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

Abstract

A series of methylene-bridged bisphenol ligands including 2,2′-CH2(6-iPr-C6H3-OH)2 (L0) and 2,2′-CH2(4-CH3-6-tBu-C6H2-OH)2 (L1), and the aryl-substituted L = CHAr(4,6-di-tBu-C6H2-OH)2 (Ar = C6H5-CH2- (L2), 4-MeO-C6H4- (L3), 3-MeO-C6H4- (L4), 4-NO2-C6H4- (L5), 2-MeO-5-Br-C6H3- (L6), 4-Me-C6H4- (L7), 4-CF3-C6H4- (L8), and 4-F-C6H4- (L9)), which were synthesized by the condensation of the respective aldehydes with 2,4-di-tert-butylphenol, afforded the corresponding [L]TiCl2 (L0−L4, L6), [L]TiCpCl (L1–L6), and [L]Ti(OiPr)2 (L1, L3, L4, L6) complexes by reaction of selected ligands with TiCl4, CpTiCl3, or Ti(OiPr)4. Following spectral and X-ray characterization, [L]TiCl2 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]TiCl2 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 (Mn) 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., Cp2TiCl2 > (iPrO)3TiCl > (iPrO)2TiCl2 > [L2]TiCl2 > [L3]TiCl2 ≥ [L4]TiCl2 > [L1]TiCl2 ≥ [L0]TiCl2 > [L6]TiCl2 > [L6]TiClCp ∼ [L2]TiCpCl. Likewise, [L0, L1, L2, L3, L4]TiCl2 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]TiCl2 ≥ [L]TiCpCl ≫ Cp2TiCl2. However, while stereoelectronic ligand effects impact both CRP and LROP, the influence of the non-coordinating CH(Ar) bridge substitution remains weak.