The Thermal and Photochemical Degradation of Poly-N-Butyl Methacrylate

MacCallum, James Richard (1961) The Thermal and Photochemical Degradation of Poly-N-Butyl Methacrylate. PhD thesis, University of Glasgow.

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An investigation of the thermal and photochemical degradation behaviour of poly-n-butyl methacrylate is described in this thesis. The object of the work was to evaluate the velocity coefficients of the various steps in the depolymerisation process with a view to extending kinetic measurements on a depolymerising methacrylate ester as a further method, in addition to polymerisation and copolymerisation, for comparing radical reactivity. The thermal and photochemical depolymerisation of polymethyl methacrylate has already been studied, A study of the pyrolysis of poly-n-butyl methacrylate is dealt with. It is shown how the original objective was baulked by the occurrence of an ester decomposition reaction within the n-butyl ester side group. The overall effect of this reaction is to introduce anhydride units into the polymer molecule, which inhibit the depolymerisation process. The following mechanism is proposed for the inhibitive action of the anhydride group. The extra link to the main polymer chain, via the anhydride function, prevents fast removal of the monomer unit from the vicinity of the depropagating radical and a pseudo equilibrium is set up, preventing further depropagation and stabilising the radical so that it can eventually react with another radical. Further evidence for this mechanism is obtained from a series of degradation experiments involving copolymers of methacrylic acid and n-butyl methacrylate. A previous observation that bubbles of gas were evolved during thermal polymerisation of n-butyl methacrylate is followed up and an ester decomposition proceeding by a radical mechanism postulated to account for this reaction. This mechanism is extended to the thermal degradation and tentatively used to account for the large effect which only a few per cent of ester decomposition has on the depolymerisation process. The photochemical degradation of poly-n-butyl methacrylate is more intensively studied because the reaction is found to involve pure depolymerisation. The mechanism of break down is in general very similar to that of polymethyl methacrylate, involving a chain reaction initiated at the ends of the polymer molecule, followed by unzipping of monomer units until mutual termination takes place. One of the main features of this work is the discovery that the initiator fragments incorporated at the polymer chain end seems to be the centres of initiation. The catalyst used in the preparation of the polymers is azoisobutyronitrile giving the fragment as the terminal unit. The stabilising effect of anhydride structures on the photochemical degradation is shown by using copolymers of methacrylic acid and poly-n-butyl methacrylate. Kinetic analysis of the degradation, both thermal and photochemical, of poly-n-butyl methacrylate is seriously complicated by the measured rate of depolymerisation being proportional to the square root of the weight of polymer sample taken. Possible explanations for this relationship are dealt with, but no completely satisfactory answer can be suggested.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Keywords: Physical chemistry, Polymer chemistry
Date of Award: 1961
Depositing User: Enlighten Team
Unique ID: glathesis:1961-79416
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 05 Mar 2020 10:01
Last Modified: 05 Mar 2020 10:01

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