Efficiency of liquid tin(ii) n-alkoxide initiators in the ring-opening polymerization of l-lactide: kinetic studies by non-isothermal differential scanning calorimetry

Novel soluble liquid tin(ii) n-butoxide (Sn(OnC(4)H(9))(2)), tin(ii) n-hexoxide (Sn(OnC(6)H(13))(2)), and tin(ii) n-octoxide (Sn(OnC(8)H(17))(2)) initiators were synthesized for use as coordination–insertion initiators in the bulk ring-opening polymerization (ROP) of l-lactide (LLA). In order to compare their efficiencies with the more commonly used tin(ii) 2-ethylhexanoate (stannous octoate, Sn(Oct)(2)) and conventional tin(ii) octoate/n-alcohol (SnOct(2)/nROH) initiating systems, kinetic parameters derived from monomer conversion data were obtained from non-isothermal differential scanning calorimetry (DSC). In this work, the three non-isothermal DSC kinetic approaches including dynamic (Kissinger, Flynn–Wall, and Ozawa); isoconversional (Friedman, Kissinger–Akahira–Sunose (KAS) and Ozawa–Flynn–Wall (OFW)); and Borchardt and Daniels (B/D) methods of data analysis were compared. The kinetic results showed that, under the same conditions, the rate of polymerization for the 7 initiators/initiating systems was in the order of liquid Sn(OnC(4)H(9))(2) > Sn(Oct)(2)/nC(4)H(9)OH > Sn(Oct)(2) ≅ liquid Sn(OnC(6)H(13))(2) > Sn(Oct)(2)/nC(6)H(13)OH ≅ liquid Sn(OnC(8)H(17))(2) > Sn(Oct)(2)/nC(8)H(17)OH. The lowest activation energies (E(a) = 52, 59, and 56 kJ mol(−1) for the Kissinger, Flynn–Wall, and Ozawa dynamic methods; E(a) = 53–60, 55–58, and 60–62 kJ mol(−1) for the Friedman, KAS, and OFW isoconversional methods; and E(a) = 76–84 kJ mol(−1) for the B/D) were found in the polymerizations using the novel liquid Sn(OnC(4)H(9))(2) as the initiator, thereby showing it to be the most efficient initiator in the ROP of l-lactide.