In contrast to the sluggish TiCl4, SbCl5 is reactive enough to initiate the polymerization of TMC in CHCl3 at 25°C. Both TiCl4 and SbCl5 form complexes with the monomer via the carbonyl group. In the case of TiCl4, a solid 1:2 complex was isolated and characterized by IR- and 13C-NMR CP/MAS spectroscopy. Furthermore, a crystalline 1:2 complex was synthesized with neopentanediol carbonate (NPC). In the case of SbCl5, no well-defined complex was obtained with TMC but a crystalline 1:1 complex was obtained with NPC. These NPC complexes proved to be useful as initiators of bulk polymerizations of TMC.