Economic pressure, environmental issues and ever increasing demand are driving the shift from oil-based polymers to those available from renewable resources. Ring opening polymerisation of cyclic esters is currently a topical field with metal complex-induced coordination/insertion type polymerisation leading the way. Such a polymerisation method offers a wide range of advantages from control over the polymer structure to kinetic enhancement; however industrially available catalysts based on tin suffer from inherent toxicity and as a result a policy change from the Food and Drug Administration (FDA) is not unrealistic. This review underlines the efforts made in the past five years or so, and shows how low toxicity metals are attracting increasing attention in the field of e-caprolactone polymerisation

Reaction of [V(X)(OR)3] (X=O, Np-tolyl; R=Et, nPr or tBu) with p-tert-butylhexahomotrioxacalix[3]areneH3, LH3, affords the air-stable complexes [{V(X)L}n] (X=O, n=1 (1); X=Np-tolyl, n=2 (2)). Alternatively, 1 is readily available either from interaction of [V(mes)3THF] with LH3, and subsequent oxidation with O2 or upon reaction of LLi3 with [VOCl3]. Reaction of [V(Np-tolyl)(OtBu)3] with 1,3-dimethylether-p-tert-butylcalix[4]areneH2, Cax(OMe)2(OH)2, afforded [{VO(OtBu)}2(mu-O)Cax(OMe)2(O)2].2 MeCN (42 MeCN), in which two vanadium atoms are bound to just one calix[4]arene ligand; the n-propoxide analogue of 4, namely [{VO(OnPr)}2(mu-O)Cax(OMe)2(O)2].1.5 MeCN (51.5 MeCN), has also been isolated from a similar reaction using [V(O)(OnPr)3]. Reaction of [VOCl3], LiOtBu, (Me3Si)2O and Cax(OMe)2(OH)2 gave [{VO(OtBu)Cax(OMe)2(O)2}2Li4O2].8 MeCN (68 MeCN), in which an Li4O4 cube (two of the oxygen atoms are derived from the calixarene ligands) is sandwiched between two Cax(OMe)2(O)2. The reaction between [V(Np-tolyl)(OtBu)3] and Cax(OMe)2(OH)2, afforded [V(Np-tolyl)(OtBu)2Cax(OMe)2(O)(OH)]5 MeCN (75 MeCN), in which two tert-butoxide groups remain bound to the tetrahedral vanadium atom, which itself is bound to the calix[4]arene through only one phenolic oxygen atom. Reaction of p-tert-butylcalix[4]areneH4, Cax(OH)4 and [V(Np-tolyl)(OnPr)3] led to loss of the imido group and formation of the dimeric complex [{VCax(O)4(NCMe)}2].6 MeCN (86 MeCN). Monomeric vanadyl oxo- and imidocalix[4]arene complexes [V(X)Cax(O)3(OMe)(NCMe)] (X=O (11), Np-tolyl (12)) were obtained by the reaction of the methylether-p-tert-butylcalix[4]areneH3, Cax(OMe)(OH)3, and [V(X)(OR)3] (R=Et or nPr). Vanadyl calix[4]arene fragments can be linked by the reaction of 2,6-bis(bromomethyl)pyridine with Cax(OH)4 and subsequent treatment with [VOCl3] to afford the complex [{VOCax(O)4}2(mu-2,6-(CH2)2C5H3N)].4 MeCN (134 MeCN). The compounds 1-13 have been structurally characterised by single-crystal X-ray diffraction. Upon activation with methylaluminoxane, these complexes displayed poor activities, however, the use of dimethylaluminium chloride and the reactivator ethyltrichloroacetate generates highly active, thermally stable catalysts for the conversion of ethylene to, at 25 degrees C, ultra-high-molecular-weight (>5, 500,000), linear polyethylene, whilst at higher temperature (80 degrees C), the molecular weight of the polyethylene drops to about 450,000. Using 1 and 2 at 25 degrees C for ethylene/propylene co-polymerisation (50:50 feed) leads to ultra-high-molecular-weight (>2,900,000) polymer with about 14.5 mol% propylene incorporation. The catalytic systems employing the methyleneoxa-bridged complexes 1 and 2 are an order of magnitude more active than the bimetallic complexes 5 and 13, which, in turn, are an order of magnitude more active than pro-catalysts 8, 11 and 12. These differences in activity are discussed in terms of the structures of each class of complex.

Two remote dialkylaluminium centres supported by a macrocyclic Schiff base ligand exhibited beneficial cooperative effects, whilst aluminoxane-type bonding proved to be detrimental to activity for the ring opening polymerisation of epsilon-caprolactone.

