Crosslinking photodimerization

One of the more interesting and successful of recent photopolymers is polyCvinyl 2-furylacrylate) (VIII). Its photosensitivity is reported to be more than an order of magnitude greater than that of II, whether sensitized or unsensitized samples are compared. The mechanism of crosslinking is proposed to involve photodimerization of pendant groups to cyclobutanes (12). 

Recent Photopolymers CrossIinked by Radical Processes. Polymers (XIV) containing the photoreactive acryloyl and methacry-oyl groups have been prepared by a cationic synthesis similar to that mentioned earlier (20). Photodimerization is not expected to play a role in crosslinking in these cases. 

Photodimerization of crystalline cinnamic acid proceeds with a constant 0 value. Photodimerization of PVCm in solution proceeds up to 100% conversion, albeit with smaller lvalues because of the low local concentration of reactive groups. In PVCm film 0 decreases in the course of the reaction, as indicated by the curves B and B. The reaction proceeds only to 50% conversion. This is a direct consequence of the heterogeneous distribution of microenvironments around the reactive sites and of the change in microenvironments as a result of crosslinks formation. 

Photocyclization is a particularly valuable route to the formation of cyclic compounds. There is a wide variety of photocyclization reactions reported in the literature of organic photochemistry, but relatively few of these have been carried out in solid polymers. The earliest reports concern the photodimerization of cinnamic acid derivatives, leading to crosslinking in solid polymers. These polymers have important applications as commercial photoresists. The chemistry has been reviewed by Delzenne (46) and Williams (47). 

On a different approach, poly(4-abietylmethylstyrene) was prepared in high yields by the reaction of poly (4-chloromethylstyrene) with sodium abietate. Near-UV irradiation of this novel polymer induced its crosslinking by photodimerization through the conjugated double bonds of the abietate moieties [110], presumably by the formation of dimeric structures similar to those described in Fig. 4.15. Abietic acid dimers were also used as diacids in the synthesis of polyamides by reaction with diamines [111]. 

The compound has functional groups that support dimerization type crosslinking and cationic polymerization upon UV exposure (A, = 300—360 nm).. Photodimerization of the chalcone-epoxy compound was confirmed by UV-visible and IR absorbance changes of the C=C double bond of the chalcone unit. Additions of small amounts of onium salts will also photoinitiate cationic polymerization of the epoxy groups present in the above chalcone-epoxy compound by exposine to UV. This ultra-violet light cured chalcone-epoxy compound was reported to possess excellent thermal stability and compares well with conventional UV-cured Bisphenol A type epoxy resins. (see Chapter 3)... 

Photocrosslinking may be carried out by exposure to UV radiation. Crosslinks are formed by 2 + 2 photodimerization between an excited cinnamoyl group with another in the ground state. It is expected that a sufficient number of these pairs will belong to the same or different molecules to form a crosslinked network. The intermolecular crosslinking reaction between the photosensitive chromophores is represented in Figure 3. 

Reactions in amorphous polymer solids are first characterized in terms of influences of molecular motion of matrix polymers and non-homogeneity of reaction sites. Specific features of photophysical processes, photoisomerization, photodimerization, chain scission and crosslinking reactions in polymer solids are then discussed separately. 

In this review article, after a description of the historical background of the present subject, some general features of reactions in amorphous polymer solids are summarized and then specific features of photo-physical processes, photoisomerization, photodimerization, chain scission, and crosslinking reactions are discussed separately. Typical theoretical treatments of solid-state reactions are reviewed in relation to the interpretation of experimental results. 

Incorporation of two photodimerization groups in a monomer molecule gives the possibility of preparing a linear polymer, and the binding of photodimerizable groups to the side chain of the polymer produces a crosslinked polymer which is practically important as a negative-type photoresist. 

This method is suited to large amounts of crosslinking and low swelling polymer gels. Examples are known of water soluble polymers, such as poly(vinyl alcohol) and poly(N-vinyl pyrrolidine), being photocrosslinked via diazo resins, bisazides, chromic acid, and photodimerization of polymers having photosensitivity, such as styrenebazolium salt, on the water soluble polymer main chain. 

Methods to introduce crosslinking between polymer molecules have been developed in the area of photosensitive resins. Although there are various photoreactions, photodimerization between side chain functional groups will be acceptable. 

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