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

Abstract

Poly(trimethylene carbonate) (PTMC) was synthesized through ring-opening polymerization by using a rare-earth borohydride initiator, [Sm(BH(4))(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 (1)H and (13)C NMR spectroscopy, (1)H-(1)H COSY, (1)H-(13)C 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(BH(4))(3)(tmc)(3)], 1. The monomer then opens up through cleavage of the cyclic ester oxygen--acyl bond and inserts into the Sm--HBH(3) bond resulting in an alkoxide complex, [Sm{O(CH(2))(3)OC(O)HBH(3)}(3)], 2, or [Sm{O(CH(2))(3)OC(O)H}(3)], 2', which then propagates the polymerization of TMC to give the active polymer [Sm({O(CH(2))(3)OC(O)}(n)O(CH(2))(3)OC(O)HBH(3))(3)], 3 or [Sm(O(CH(2))(3)OC(O){O(CH(2))(3)OC(O)}(n)O(CH(2))(3)OC(O)H)(3)], 3'. Finally, acidic hydrolysis of 3 or 3' gives HO(CH(2))(3)OC(O)[O(CH(2))(3)OC(O)](n)O(CH(2))(3)OC(O)H, 4. This novel alpha-hydroxy,omega-formatetelechelic PTMC represents the first example of a formate-terminated polycarbonate. TMC and epsilon-caprolactone (CL) were copolymerized to afford both random PTMC-co-PCL and block PTMC-b-PCL copolymers that were characterized by (1)H 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.

Bis(phosphinimino)methanide Borohydride Complexes of the Rare‐Earth Elements as Initiators for the Ring‐Opening Polymerization of ε‐Caprolactone: Combined Experimental and Computational Investigations

“…[38,39,[44][45][46][47][48][49][50] This relies on the in situ reduction of the carbonyl group of the first inserted cyclic ester molecule (CL or lactide (LA)) by the BH 4 À function (see below). [38,[44][45][46][47][48][49][50][51][52] Indeed, dihydroxy-terminally functionalized PCLs (HO-PCL-OH) [44][45][46][47][48][49][50] or PLAs (HO-PLA-OH) [38,39] have been easily prepared without the inherent constraints associated with post-polymerization chemical modification, as fully evidenced by 1 H and 13 C NMR spectroscopy and MALDI-TOF MS analyses. Indirect support for the formation of dihydroxy-terminated polyesters was also provided by the successful chain-extension synthesis of AB 2 triblock copolymers, poly(caprolactone)-b-[poly(benzylglutamate)] 2 and poly(caprolactone)-b-[poly(methyl methacrylate)] 2 [54] or of poly(ester-urethane).…”

“…[21] In the 1980s Mirsaidov and co-workers developed a more convenient approach starting from LnCl 3 (Ln = La, Ce, Pr, Nd, Sm) and NaBH 4 . [22][23][24] Later on, greater value was given to these original species in the promotion of rare-earth borohydride organometallic chemistry [25][26][27][28] and in their use as efficient catalysts for the (co)polymerization of ethylene, [29][30][31] isoprene, [31][32][33][34][35] styrene, [34,36] and some polar monomers such as lactide, [37][38][39][40][41][42] e-caprolactone, [37][38][39][43][44][45][46][47][48][49][50][51][52] trimethylene carbonate, [52] and methyl methacrylate. [53][54][55][56][57] Also, some rare-earthmetal borohydride derivatives such as metallocenes, [31,46] mono-cyclopentadienyl complexes, …”

A DFT Study of the Mechanism of Polymerization of ε‐Caprolactone Initiated by Organolanthanide Borohydride Complexes

“…[10][11][12][13][14][15][16] Besides being highly efficient and enabling the controlled and "living" ROP of cyclic esters, the most distinctive feature of rare earth borohydrides, and thus their major technological impact, is that they provide easy access to terminally functionalized polymers. This in situ reduction process (see below) provides a direct procedure to prepare a,w-functionalized polymers, thus avoiding the post-polymerization chemical modification of polymers and all of the associated inherent constraints.…”

Poly(trimethylene carbonate) from Biometals‐Based Initiators/Catalysts: Highly Efficient Immortal Ring‐Opening Polymerization Processes

“…[14] Other reports, yet scarce, relate to the use of group III metals for the ROP of TMC; while most of these initiators are discrete alkoxide complexes, some guanidinate derivatives have also shown high activity. [15] The activities (TOF = mol monomer ·mol initiator À1 ·h À1 ) reported for the ROP of TMC with the above-mentioned systems remain, however, in general quite low (TOF 1790). [7,15] To achieve conversion of very large amounts of cyclic monomer using as little as possible of the metal component, i.e., to improve on the catalytic activity and productivity and to minimize as well the residual metal content in the final polymers while maintaining a good control over the polymerization, the concept of "immortal" polymerization is very appealing.…”

“…[15] The activities (TOF = mol monomer ·mol initiator À1 ·h À1 ) reported for the ROP of TMC with the above-mentioned systems remain, however, in general quite low (TOF 1790). [7,15] To achieve conversion of very large amounts of cyclic monomer using as little as possible of the metal component, i.e., to improve on the catalytic activity and productivity and to minimize as well the residual metal content in the final polymers while maintaining a good control over the polymerization, the concept of "immortal" polymerization is very appealing. In such an approach, the presence within the catalytic system of added protic sources such as alcohols that act as chain transfer agents (CTAs) enables the growth of several polymer chains per metal center.…”