Jean‐Alain Bousquet

Photocrosslinking of elastomer systems—III Ultraviolet light induced reactions in EPDM and EPR blended with or modified by grafting of derivatives of benzophenone

Abstract Photocrosslinking experiments were performed in elastomeric polymer films (EPR or EPDM) that either contained benzophenone or some of its derivatives, or had been previously modified (EPDM) by grafting of benzophenone. This latter modification was performed by a series of Friedel-Crafts reactions. The resulting materials were characterized by i.r. and u.v. spectroscopy in conjunction with physical property measurements. The results allowed relationships between the properties of the photosensitizers in their excited states and their efficiency in the crosslinking processes to be established. The results suggest that each coupled polymer-photoinitiator exhibits a particular behaviour. Thus it appeared that, in the presence of oxygen, the hexadiene branches of the terpolymer can undergo various types of reactions. Moreover, the grafting of the photosensitizer onto the elastomer backbone was shown to enhance considerably the crosslinking reaction. It is possible to conclude that oxygen plays a substantial role in the photocrosslinking. Indeed it always enhances crosslink formation. It can be assumed that the photooxidation of the polymer backbone yields alkoxy and hydroperoxy radicals which can act as crosslink sites.

Photo-oxidation of a random styrene-butadiene copolymer: Role of hydroperoxides and behaviour of butenyl segments

Abstract The photo-oxidation of a random SBR (styrene-butadiene rubber) was investigated quantitatively. The determination of quantum yields allowed elucidation of both the nature of oxidation reactions involved and their respective importance in the overall photo-oxidation process. The key role played by polymeric hydroperoxides was emphasized by clarifying their behaviour either as “active” groups, localized close to excited phenyl rings and prone to degradation after energy transfer, or as dispersed “inactive” groups remaining stable after their formation. Two consecutive reaction phases, distinguished by kinetic features, were observed. The first (photosensitized auto-oxidation: PAO) is governed by the auto-oxidation of butenyl segments arising from the propagation of macroperoxyradicals (POO • ) and the second (regularly maintained photosensitized oxidation: RMPO) depends mainly on photosensitized decomposition of “active” hydroperoxides. Participation of chromophoric oxidation products, especially unsaturated carbonyl functions, is also worthy of note. Finally, the mechanism of double bond epoxidation was shown to proceed through a radical addition of propagative POO • to unsaturated bonds.

Photochemistry of benzoin derivatives in an oxygenated polymeric medium

SummaryInvestigation of the photochemistry of benzoin derivatives in a polymer medium leads to the suggestion that oxygen play an important part in the first processes which occur under U.V. irradiation. This is of interest for the understanding of the photocrosslinking process.