Synthesis and characterizations of bis(phenoxy)‐amine tin(II) complexes for ring‐opening polymerization of lactide

Abstract

A series of tin(II) complexes supported by N 2 O 2 bis(phenol)-amine ligands were prepared from the reactions of the corresponding ligands with Sn[N(SiMe 3 ) 2 ] 2 in benzene at room temperature. The ligands were designed to have different substituted group at the ortho-position on the aryl rings (R = t Bu, CH 3 ) and N-containing side arm (E = CH 2 NEt 2 and pyridine) giving a variation of tin(II) complexes (R = t Bu, E = CH 2 NEt 2 , 2a; R = t Bu, E = py, 2b; R = CH 3 , E = CH 2 NEt 2 , 2c; R = CH 3 , E = py, 2d). All complexes were characterized by NMR spectroscopy and single-crystal X-ray analysis. The single-crystal X-ray crystallography revealed that all complexes have a monomeric four-coordinate tin center with a distorted seesaw structure. All complexes are active for solvent-free polymerization of L-lactide at 120 C giving poly(L-lactide) with narrow to moderate dispersity (Ð = 1.12-1.56). In the presence of benzyl alcohol during the polymerization, the resulting polymer was found to be linear having benzyl alcohol as the end group while, in the absence of benzyl alcohol, the polymer was cyclic. The large t Bu group at the ortho-position was found to decrease polymerization activity while the more basic CH 2 NEt 2 group was found to increase the polymerization activity. The polymerization of rac-lactide under a similar condition gave PLA having a slight heterotactic bias for all catalysts.

Syntheses of polylactides by means of tin catalysts

“…Cyclic tin(II) complexes such as the bisphenoxide complexes 1a-g or the salicylaldiminato complexes 2a-c in Scheme 48 were synthesized by the Phomphrai group and characterized by various methods including X-ray structure analyses. [209][210][211] In most polymerization experiments with LLA or εCL cyclic polyesters were obtained and their formation was ascribed to a zwitterionic mechanism, which is another version of ROPPOC mechanism as explained and discussed before. Those authors assumed that the lone electron pair of the tin atom stabilizes the acylium ion of the electrophilic chain end in analogy to a carbene.…”

The Ring‐Opening Polymerization–Polycondensation (ROPPOC) Approach to Cyclic Polymers

“…Finally, a paper of Phomphrai and coworkers describing syntheses of cyclic polylactides by means of a tin(II)complex derived from a bisphenol ( Scheme ) needs discussion. [ 53 ] Those authors proposed a ZIP ((a) in Scheme 28) saying that the lone electron pair of the Sn(II) atom plays the role of a carbene. However, an experimental evidence for a ZIP mechanism was not provided.…”

Reversible polycondensations outside the Jacobson–Stockmayer theory and a new concept of reversible polycondensations

“…Such extreme conversions are interesting, because the modern theories of reversible and irreversible polycondensations predict that the fraction of cycles increases with the conversion until all reaction products have a cyclic topology at 100% conversion. For ROPPOC syntheses of cyclic polyesters two classes of catalysts have been found, namely strong nucleophiles, such as pyridines [10][11][12][13][14] heterocyclic carbenes [15][16][17][18][19][20][21][22] on the one hand, and covalent tin(II) or tin(IV) compounds [23][24][25][26] on the other. A simplified polymerization mechanism involving covalent end groups catalyzed by dibutyltin bis( pentafluorophenoxide) BuSnOPF is outlined in Scheme 3.…”