Ring-Opening Polymerization of rac-Lactide by Bis(phenolate)amine-Supported Samarium Borohydride Complexes: An Experimental and DFT Study

Abstract

The synthesis and ring-opening polymerization (ROP) capability of bis(phenolate)amine-supported samarium borohydride and amide complexes are reported, together with a DFT study. Reaction of Na 2 -O 2 N L (L=OMe, NMe 2 , py, or Pr) with Sm(BH 4 ) 3 (THF) 3 gave the borohydride complexes Sm(O 2 N L )-(BH 4 )(THF) (L = OMe (2), NMe 2 (3), or py (4)) or Sm(O 2 N Pr )(BH 4 )(THF) 2 (5). Compounds 4 and 5 lost THF in vacuo, forming phenolate O-bridged dimers 1 and 6, respectively. Reaction of H 2 O 2 N L with Sm{N(SiHMe 2 ) 2 } 3 (THF) 2 formed monomeric Sm(O 2 N L ){N(SiHMe 2 ) 2 }(THF) (L = OMe (7), NMe 2 (8), or py ( 9) 12) or py (13); n=0, L=NMe 2 ( 14)) were prepared by reaction of Sm(O 2 N L )(BH 4 )(THF) with KN(SiMe 3 ) 2 . The X-ray structures of 2, 5, 6, 7, 10, 13, and 14 were determined. The borohydrides 2-5 were very efficient initiators for the ROP of ε-CL, giving linear dihydroxytelechelic poly(ε-CL). Selected amide initiators were also assessed but gave poorer control, as judged by broad PDI (M w /M n ) values and significant amounts of cyclic poly(ε-CL)s. Of the borohydrides, only 2-4 were active for the ROP of rac-LA, and activity increased in the order O 2 N L = O 2 N OMe ≈ O 2 N py < O 2 N NMe2 . The latter ligand also gave the best control of the ROP, as judged by the PDIs and M n values. All gave heterotactically enriched poly(rac-LA) with P r values in the range 0.82-0.84. The ROP of rac-LA with the amides 7, 9, and 12 was faster but much less well controlled. Overall, the borohydride initiators were superior for the ROP of both ε-CL and rac-LA when compared to otherwise identical amide initiators. MALDI-ToF MS analysis of the poly(rac-LA) formed with 3 showed both -CH(Me)CHO and -CH(Me)CH 2 OH end groups originating from the insertion of the first LA monomer into the Sm-BH 4 moiety of 3. In contrast, 2 and 4 formed only R,ω-dihydroxy-terminated polyesters with -CH(Me)CH 2 OH and -CH(Me)OH end groups. DFT calculations on Eu(O 2 0 N NMe2 )(BH 4 ) found two mechanisms for the initial ring-opening step of LA by the borohydride group, giving pathways leading to either aldehyde-or alcohol-terminated poly(lactide)s. Of these two pathways, the one giving R,ω-dihydroxy-terminated polymers was the most favored, in agreement with experiment. (Ligand abbreviations: O 2 N L = RCH 2 N(CH 2 -2-O-3,5-C 6 H 2 t Bu 2 ) 2 where R = CH 2 OMe, CH 2 NMe 2 , py, or Et for L = OMe, NMe 2 , py, or Pr, respectively; O 2 0 N NMe2 = Me 2 -NCH 2 CH 2 N(CH 2 -2-O-C 6 H 4 ) 2 .

A Comparison of Gallium and Indium Alkoxide Complexes as Catalysts for Ring-Opening Polymerization of Lactide

“…Metal alkoxide species, M-OR, have been shown to be the true active species in numerous ROP processes. [1][2][3][4][5][6][7][8][9][10][11][12][13][18][19][20][21][22][23][24] For example, in 2004, Lewiński and co-workers reported the first structurally characterized aluminum ε-caprolactone complex, indicating that the Al-Me group has poor reactivity with CL. 77 Thus, in most ROP cases, an alcohol is used as an initiator and is essential in order to start the polymerization.…”

Computational Studies on the Selective Polymerization of Lactide Catalyzed by Bifunctional Yttrium NHC Catalyst

“…Theoretical calculations have shown their ability to efficiently describe and explain the ROP mechanism of cyclic esters, such as lactones and lactide at the molecular level [36][37][38][39][40][41]. For instance, Hormnirun et al conducted a DFT study on the rac-lactide ring-opening mechanism initiated by a series of bis(pyrrolidene) Schiff-base aluminium complexes to reveal the correlation between the structure of backbone linker and the polymerization activity and stereoselectivity [42].…”

Redox Control of Aluminum Ring-Opening Polymerization: A Combined Experimental and DFT Investigation

“…On the other hand, based on previous reports, [5][6][7][8][9][10][11][12][13][18][19][20][21][22][23][24][25][26][27] after the insertion of the first equivalent of other monomers (CL, VL, and TMC), the rates of polymerization will not decrease remarkably, indicating that the insertion of the first monomers will not cause such a significant chelation effect like LA. Thus, in these cases, the energy of the initiation step can be used to estimate the energy of propagation.…”