Synthesis of functional stimulus responsive oligovinyl ethers and towards highly branched polymers using ab initio cationic polymerisation.
Studing for a doctor of philosophy (PhD) at the University of Sheffield under Prof. S. Rimmer [2012 – 2015]
Oligo(vinyl ethers) with functionalised end groups were produced through the alkylation of silyl enol ethers in ab initio cationic polymerisations. In order to obtain high degrees of chain end functionality a great deal of care was taken in choosing the monomer concentration, the reaction temperature, Lewis acids and the concentration of silyl enol ether. The experiment also required the use of highly nucleophilic species such as silyl ketene acetal, which react with carbocations at similar rates to vinyl ethers. Rapid elimination of the primary adduct produced the ester end-groups. MALDI-TOF mass spectrometry was used to analyse the oligomers obtained and to assess the side reactions and chain end functionalisation processes. Oligo(methyl vinyl ether)s were synthesised in cationic polymerisations using the HCl-isobutyl vinyl ether/SnCl4 initiating system within the polymerisation temperature range -26 °C to -78 °C, and in the absence of an end-capping agent. It was observed that the use of a lower temperature yielded well-defined oligomers. Furthermore, during the polymerisation the silyl enol ether reactivity suppressed the termination rate and oligomethyl vinyl ether with increased levels of ester end groups were then produced. In addition, a size exclusion chromatograph was calibrated using MALDI-TOF MS to obtain molecular weights of narrow fractions (SEC-MALDI TOF). The Kuhn-Mark-Houwink relation for oligo(methyl vinyl ether) was then established. In the light of the above, it was possible to infer that the solution coil conformation is dependent on the end group structure. Oligo(isobutyl vinyl ethers) were also prepared successfully at different temperatures (-26 and -78 °C) in the presence of a silyl enol ether using HCl-iBVE/TiCl4 as the initiating system. At a lower temperature, and with a high concentration of silyl enol ether, the normal termination reactions were suppressed and oligo(isobutyl vinyl ethers) with a high ester group chain end functionality were produced. This phenomenon can be attributed to the fact that, at lower reaction temperatures, the rate of propagation relative to the rate of termination and the control of polymerisation was observed. Another means by which oligo(isobutyl vinyl ethers) were prepared in this experiment with ester end groups was by using continuous reaction processes. These processes are commonly applied in many industrial applications as they allow for the production of large quantities of polymer in a short period of time. These particular polymerisations were applied by using HCl-iBVE as an initiating system with SnCl4 and TiCl4 as Lewis acids at -15 °C. The production of oligomers with a high degree of ester group functionality was carried out by the use of SnCl4 with a high concentration of silyl enol ether at a slow flow rate. It was also possible to produce highly branched functional polymers by using continuous processes. In this instance, isobutyl vinyl ether was used as a first monomer with diethyl divinyl ether (DEDVE) and divinyl benzene (DVBz) being used as second monomers in the presence of silyl enol ether at a temperature of -15 °C. Many different Lewis acids were used in these polymerisation reactions It was observed that the polymerisation of iBVE with DEDVE was only successful by using Sc(OTf)3 as a Lewis acid. Highly branched polymers with ester group functionality were detected at a monomer ratio of 9:1 with a low concentration of silyl enol ether at a high feed rate. In the polymerisation of iBVE with DVBz, only two Lewis acids successfully aided these reactions (ZnI2/HI and Sc(OTf)3). In ZnI2/HI, a low concentration of silyl enol ether with a high flow time was found to form highly branched polymers with the chain end group. In using Sc(OTf)3, a high concentration of SEE with a low feed rate was shown to produce highly branched functional polymers. By analysing the aforementioned information it was possible to deduce that the feed rate and the concentration of silyl enol ether had a substantial effect on the chain end attained.
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