Radical and Ring-Opening Polymerizations with Aryl-Substituted Methylene-Bridged Titanium Bisphenolates

Le résumé fourni par la source

A series of methylene-bridged bisphenol ligands including 2,2′-CH 2 (6- i Pr-C 6 H 3 -OH) 2 ( L0 ) and 2,2′-CH 2 (4-CH 3 -6- t Bu-C 6 H 2 -OH) 2 ( L1 ), and the aryl-substituted L = CH Ar (4,6-di- t Bu-C 6 H 2 -OH) 2 ( Ar = C 6 H 5 -CH 2 - ( L2 ), 4-MeO-C 6 H 4 - ( L3 ), 3-MeO-C 6 H 4 - ( L4 ), 4-NO 2 -C 6 H 4 - ( L5 ), 2-MeO-5-Br-C 6 H 3 - ( L6 ), 4-Me-C 6 H 4 - ( L7 ), 4-CF 3 -C 6 H 4 - ( L8 ), and 4-F-C 6 H 4 - ( L9 )), which were synthesized by the condensation of the respective aldehydes with 2,4-di- tert -butylphenol, afforded the corresponding [ L ]TiCl 2 ( L0 − L4, L6 ), [ L ]TiCpCl ( L1–L6 ), and [ L ]Ti(O i Pr) 2 ( L1, L3, L4, L6 ) complexes by reaction of selected ligands with TiCl 4, CpTiCl 3, or Ti(O i Pr) 4 . Following spectral and X-ray characterization, [ L ]TiCl 2 and [ L ]TiCpCl subsets were investigated in both the controlled radical polymerization (CRP) of styrene and the living ring-opening polymerization (LROP) of ε-caprolactone (ε-CL). Zn reduction of [ L0 − L4, L6 ]TiCl 2 and [ L2, L6 ]TiCpCl produces Ti(III) • metalloradicals, which initiate via radical ring opening of epoxide precursors and mediate CRPs by reversible endcapping of polymer radicals. A linear dependence of molecular weight ( M n ) on conversion, moderate polydispersities (PDI ∼ 1.5), and low initiator efficiency (IE < 0.1) result in all cases, but these bisphenolates remain inferior to Cp and alkoxide derivatives, i.e., Cp 2 TiCl 2 > ( i PrO) 3 TiCl > ( i PrO) 2 TiCl 2 > [ L2 ]TiCl 2 > [ L3 ]TiCl 2 ≥ [ L4 ]TiCl 2 > [ L1 ]TiCl 2 ≥ [ L0 ]TiCl 2 > [ L6 ]TiCl 2 > [ L6 ]TiClCp ∼ [ L2 ]TiCpCl. Likewise, [ L0, L1, L2, L3, L4 ]TiCl 2 and [ L2, L5, L6 ]TiCpCl initiate and catalyze ε-CL LROPs, which proceed similarly (PDI < 1.1–1.2, IE ∼ 1–2) even without alcohol coinitiators, and are likely mediated by trace water-promoted anionic-coordination and activated monomer mechanisms, with [ L ]TiCl 2 ≥ [ L ]TiCpCl ≫ Cp 2 TiCl 2 . However, while stereoelectronic ligand effects impact both CRP and LROP, the influence of the non-coordinating CH(Ar) bridge substitution remains weak.

Ce résumé expose les affirmations des auteurs. BNTIC ne l’interprète pas comme une validation indépendante des résultats.