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(mu-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(mu-Cl)(mu-OH)} 6∙7MeCN and {[TiL(O)3(On-pentyl)]2(mu-Cl)(mu-OH)}∙7.5MeCN (7∙7.5MeCN), respectively; [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). Type I and II complexes have been screened for their ability to act as catalysts in the ring opening polymerization (ROP) of epsilon-caprolactone (epsilon-CL), delta-valerolactone (delta-VL), omega-pentadecalactone (omega-PDL) and rac-lactide (r-LA), both with and without benzyl alcohol present and either under N2 or in air. The copolymerization of epsilon-CL with delta-VL and with r-LA has also been investigated. For the ROP of epsilon-CL, all were highly active (>99% conversion) at 130 oC over 24 h both under N2 and in air, whilst over 1h, for the type I complexes the trend was 3>2>1 but all were poor (≤12% conversion). By contrast, 5 over 1h at 130 oC was highly active (85% conversion). At 80oC, the activity trend followed the order 5 ≈ 4 > 3 > 2 > 1. For delta-VL, at 80 oC the activity trend 5 ≈ 4 > 1 > 2 > 3 was observed. ROP of the larger omega-PDL was only possible using 5 at 130 oC over 24 h with moderate activity (48 % conversion). For r-LA, only low molecular weight products were obtained, whilst for the co-polymerization of epsilon-CL with delta-VL using 5, high activity was observed at 80 oC affording a polymer of molecular weight > 23,000 and with equal incorporation of each monomer. In the case of epsilon-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 to that of three benchmark species.