Sulfonamide, Phenolate, and Directing Ligand-Free Indium Initiators for the Ring-Opening Polymerization of rac-Lactide

Reaction of In(CH2SiMe3)(3) with (H2N2Npy)-N-Ts or (H2N2NOMe)-N-Ts gave the five-coordinate indium alkyls In((N2Npy)-N-Ts) (CH2SiMe3) (14) and In((N2NOMe)-N-Ts)(CH2SiMe3) (15). The corresponding reaction with (H2N2Nph)-N-Ts gave four-coordinate In((N2NPh)-N-Ts)(CH2SiMe3) (16). Reaction of (H2N2Npy)-N-Ts with In{N(SiMe3)(2)}(3) gave the amide In((N2Npy)-N-Ts){N(SiMe3)(2)} (19). Reaction of (Na2N2Npy)-N-Ts with InCl3 in THF followed by (LiOPr)-Pr-i gave the ate product In((N2Npy)-N-Ts)((OPr)-Pr-i)Cl{Li(THF)} (21). The chloride complex In((N2Npy)-N-Ts)Cl(py) (22) was also isolated. Reaction of In(CH2SiMe3)(3) with (H2O2Npy)-N-Me gave the bis(phenolate)amine compound In((O2Npy)-N-Me)(CH2SiMe3) (23), and reaction with (PrOH)-Pr-i or Me2NCH2CH2OH gave the mixed alkyl-alkoxides In(CH2SiMe3)(2)((OPr)-Pr-i) (17) and In(CH2SiMe3)(2)(OCH2CH2NMe2) (18), which are dimeric in the solid state. The X-ray structures of 14, 15, 17-19, and 21-23 have been determined. The compounds were evaluated as initiators for the ring-opening polymerization of rac-lactide. With the alkyl and amide initiators, the M-n control was poor because of an unfavorable mismatch between initiation and propagation rates. The molecular weight control with the ate complex 21 was better, but the ROP was slow. The best-behaved initiator in terms of living ROP was In(CH2SiMe3)(2)((OPr)-Pr-i) (17). When used alone, In(CH2SiMe3)(3) was a very poor initiator, but in the presence of BnNH2 amine-co-initiated immortal ROP occurred to give atactic amine-terminated PLA in a controlled manner. Use of InCl3 and Et3N in place of the indium alkyl gave a 4-fold increase in activity and a substantial increase in heterotacticity (P-r = 0.85). Initial studies showed that BnNH2 and InCl3 alone could also produce heterotactically enriched, amine-terminated PLA at room temperature, albeit very slowly. ((H2N2Npy)-N-Ts = (2-NC5H4)CH2N(CH2CH2NHTs)(2); (H2N2NOMe)-N-Ts = MeOCH2CH2N(CH2CH2NHTs)(2); (H2N2NPh)-N-Ts = PhCH2N(CH2CH2NHTs)(2); (H2O2Npy)-N-Me = (2-NC5H4)CH2N(CH2ArOH)(2) (Ar = C6H2Me2).)