Titanium Complexes of Tridentate Aminebiphenolate Ligands Containing Distinct N-Alkyls: Profound N-Substituent Effect on Ring-Opening Polymerization Catalysis

Abstract

The synthesis, structural characterization, and reactivity studies of titanium complexes supported by tridentate amine biphenolate ligands of the type [RN(CH(2)-2-O-3,5-C(6)H(2)(tBu)(2))(2)](2-) {[R-ONO](2-); R = tBu (1a), iPr (1b), nPr (1c)} are described. Alcoholysis of Ti(OiPr)(4) with H(2)[1a-1c] in diethyl ether solutions at 25 °C generates quantitatively the corresponding [R-ONO]Ti(OiPr)(2) (2a-2c) as a yellow crystalline solid. X-ray diffraction studies of 2b and 2c showed them to be five-coordinate, trigonal-bipyramidal species. Ring-opening polymerization of ε-caprolactone (ε-CL) catalyzed by 2b and 2c proved to be living, as evidenced by the narrow molecular weight distributions of the derived polymers and the linear dependence of number-averaged molecular weights on the monomer-to-catalyst ratios or polymerization time. Kinetic studies revealed that the polymerization rates are first-order in the concentration of ε-CL and first-order in that of 2b and 2c. The propagation rate of 2c is ca. 15 times faster than that of 2b, highlighting a profound substituent effect of primary versus secondary N-alkyls. In sharp contrast, reactions employing catalytic 2a produce either low-molecular-weight oligomers or polymers characteristic of somewhat wider molecular weight distributions, depending on the polymerization temperatures.

Heterodinuclear titanium/zinc catalysis: synthesis, characterization and activity for CO2/epoxide copolymerization and cyclic ester polymerization

“…Heterodinuclear 3 displayed moderate activities (refer to ESI, Fig. S13 and Table S3 †) 25,27 but also showed a clear induction period of approximately 45 minutes, during which very low conversions were observed. Such induction or initiation periods are common for ε-CL polymerization with other Ti and group 4 iso-propoxide catalysts, 27,113,114 and are attributed to a structural rearrangement of the ligands of the initiator, allowing the monomer to access the metals coordination sphere prior to insertion.…”

“…Titanium catalysts are useful in various transformations including hydroaminoalkylation reactions, 18 aldol and allylic additions to ketones and aldehydes, 19,20 and the epoxidation of alkenes. 21 In polymerization catalysis, titanium complexes are particularly effective single site catalysts for olefin polymerization, [22][23][24] and active titanium catalysts have been reported for oxygenated monomers including ε-caprolactone, 15,[25][26][27][28] rac-lactide and, more recently, CO 2 /epoxide ring opening co-polymerization (ROCOP). 27,[50][51][52][53][54][55][56][57] Heterodinuclear complexes have shown great promise in polymerization catalysis and have allowed greater activities and selectivities to be achieved.…”

“…21 In polymerization catalysis, titanium complexes are particularly effective single site catalysts for olefin polymerization, [22][23][24] and active titanium catalysts have been reported for oxygenated monomers including ε-caprolactone, 15,[25][26][27][28] rac-lactide and, more recently, CO 2 /epoxide ring opening co-polymerization (ROCOP). 27,[50][51][52][53][54][55][56][57] Heterodinuclear complexes have shown great promise in polymerization catalysis and have allowed greater activities and selectivities to be achieved. 14,[58][59][60][61][62][63][64][65] Recent breakthroughs include elegant heterodinuclear transition metal complexes based on Ti/Cr and Ti/Zr, developed by Marks and co-workers, which show higher activities in olefin polymerization than previous generations of homodinuclear analogues.…”

Titanium pyridonates for the homo- and copolymerization of rac-lactide and ε-caprolactone

“…7 Some examples of random co-polymer synthesis have been reported, including our initially communicated titanium pyridonate system. [12][13][14][15] The investigation of titanium complexes as initiators [12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29] for the ring-opening polymerization of cyclic esters has been largely driven by the minimal cost, high abundance, and low toxicity 30 of the source metal. While they are as yet less competitive than the leading initiator systems, these practical advantages have motivated vigorous research interest into a variety of ligand scaffolds in pursuit of achieving high molecular weight polymers with control of dispersity, co-monomer incorporation and tacticity.…”

Copper(II) Complexes with Tridentate Bis(pyrazolylmethyl)pyridine Ligands: Synthesis, X‐ray Crystal Structures and ϵ‐Caprolactone Polymerization

“…Gel permeation chromatography (GPC) analysis of the resulting polymers shows a monomodal molecular weight distribution (Mw/Mn) that correspond to a single‐site catalyst, suggesting control over the polymerization process and less transesterification side reactions . Furthermore, the comparison among PDI from polymers obtained with 4–6 and related copper complexes with bidentate azole ligands (2.6‐3.9), supports the statement that a sterically demanding tridentate ligand scaffold could lock the active species in a rigid arrangement which favors a controlled polymerization behavior . As a consequence, a possible proposal for further improvement could be increasing the steric hindrance at ligands by changing methyl substituents for bigger groups like phenyls.…”