Synthesis of oligo (vinyl ether)s in ab initio cationic polymerisation
Terminally functionalised oligo(vinyl ether)s were produced in ab initio cationic polymerisations. Various polymerisations and chain end functionalisation systems were investigated. MALDI-TOF mass spectrometry was applied to analyse the obtained oligomers and thus the polymerisation and chain end functionalisation process. Oligo(isobutyl vinyl ether), also oligo(ethyl vinyl ether) and oligo(methyl vinyl ether) were synthesised in cationic polymerisation. Silyl enol ethers were added to the polymerisation as end-capping agents before initiations and they compete with monomer to cap the carbocationic chain end. The methodology relies on a comparable end capping rate to chain propagation rate so that oligomers can still be produced in the presence of reactive end-capping agents whilst other side reactions are suppressed. Polymerisation temperatures investigated range from -78°C to the room temperature (21 °C), 4 out of 6 silyl enol ethers were applied and are proved to be reactive in the end-capping. Initiation systems investigated include iBVEHCl/Yb(OTf)3 and iBVE-HCl/SnC14 and both produced oligomers with high chain end functionalities. Generally silyl enol ether functionalised oligo(vinyl ether)s have lower molecular weights and broader molecular weight distributions than their identical control polymerisations without end-capping. The iBVE-HCl/SnCI4 initiation system produced functionalised oligomers with narrower molecular weight distribution than iBVE-HCl/Yb(OTf)3 initiation system. Different silyl enol ether reactivities in end-capping were observed. It was also observed that ab initio chain end functionalisation by reactive silyl enol ethers largely suppressed the majority of side reactions during polymerisation. This suppression was attributed to the higher rates of end-capping than side reaction rates. When (1-tert-butyl-vinyloxy)trimethyl-silane was applied as end-capping agent, the polymerisation system produced oligomers with narrower molecular weight distributions than the control polymerisations without end-capping while the chain end functionalities were also obtained. This indicates the possibility of setting up a controlled ab initio chain end functionalisation cationic polymerisation system in which the oligomer's molecular weight, polydispersity and chain end functionality can be regulated at the same time. MALDI-TOF MS, NMR and SEC are mainly applied in the oligomer characterisation. Side reactions in this polymerisation system were examined from these analyses. Under less critical polymerisation conditions 7 different chain ends from side reactions are observed in MALDI-TOF mass spectra. Based on the MS and NMR observation the various chain end structures are identified. Side reactions are also postulated which mainly include ß-proton elimination, water capping of the carbocationic chain end and combinations of these. Sample preparation for MALDI-TOF MS analysis of oligo(vinyl ether)s is investigated. Direct laser desorption of oligo(isobutyl vinyl ether) was observed for the samples of molecular weight of up to 2k Daltons. Complementary information of oligo(isobutyl vinyl ether)s obtained from ESI MS indicates a serious mass discrimination in MALDI-TOF MS technique and thus it is regarded that MALDITOF MS can not provide reliable molecular weight distributions for polymers with broad molecular weight distributions. Combination of SEC and MALDI-TOF MS to calibrate SEC columns and quantitative application of MALDI-TOF MS to analyse the oligomers' chain end functionalities were also explored.