The Dual‐Stereocontrol Mechanism: Heteroselective Polymerization of rac‐Lactide and Syndioselective Polymerization of meso‐Lactide by Chiral Aluminum Salan Catalysts

Abstract

We describe alkoxo‐aluminum catalysts of chiral bipyrrolidine‐based salan ligands that follow the dual‐stereocontrol mechanism wherein a given combination of stereogeneities at the metal site and the proximal center of the last inserted lactidyl (“match”) is active towards lactide having a proximal stereogenic center of the opposite configuration, while the diastereomeric combination of stereogeneities (“mismatch”) is inactive towards any lactide. Polymerization of rac‐LA by the enantiomerically pure catalysts was sluggish and gave stereoirregular poly(lactic acid) (PLA) because selective insertion to a match diastereomer gives a mismatch diastereomer. The racemic catalysts showed higher activity and led to highly heterotactic PLA following polymeryl exchange between two mismatched catalyst enantiomers. A succession of match diastereomers in selective meso‐LA insertions led to syndiotactic PLAs reaching a syndiotacticity degree of α=0.96. This polymer featured a Tm of 153 °C matching the highest reported value, and the highest crystallinity (ΔHm=56 J g−1) ever reported for syndiotactic PLA.

Isoselective Polymerization of rac‐Lactide by Highly Active Sequential {ONNN} Magnesium Complexes

“…Polymerization of meso-LA may also be employed for distinguishing between the two stereocontrol mechanisms, as the enantiomorphic-site control should lead to syndiotactic PLA (-RS-RS-RS-RS-s equence;d ominant rrr tetrad in the HD 1 HNMR spectrum)w hile the chain-end control should lead to heterotactic PLA (-RS-SR-RS-SR-s equence;a % 1:1r atio of rmr and mrm tetrads in the HD 1 HNMR spectrum). [71,72] Polymerization of 300 equiv.o fmeso-LA by Lig 1 Mg-HMDS resulted in full monomer consumption within 10 min, and the obtaining of ah eterotactically inclinedP LA (P r = 0.78) according to the HD 1 HNMR spectrum ( Figure S41), thus supporting our chain-ends tereocontrol mechanism proposal. Complexes Lig 2 Mg-HMDS and Lig 3 Mg-HMDSe xhibited similar isoselectivities in rac-LA polymerization andl ed to P m values of 0.88-0.90.…”

“…A polymerization run of rac ‐LA in the presence of a racemic mixture of Lig 1 Mg‐HMDS led to the same degree of isotacticity and similar HD 1 H NMR spectra (Table S3, entry 11). Polymerization of meso ‐LA may also be employed for distinguishing between the two stereocontrol mechanisms, as the enantiomorphic‐site control should lead to syndiotactic PLA (‐ RS ‐ RS ‐ RS ‐ RS ‐ sequence; dominant rrr tetrad in the HD 1 H NMR spectrum) while the chain‐end control should lead to heterotactic PLA (‐ RS ‐ SR ‐ RS ‐ SR ‐ sequence; a ≈1:1 ratio of rmr and mrm tetrads in the HD 1 H NMR spectrum) [71, 72] . Polymerization of 300 equiv.…”

Make or break: Mg(ii)- and Zn(ii)-catalen complexes for PLA production and recycling of commodity polyesters

“…3,[10][11][12][13][14] Recent research has primarily sought to address toxicity concerns associated with the industry standard, Sn(Oct) 2 , through the development of biocompatible and environmentally benign alternatives. [15][16][17] Consequently, a diverse range of metals have been explored, including Al(III) [18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35] and group(IV), [36][37][38][39][40][41][42][43][44][45][46][47][48][49] with a particular focus on retaining activity whilst maintaining stereocontrol. Metals most pertinent to this report include Zn(II) and Mg(II), which are cheap and non-toxic.…”

Controlled Ring-Opening Polymerization of O-Carboxyanhydrides to Synthesize Functionalized Poly(α-Hydroxy Acids)

“…91 Therefore, the search for a new catalytic system for enriched control of the microstructure of copolymers is crucial. Despite efforts devoted to the synthesis of stereoregular polyesters by stereoselective ROP of racemic LA and lactones, [92][93][94][95][96][97][98][99][100][101] relatively few well-defined metal catalysts are available for stereoselective polymerization of racemic OCA monomers.…”