Treatment of the heterobimetallic iron(II) alkoxides [(THF)MFe(OtBu)(3)](2) with p-tert-butylcalix[4]areneH(4) (L(1)H(4)) affords the oxo-bridged diiron(III) complexes {Fe[M(NCMe)(x)](2)L(1)}(2)(mu-O), M = Na, x = 2 1 x 8(CH(3)CN), M = K, x = 3 2 x 3.5(CH(3)CN); similar use of p-tert-butylcalix[6]areneH(6) (L(2)H(6)) afforded [{Fe(2)(mu-O)Na(2)(OH(2))(NCMe)(2)L(2)}(2)][{Fe(2)(mu-O)Na(OH(2))(NCMe)(6)L(2)}(2)](2-)[Na(NCMe)(5)](2)(2+) 3 x 9.46(CH(3)CN) and [{Fe(2)(mu-O)L(2)(K(NCMe)(2))(2)}(2)] 4 x 10.8(MeCN), respectively. In the case of 4, a minor product {(L(2)(2)Fe(8)O(8))[K(NCMe)(1.5)K(H(2)O)(NCMe)(2.5)](2)} 5 x 6(CH(3)CN), which is comprised of chains of (L(2)(2)Fe(8)O(8)) clusters bridged by K/MeCN fragments, is also isolated. Use of p-tert-butylcalix[8]areneH(8) (L(3)H(8)) and two equivalents of [(THF)KFe(OtBu)(3)](2) affords [(K(2)(mu-NCCH(3))(4)(mu-OH(2)))(2)(Fe(2)(mu-O)L(3)H(2))(2)(CH(3)CN)(2)] 6 x 9(CH(3)CN). In the case of p-tert-butyltetrahomodioxacalix[6]areneH(6) (L(4)H(6)), reaction with [(THF)MFe(OtBu)(3)](2) (two equivalents) leads to isolation of the pseudoisomorphic complexes [M(2)(CH(3)CN)(4)L(4)Fe(2)(mu-O)] x 4 CH(3)CN M = Na 7 x 4(CH(3)CN), M = K 8 x 2(CH(3)CN); similar use of p-tert-butylhexahomotrioxacalix[3]areneH(3) (L(5)H(3)) led to [Na(2)Fe(2)(mu-OH)(2)(L(5))(2)(CH(3)CN)(4)] 9 x 2(CH(2)Cl(2)). The complex [L(4)(ZnEt)(4)Zn(2)(CH(3)CN)(4)(mu-OEt)(2)], 10 x 2(CH(3)CN), isolated from the reaction of L(4)H(6) and ZnEt(2) is also reported. Complexes 1-10 are structurally characterized (partially in the case of 4) and screened (not 5) as catalysts for the ring opening polymerization of epsilon-caprolactone.

The pro-catalysts [V(NAr)(L)(OR)] (Ar = p-tolyl, p-ClC(6)H(4), p-(OMe)C(6)H(4), p-(CF(3))C(6)H(4); R = t-Bu, i-Pr, n-Pr, Et, C(CH(3))(CF(3))(2)) have been prepared in good yields from the reaction of [V(NAr)(OR)(3)] and the bisphenol 2,2'-CH(3)CH[4,6-(t-Bu)(2)C(6)H(2)OH](2) (LH(2)). X-Ray crystal structure determinations for the Ar = p-tolyl, R = t-Bu (1), R = C(CH(3))(CF(3))(2) (2) and Ar = p-ClC(6)H(4), R = t-Bu (3) derivatives revealed monomeric complexes, whereas use of R = i-Pr, n-Pr or Et led to alkoxide-bridged dimeric structures of the form [V(NAr)(L)(mu-OR)](2) (R = i-Pr, Ar = p-tolyl (4), p-ClC(6)H(4) (5), p-(CF(3))C(6)H(4) (6), p-(OMe)C(6)H(4) (7); R = n-Pr, Ar = p-tolyl (8), p-(CF(3))C(6)H(4) (9); R = Et, Ar = p-ClC(6)H(4) (10), p-tolyl (11)). Complexes 1-11 yield highly active ethylene polymerisation catalysts when treated with DMAC (dimethylaluminium chloride) in the presence of ETA (ethyltrichloroacetate), with activities in the range 38,800 to 75,200 g mmol(-1) h(-1) bar(-1). The molecular weights of the resultant polymers were in the range 37,000 to 411,000 g mol(-1), with molecular weight distribution 2.2 to 4.7. The effect of the nature of the para-arylimido substituent and the alkoxide group OR upon the catalytic activity has been investigated. For epsilon-caprolactone polymerisation, mononuclear 1-3 exhibit low conversion (< or = 25%; 0% for 2), whereas use of the dimeric species 4-11 led to higher conversions (41-78%).