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Abstract

The chemistry and photochemistry of [2+2] cyclo-addition reactions used in photocrosslinking is briefly outlined.

A spectroscopic investigation into 2- and 4-styrylpyridinium modified polyvinyl alcohol for use in the screen printing industry is undertaken. It is shown that insolubilisation of lithographic systems is due to "cyclobutane" dimer formation and not to a crosslinking process involving nucleophilic attack by pendant hydroxyl groups upon pendant styrylpyridinium groups.

The 4-styrylpyridinium modified polyvinyl alcohol crosslinks at a faster rate than the 2-substituted derivatives. Evidence is provided for the enhanced aggregation of the 4-substituted polymer over the 2-substituted polymer. It is postulated that the quaternised nitrogen interacts with the hydroxyl groups of the polymer backbone such that the 4-styrylpyridinium units can easily align themselves in the correct orientation for dimerisation. UV (diffuse reflectance) and excitation spectroscopy indicate that the cyclo-addition reaction occurs via excited charge transfer complex formation.

Simple ethylenic aldehydes can be grafted onto polyvinyl alcohol. Evidence is provided for a mixed cyclo-addition reaction occurring when these modified polyvinyl alcohol solutions are physically mixed with styrylpyridinium modified polyvinyl alcohol. Thus the photosensitivity of the solution can be maintained with less styrylpyridinium salt. Again an excited charge transfer complex mechanism is proposed.

A new way of linking the styrylpyridinium group to the polyvinyl alcohol is investigated. Attachment via the borate group capitalises upon the fact that the predominant repeat group in polyvinyl alcohol is a 1,3-diol, since the 6 membered cyclic borate ester is more stable than the 5- membered ring system. Furthermore, attachment via the borate group occurs under neutral to basic conditions thereby avoiding unwanted acid catalysed condensation of the polyvinyl alcohol which occurs when the complexes with the aldehyde group are made.

Publication Type:	Thesis (Doctoral)
Subjects:	Q Science > QD Chemistry
Departments:	Doctoral Theses School of Science & Technology School of Science & Technology > School of Science & Technology Doctoral Theses

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