Unprecedented Polymerization of Trimethylene Carbonate Initiated by a Samarium Borohydride Complex: Mechanistic Insights and Copolymerization with ε‐Caprolactone

Publisher Website
Google Scholar
Abstract
Poly(trimethylene carbonate) (PTMC) was synthesized through ring-opening polymerization by using a rare-earth borohydride initiator, [Sm(BH4)3(thf)3]. This initiator shows a high activity to give high-molar-mass PTMCs with molar-mass distributions ranging from 1.2 to 1.4, and with a regular structure void of ether linkages. The polymers were characterized by 1H and 13C NMR spectroscopy, 1H-1H COSY, 1H-13C HMQC NMR spectroscopy, size-exclusion chromatography (SEC), viscosimetry, and MALDI-TOF MS analyses. A coordination-insertion mechanism was established based on detailed NMR characterizations, especially of the polymer chain end-functions. The monomer initially coordinates the samarium to give [Sm(BH4)3(tmc)3], 1. The monomer then opens up through cleavage of the cyclic ester oxygenacyl bond and inserts into the SmHBH3 bond resulting in an alkoxide complex, [Sm{O(CH2)3OC(O)HBH3}3], 2, or [Sm{O(CH2)3OC(O)H}3], 2′, which then propagates the polymerization of TMC to give the active polymer [Sm({O(CH2)3OC(O)}nO(CH2)3OC(O)HBH3)3], 3 or [Sm(O(CH2)3OC(O){O(CH2)3OC(O)}nO(CH2)3OC(O)H)3], 3′. Finally, acidic hydrolysis of 3 or 3′ gives HO(CH2)3OC(O)[O(CH2)3OC(O)]nO(CH2)3OC(O)H, 4. This novel α-hydroxy,ω-formatetelechelic PTMC represents the first example of a formate-terminated polycarbonate. TMC and ε-caprolactone (CL) were copolymerized to afford both random PTMC-co-PCL and block PTMC-b-PCL copolymers that were characterized by 1H NMR spectroscopy, SEC, and differential scanning calorimetry (DSC). The structure of the block copolymers depends on the order of addition of monomers: if CL is introduced first, dihydroxytelechelic HO-PTMC-b-PCL-OH polymers are formed, whereas introduction of TMC first or simultaneous addition of comonomers leads to hydroxyformatetelechelic HC(O)O-PTMC-b-PCL-OH analogues.

This publication has 56 references indexed in Scilit: