Catalytic ring-opening polymerization of epoxides, anhydrides and CO2 : towards polyesters and poly(ester-co-carbonate)s

Increasing oil prices, rising environmental concerns and facing increasing interest in possibly unprecedented properties of bio-based polymer products have raised the attention for exploring the opportunities of manufacturing materials from bio-based monomers and through new olymerization pathways. There are several methods known for polyester synthesis, e.g. polycondensation polymerization which suffers from side reactions and requires long reaction times and high temperatures in order to reach high conversions necessary to produce polymers with high molecular weights. On the other hand, catalytic ring-opening polymerization (cROP) of lactones or copolymerization of epoxides plus anhydrides provides alternative routes for producing AB and AABB type of polyesters, respectively. The major advantage of ROP is that it can be carried out under milder conditions (shorter reaction times and lower temperatures), which is not only beneficial with respect to avoiding thermally induced side reactions but which is also attractive from an energy consumption point of view and provides excellent control over the polymer microstructure. The cROP technique provides the opportunity to synthesize various well-defined polymer architectures due to the broad range of monomers available. Although the polyesters prepared by the cROP technique have lower molecular weights than the values calculated from the applied monomer/catalyst ratios, the low PDI values along with the linearity observed for number average molecular weights development versus conversion enable us to synthesize for example block copolymers. One has to keep in mind that this cROP of epoxides, anhydrides and CO2 is not truly living, but the occurring chain transfer reactions are much faster than the propagation reactions, which explains the earlier mentioned relationship between average molecular weights and monomer conversion. The goals of this thesis were to synthesize polyesters and poly(ester-co-carbonate)s via a catalytic route, investigating the catalytic activity of different catalysts, such as metal salen and metal porphyrin complexes and bis (phenoxy) zinc species in this type of polymerization, studying activities of both different petrochemical and bio-based monomers in this type of polymerization, studying the role and effect of chain transfer agents in the epoxide/anhydride cROP and full characterization of the obtained polymers. Ring-opening co- and terpolymerization of cyclohexene oxide (CHO) with CO2 and various anhydrides using salophen and TPP chromium catalysts resulted in polyesters and poly(ester-co-carbonate)s. Intensive MALDI-ToF-MS analysis further explained the polymers’ chain termini and topology. As the ligand backbone structure plays a significant role in the salen catalyst activity, various metal salen catalysts were designed and the effect of different diimine backbone structure of the salen ligand and/or the metal center on the cROP of CHO and various anhydrides was investigated. As already reported for similar epoxide/CO2 polymerizations, the presence of a cocatalyst has a significant effect on the catalyst’s activity and selectivity. Therefore the role and the required amount of various types of cocatalysts, such as N-heterocyclic amines, phosphines and bis(triphenylphosphoranylidene)ammonium chloride, were investigated. Regarding the high activity of metal salen complexes in CHO/anhydride cROP and considering the similarities between CHO and styrene oxide (SO) as monomers, the cROP of SO with different metal salen and porphyrin catalysts as well as the effect of different cocatalysts were the next focus in this study. In addition to the alicyclic anhydrides, two extra anhydrides, namely the unsaturated maleic anhydride and the bio-based citraconic anhydride, were investigated using this system. It was shown that the copolymerization reactions of these two monomers with PA are highly dependent on the temperature, time, type of cocatalyst and solvent used. The exciting renewable epoxide limonene oxide (LO) was the next in line to be investigated in the cROP with anhydrides, utilizing metal salen complexes, and showed high activities of certain metal salen complexes for LO/anhydride copolymerization. Considering dihydroxy end-capped polyesters as a requirement for coating resin applications, the role of difunctional chain transfer agents (CTAs) was also investigated in this system and it turned out that the polymer molecular weights can be tuned in the presence of these CTAs. The salen chromium catalyst proved to be one of the most stable catalysts in the presence of such CTAs. A different type of catalyst, viz. a bis (phenoxy) zinc complex which produces not only poly(ester-co-ether)s but also in some cases can result in pure polyesters depending on the conditions, was also applied in this polymerization system. Although there is still a long path to go before fully bio-based polymeric materials can be produced in an industrially feasible way via this catalytic route, cROP of epoxides/anhydrides definitely has potential and the initial results are quite promising and might very likely generate more inspiration along the way