Selective homopolymerization of cyclohexene oxide in presence of CO2 catalyzed by hexafluorophosphate salts

In the last forty years, CO2 has become the subject of study as a carbon source, being a non-toxic, renewable and easily available resource in large quantities and high purity. It has largely been shown that one of the ways to use carbon dioxide is in the alternating copolymerization with epoxides. Depending on the epoxide used, the resulting materials are compatible with a wide variety of applications such as ceramic binders, adhesives, coatings and packaging materials. In the frame of the research of novel cocatalysts for epoxide/CO2 copolymerization, recently it was shown that organic quaternary phosphonium salts can be used as novel cocatalyst in conjunction with chromium salen complexes. Highly efficient formation of polycarbonate with narrow distribution and molecular weights comparable or superior to those obtained with the well-known bis(triphenylphosphine)iminium (PPN+) salts was achieved with these salts. Nonetheless, the most common anions are usually Cl-, N3-, Br-, or OAc while hexafluorophosphate anions, largely used as highly efficient coupling agents for peptides, are commonly regarded as poorly coordinating ligands unable (or hardly able) to activate the catalyst in the coupling of epoxide with CO2. Notwithstanding their substantial inability to bind a five-coordinated metal center to give rise to the octahedral molecule active in the epoxide ring opening, the phosphonium hexafluorophosphates tested and shown in Figure 1 are able to initiate and carry on the homopolymerization of CHO to PCHO in almost every experimental condition. Several experiments, performed both in presence or absence of a metal Salen-type catalyst and/or under pressure, selectively led to the unique formation of pure polyether with molecular weight up to 10.000 g/mol and selectivity higher than 97% over the cyclic by-product, with similar properties despite the different used cations, with the cation-bound halide being reasonably disregarded as initiator. Such a behavior was not observed when propylene oxide was used as a monomer: a very modest catalytic activity in the copolymerization of PO and CO2 to poly(propylene carbonate) was detected, the conversion of propylene oxide did not exceed 20% , low selectivities with a medium-to-high content of ether linkage, ranging from 50% to 60% have been observed. FTIR/ATR, 1H-NMR, SEC and DSC studies on the afforded homopolymers were accomplished.