Early studies using calcium oxide, carbonate, and carboxylates repo..., even in bulk at 120–180 °C. PolyGA and copolymers of GA with CL and L-LA have been prepared using Ca(acac)2, but again high temperatures (150–200 °C) are required. Under these conditions transesterification occurs, although to a lesser extent than in analogous Sn(Oct)2-initiated polymerizations.
Activities using calcium alkoxides are somewhat higher. Commercially available Ca(OMe)2 polymerizes both CL and LA in bulk at 120 °C. In both cases Mw/Mn values imply a significant degree of control (1.25 for PCL, 1.43 for PLA), although racemization of the lactide stereogenic centers is also observed. Molecular weight data are also higher than calculated values, and this has been attributed to aggregation of the active species, rendering some of the alkoxides inactive.
Higher activities may be obtained if the bis(alkoxide) is generated in situ. For example, addition of 100 equivalents L-LA to a 1:2 mixture of Ca(NTMS2)2 and MeOH in THF generates PLA to 97% conversion in 18 min at room temperature (c.f. 66% conversion after 90 min in bulk at 120 °C using commercial Ca(OMe)2). Furthermore, no epimerization is observed. Polydispersities are narrow (1.1), with Mn values slightly higher than those predicted from the monomer:initiator ratios. Initiator efficiency may be improved by adding excess MeOH which promotes rapid, reversible transfer between dormant and active species.
Bulkier alcohols provide improved levels of control. For instance, Ca(NTMS2)2.THF2/2iPrOH consumes 100 equivalents L-LA in 35 min at 25 °C (Mn = 6,900, Mn calc = 7,200, Mw/Mn = 1.05). Chain lengths determined by NMR spectroscopy agree closely with GPC data; NMR spectra also confirm that only the isopropoxide groups initiate polymerization since no silylamide end-groups are detected. The absence of an induction period suggests that in situ formation of calcium alkoxides suppresses aggregation; a similar effect using in situ-generated yttrium alkoxides has also been observed.
If mono-hydroxyl functionalized poly(ethylene glycol), HO-PEG, is added to Ca(NTMS2)2.THF2, then addition of LA affords the diblock PEG-b-PLA (Mn = 15,500, Mn calc = 15,500, Mw/Mn = 1.03). Using a similar strategy the reaction of CaH2 with telechelic diol HO-(PEG)-OH, followed by polymerization of L-LA results in a triblock structure, PLA-b-PEG-b-PLA of narrow polydispersity (1.02–1.08). Triblock copolymers of morpholine-2,5-diones with PEO have also been prepared in this manner.