Modification of polydienes via homogenous catalysis
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Our investigations into the synthesis of polyketones have involved the catalytic functionalisation of polydienes with particular emphasis on routes via a polyepoxide and a polyalcohol. Initially, a route via the polyepoxide was studied, in which polybutadiene (various micro structures and molecular weights) and polyisoprene were epoxidised (equation 1) in high conversion using a [MoO₂CI₂ [(lR)-endo]-(+)-3(diethoxyphosphoryl) camphor] catalyst (MoCI₂O₂L) and subsequently isomerised to the polyketone using lithium bromide [(equation 2), see Scheme 1 in thesis]. Lithium bromide requires a solublising agent in reactions carried out in dichloromethane, however this is not the case for reactions carried out in tetrahydrofuran as lithium bromide is readily dissolved in this solvent. (See abstract in full thesis for equation). Polybutadiene and polyisoprene can be easily converted to their respective polyepoxide. The backbone double bonds of the polydienes show from ¹H nmr 100% conversion to the epoxide with greater than 95% selectivity. The polyepoxides are easily purified after the removal of HCl (produced by the catalyst) as this causes crosslinking. Subsequent isomerisation of the purified poly epoxides show from ¹³C and ir that characteristic ketone units can be formed along with the ketal resonances attributed to brevicomin type moeties, formed from the reaction of a ketone with a neighbouring epoxide group. Problems with the insolubility of the polyketone led to the preparation of a polyalcohol, via hydroboration using 9-borabicyclononane (equation 3) followed by oxidation using an alkaline solution of hydrogen peroxide (equation 4). The attempted dehydrogenation of the polyalcohol to give the resultant polyketone using various oxidants was studied, see Scheme 2. Conversion of the polybutadiene to the alcohol via hydroboration-oxidation using 9-BBN/NaOH-H₂O₂ showed by ¹H and ¹³C nmr a 50% conversion of the backbone double bonds at a low temperature and 100% conversion at a high temperature. The microstructure of the resultant polyalcohol showed 1,4: 1,5: 1,6 segments in a 1:2:1 ratio, these different segment impart irregularity into the polymer chain and could consequently be carried through to the polyketone giving rise to lower crystallinity and a more processable polymer. Although various oxidants has been investigated for the dehydrogenation of the polyalcohol a suitable oxidant has not yet been found.
Thesis, PhD Doctor of Philosophy
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