Co‐polymerization of propylene oxide and CO₂ using early transition metal (groups IV and V) metallocalix[n]arenes (n = 4, 6, 8)

Abstract

A number of metallocalix[n]arenes, where n = 4, 6, or 8, of titanium and vanadium have been screened for their ability to act as catalysts for the co‐polymerization of propylene oxide and CO2 to form cyclic/polycarbonates. The vanadium‐containing catalysts, namely [VO(L1Me)] (1), [(VO2)L2H6] (2), [Na(NCMe)6]2[(Na[VO]4L2)(Na(NCMe))3]2 (3), [VO(μ‐OH)L3/H2]2∙6CH2Cl2 (4), {[VO]2(μ‐O)L4[Na(NCMe)2]2} (5), {[V(Np‐tolyl)]2L4} (6) and [V(Np‐RC6H4)Cl3] (R = Cl (7), OMe (8), CF3 (9)), where L1H3 = methylether‐p‐tert‐butylcalix[4]areneH3, L2H8 = p‐tert‐butylcalix[8]areneH8, L3H4 = p‐tert‐butylthiacalix[4]areneH4, L4H6 = p‐tert‐butyltetrahomodioxacalix[6]areneH6, performed poorly, affording, in the majority of cases, TONs less than 1 at 90°C over 6 h and low molecular weight oligomeric products (Mn ≤ 1665). In the case of the titanocalix[8]arenes, {(TiX)2[TiX(NCMe)]2(μ3‐O)2(L2)} (X = Cl (10), Br (11), I (12)), which all adopt a similar, ladder‐type structure, the activity under the same conditions is somewhat higher (TONs >6) and follows the trend Cl > Br > I; by comparison the non‐calixarene species [TiCl4(THF)2] was virtually inactive. In the case of 10, it was observed that the use of PPNCl (bis[triphenylphosphine]iminium chloride) as co‐catalyst significantly improved both the polymer yield and molecular weight (Mn 3515). The molecular structures of the complexes [HNEt3]2[VO(μ‐O)L3H2]2∙3CH2Cl2 (4∙3CH2Cl2), [VO(μ‐OH)L3/H2]2∙6CH2Cl2 (4/) (where L3/H2 is a partially oxidized form of L3H4) and {(TiCl)2[TiCl(NCMe)]2(μ3‐O)2(L2)}·6.5MeCN (10·6.5MeCN) are reported.

Metallocalix[n]arenes in catalysis: A 13-year update

“…In particular, the tetranucluear complexes were found completely inactive, regardless of the halide ancillary ligand, suggesting that for this particular catalyst system, the presence of multiple metal centers might be detrimental for the polymerization process. These tetrametallic species have also been tested in the ring opening co-polymerization of CO 2 and propylene oxide to afford cyclic polycarbonates [36]. Turn Over Numbers spanning from 2 to 6 g/g cat were observed at 90 °C at a CO 2 pressure of 6 MPa after 5 h; the catalyst activity proved dependent on the nature of the halide ligand (Cl > Br > I) present.…”

“…None of the compounds proved active in the ROP of r-LA. Finally, complex 41 was also poorly active in the synthesis of cyclic polycarbonates by the ring opening co-polymerization of CO 2 with propylene oxide [36].…”

Metallocene catalysts for the ring-opening co-polymerisation of epoxides and cyclic anhydrides

“…39–41 However, there are only limited reports of the Group 4 metals (Ti, Zr, Hf) being used in this transformation; perhaps surprising given there are multiple studies detailing their efficacy in the ROP of cyclic esters 42–46 and the ROCOP of epoxides and CO 2 . 47–55 In 1999, Endo and co-workers reported the use of titanium alkoxide catalysts in the ROCOP of oxetane and anhydrides, whilst heterogeneous zirconium phosphate materials with intercalated amines have also been demonstrated to catalyse the ROCOP of glycidyl phenyl ether and hexahydro-4-methylphthalic anhydride. 56–59 Furthermore, Chakraborty, Chand and colleagues have detailed the use of 8-hydroxyquinolinate based ligands complexed to titanium and zirconium in the copolymerisation of maleic anhydride and tert -butyl glycidyl ether.…”