Issue 6, 2020

Use of titanocalix[4]arenes in the ring opening polymerization of cyclic esters


The known dichloride complexes [TiCl2L(O)2(OR)2] (type I: R = Me (1), n-Pr (2) and n-pentyl (3); L(OH)2(OR)2 = 1,3-dialkyloxy-p-tert-butylcalix[4]arene), together with the new complexes {[TiL(O)3(OR)]2(μ-Cl)2}·6MeCN (R = n-decyl (4·6MeCN)), and [Ti(NCMe)Cl(L(O)3(OR))]·MeCN (type II: R = Me, 5·MeCN) are reported. Attempts to prepare type II for R = n-Pr and n-pentyl using [TiCl4] resulted in the complexes {[TiL(O)3(On-propyl)]2(μ-Cl)(μ-OH)} 6·7MeCN and {[TiL(O)3(On-pentyl)]2(μ-Cl)(μ-OH)}·7.5MeCN (7·7.5MeCN), respectively; use of [TiCl4(THF)2] resulted in a co-crystallized THF ring-opened product [Ti(NCMe)(μ3-O)L(O)4TiCl(O(CH2)4Cl)]2–2[TiCl(NCMe)(L(O)3(On-Pr))]·11MeCN (8·11MeCN). The molecular structures of 2·2MeCN, 4·6MeCN, and 5·MeCN together with the hydrolysis products {[TiL(O)3(OR)]2(μ-Cl)(μ-OH)} (R = n-Pr 6·7MeCN; n-pentyl, 7·7.5MeCN, 9·9MeCN); R = n-decyl 10·8.5MeCN) and that of the ring opened product 8·11MeCN and the co-crystallized species [Ti2(OH)Cl(L(O)3(OR))][L(OH)2(OR)2]·2.85(C2H3N)·0.43(H2O) (R = n-pentyl, 11·2.85(C2H3N)·0.43(H2O)) are reported. Type I and II complexes have been screened for their ability to act as catalysts in the ring opening polymerization (ROP) of ε-caprolactone (ε-CL), δ-valerolactone (δ-VL), ω-pentadecalactone (ω-PDL) and rac-lactide (r-LA), both with and without benzyl alcohol present and either under N2 or in air. The copolymerization of ε-CL with δ-VL and with r-LA has also been investigated. For the ROP of ε-CL, all performed efficiently (>99% conversion) at 130 °C over 24 h both under N2 and in air, whilst over 1 h, for the type I complexes the trend was 3 > 2 > 1 but all were poor (≤12% conversion). By contrast, 5 over 1 h at 130 °C was highly active (85% conversion). At 80 °C, the activity trend followed the order 54 > 3 > 2 > 1. For δ-VL, at 80 °C the activity trend 54 > 1 > 2 > 3 was observed. ROP of the larger ω-PDL was only possible using 5 at 130 °C over 24 h with moderate activity (48% conversion). For r-LA, only low molecular weight products were obtained, whilst for the co-polymerization of ε-CL with δ-VL using 5, high activity was observed at 80 °C affording a polymer of molecular weight >23 000 Da and with equal incorporation of each monomer. In the case of ε-CL/r-LA co-polymerization using 5 either under N2 or air, the polymerization was more sluggish and only 65% conversion of CL was observed and the resultant co-polymer had 65 : 35 incorporation. Complex 5 could also be supported on silica, however this system was not as active as its homogeneous counterpart. Finally, the activity of these complexes is compared with that of three benchmark species: a di-phenolate Ti compound {TiCl2(2,2′-CH3CH[4,6-(t-Bu)2C6H2O]2)} (12) and a previously reported NO2-containing titanocalix[4]arene catalyst, namely cone-5,17-bis-tert-butyl-11,23-dinitro-25,27-dipropyloxy-26,28-dioxo-calix[4]arene titanium dichloride (13), as well as [Ti(Oi-Pr)4]; the parent calixarenes were also screened.

Graphical abstract: Use of titanocalix[4]arenes in the ring opening polymerization of cyclic esters

Supplementary files

Article information

Article type
20 Dec 2019
25 Jan 2020
First published
27 Jan 2020
This article is Open Access
Creative Commons BY license

Catal. Sci. Technol., 2020,10, 1619-1639

Use of titanocalix[4]arenes in the ring opening polymerization of cyclic esters

Z. Sun, Y. Zhao, O. Santoro, M. R. J. Elsegood, E. V. Bedwell, K. Zahra, A. Walton and C. Redshaw, Catal. Sci. Technol., 2020, 10, 1619 DOI: 10.1039/C9CY02571E

This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. You can use material from this article in other publications without requesting further permissions from the RSC, provided that the correct acknowledgement is given.

Read more about how to correctly acknowledge RSC content.

Social activity