<p> The key objective of this research was to study the rates of the chemical cleavage of benzyl-silicon bonds in small model molecules, oligomers, polymers, copolymers, and crosslinked microspheres and microgels.</p> <p> Substrate species including benzyltrimethylsilane (BTMS), p-isopropylbenzyltrimethylsilane (ISO-BTMS), oligomeric and polymeric vinylbenzyltrimethylsilane (VBTMS), and their copolymers with styrene and methyl methacrylate as well as microspheres and microgels of bis(vinylbenzyl)dimethylsilane (BVBDMS) were synthesized using Grignard reaction, free radical polymerization, and precipitation polymerization.</p> <p> Narrow dispersed microspheres were synthesized from bis(vinylbenzyl)dimethylsilane (BVBDMS) by precipitation polymerization in acetonitrile. The reactivities of para/para, meta/meta, and meta/para isomers of BVBDMS in precipitation polymerization were found to be similar and to obey first-order kinetics. Their apparent rate of polymerization is comparable with that of meta and para divinylbenzene isomers under identical polymerization conditions. FT-IR analysis of BVBDMS microspheres shows that there are only few pendant double bonds in the particles. This is likely due to the similar reactivity of isolated double bonds of BVBDMS.</p> <p> Two nucleophilic (hydroxide and fluoride ion) and one oxidative (ceric ammonium nitrate) reagents have been used to cleave the benzylic-silicon bonds of the substrates. The cleavage reactions were quantitatively monitored by 1H-NMR / 29Si-NMR or FT-IR to derive the reaction kinetic parameters. The reaction behavior of most of the substrates differed from that expected based on the Flory's principle of equal reactivity.</p> <p> Among the hydroxide ion initiated cleavage reactions, the small molecules and the oligomeric analogs obeyed first-order kinetics, but the homopolymer and the copolymers deviated from first-order kinetics. This could be due to the low concentration of hydroxide ion in the polymer matrix, arising from the exclusion of polar hydroxide ion from the hydrophobic polymer matrix. The p-isopropylbenzyltrimethylsilane exhibited a lower pseudo first-order rate than benzyltrimethylsilane. This is attributed to an electron releasing substituent effect. Methyl methacrylate can accelerate the reaction on poly(vinylbenzyltrimethylsilane-co-methyl methacrylate) by increasing the overall copolymer polarity. The reverse is true for the corresponding styrene copolymers owing to the steric hindrance offered by the phenyl ring of styrene, and the enhanced hydrophobic repulsion against the access of hydroxide ion into the polymer matrix. An electrophilically assisted process was proposed as a principal reaction mechanism for this cleavage reaction. It was found that the only nucleophile attacking on silicon would be the hydroxide ion in KOH/EtOH/THF promoted reactions.</p> <p> Fluoride ion initiated cleavage reactions of substrates containing benzylic-silicon bonds were found to follow first-order kinetics. The reaction on small molecules was not studied due to their very rapid reaction at room temperature. The homopolymer of vinylbenzyltrimethylsilane exhibited a higher rate of reaction than the corresponding oligomer. However, the change in the reaction rate within a copolymer series, differing in molecular weight and composition, was not significant. Poly(VBTMS-co-MMA) exhibited a rate higher than that of styrene copolymers for polarity or steric reasons.</p> <p> Oxidative cleavage of benzylic-silicon bond by ceric ammonium nitrate (CAN) was found to obey first-order kinetics at 1:3 substrate to cerium(IV) ratio, and did not show any deviation in reaction order even at higher CAN concentration. The electron releasing isopropyl group reduces the oxidation potential of p-isopropylbenzyltrimethylsilane (ISO-BTMS), resulting in an enhanced reaction rate compared to benzyltrimethylsilane (BTMS). This rate accelerating substituent effect, together with a much higher negative value (-5.4) of Hammett reaction constant p is in accordance with the radical-cation mechanism operating in ceric ammonium nitrate promoted oxidation reactions. Significant loss of silane functionality was observed in reactions with polymeric substrates. This is attributed to the benzylic radical coupling reactions. The possibility of polymer backbone cleavage is ruled out for the following reasons: A) lack of significant molecular weight reduction in the oxidation products of polymeric substrates. B) about 100 times easier breaking of benzylic-silicon bond as trimethylsilyl cation than a hydrogen from carbon as proton, and the steric congestion offered by the polymer chain favors the benzylic radical formation only at the primary carbon, not on tertiary methine (C-H) on the chain.</p> / Thesis / Master of Science (MSc)
| Identifer | oai:union.ndltd.org:mcmaster.ca/oai:macsphere.mcmaster.ca:11375/19305 |
| Date | 01 1900 |
| Creators | Kavalakatt, Pauly |
| Contributors | Stöver, H. D. H., Chemistry |
| Source Sets | McMaster University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis |
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