Kinetics and mechanism of cyclic esters polymerization initiated with covalent metal carboxylates, 5

Ring-opening polymerizations of ε-caprolactone (CL) and L,L-dilactide (LA) initiated by tin(II) octoate (Sn(Oct)2) and zinc octoate (Zn(Oct)2) and co-initiated with butyl alcohol (BuOH) carried out in tetrahydrofuran as a solvent at 80 °C were studied. By means of MALDI-TOF mass spectrometry, the formation of several populations of polyester macromolecules bearing various end-groups was revealed, namely for poly(ε-caprolactone) (PCL): BuO(O)CPCLOH (A), BuO(O)CPCLOct (B), HO(O)CPCLOH (C), HO(O)CPCLOct (D), and PCL cyclics (E), and for poly(L-lactide) (PLA): BuO(O)CPLAOH (A′), BuO(O)CPLAOct (B′), HO(O)CPLAOH (C′), and HO(O)CPLAOct (D′) (where Bu = C4H9 and Oct = O(O)CCH(C2H5)C4H9). In these polymerizations the end-groups in the originally formed macromolecules change slowly with time. In the LA/Sn(Oct)2/BuOH system at the beginning of polymerization almost exclusively macromolecules of the structure A′ are formed and then structures B′, C′, and D′ start to appear, however, after a period more than 300 times (at 80 °C) longer than that required for full monomer conversion, these macromolecules give exclusively esterified B′ and D′ chains. With Zn(Oct)2/BuOH all of these processes are much slower and less selective. Dependencies of mass fraction of macromolecules with various end-groups on time in L,L-dilactide polymerization initiated with Sn(Oct)2/BuOH system. Symbols ▵, ▴, ○, • correspond to PLA populations A′, B′, C′, and D′.