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)}center dot 6MeCN (R = n-decyl (4 center dot 6MeCN)), and [Ti(NCMe)Cl(L(O)(3)(OR))]center dot MeCN (type II: R = Me, 5 center dot 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 center dot 7MeCN and {[TiL(O)(3)(On-pentyl)](2)(mu-Cl)(mu-OH)}center dot 7.5MeCN (7 center dot 7.5MeCN), respectively; use of [TiCl4(THF)(2)] resulted in a co-crystallized THF ring-opened product [Ti(NCMe)(mu(3)-O)L(O)(4)TiCl(O(CH2)(4)Cl)](2)-2[TiCl(NCMe)(L(O)(3)(On-Pr))]center dot 11MeCN (8 center dot 11MeCN). The molecular structures of 2 center dot 2MeCN, 4 center dot 6MeCN, and 5 center dot MeCN together with the hydrolysis products {[TiL(O)(3)(OR)](2)(mu-Cl)(mu-OH)} (R = n-Pr 6 center dot 7MeCN; n-pentyl, 7 center dot 7.5MeCN, 9 center dot 9MeCN); R = n-decyl 10 center dot 8.5MeCN) and that of the ring opened product 8 center dot 11MeCN and the co-crystallized species [Ti-2(OH)Cl(L(O)(3)(OR))][L(OH)(2)(OR)(2)]center dot 2.85(C2H3N)center dot 0.43(H2O) (R = n-pentyl, 11 center dot 2.85(C2H3N)center dot 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 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 N-2 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 performed efficiently (>99% conversion) at 130 degrees C over 24 h both under N-2 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 degrees C was highly active (85% conversion). At 80 degrees C, the activity trend followed the order 5 approximate to 4 > 3 > 2 > 1. For delta-VL, at 80 degrees C the activity trend 5 approximate to 4 > 1 > 2 > 3 was observed. ROP of the larger omega-PDL was only possible using 5 at 130 degrees 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 epsilon-CL with delta-VL using 5, high activity was observed at 80 degrees C affording a polymer of molecular weight >23 000 Da and with equal incorporation of each monomer. In the case of epsilon-CL/r-LA co-polymerization using 5 either under N-2 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)(2)C6H2O](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.