Vinylation 2+2 photodimerization

Photosensitized Reactions for Polymers. The economic and technical features for photocross-linking, photosolubilizafion, and photopolymerization reactions have been reviewed (55). The widely used poly(vinyl ciunamates) (PVCN) photocross-link by a photodimerization reaction. 

An obvious extension of the studies on photodimerization of crystalline olefins is to solid-state vinyl polymerization (with light, if absorbed, or y-irradiation), with the aim of achieving stereoregular polymers. In fact, an immense effort has been made in this direction, but with singular lack of success. The explanation is that, for various reasons, the lattice in the vicinity of the chain front becomes progressively more damaged as polymerization proceeds, so that after relatively few steps there is loss of stereochemical control. 

In contrast to these ring openings, only ring closure could be observed in the photodimerization of phenyl vinyl ether, Eq. (40), which proceeded only in one direction... 

The photocyclodimerization of JV-vinylcarbazole, which was reported by Ledwith and Shirota, can be accounted for by this mechanism [71-73]. A chain process is involved in this photoreaction, and the quantum yield exceeds unity (the maximum quantum yield is 66). The hole transfer from the cyclobutane radical cation to a neutral JV-vinylcarbazole is a key reaction for the chain process. Similar photodimerizations of electron-rich alkenes such as aryl vinyl ethers [74-76], indenes [29, 77, 78], styrenes [79-80] and enamines [71] have been reported by several groups. The DCA-sensitized photodimerization of phenyl vinyl ether gives cis- and trans-1,2-diphenoxycyclobutanes. This photoreaction also involves a chain process although the chain length is short [75]. 

The 1,4-dicyanonaphthalene-sensitized photocyclodimerization of the vinyl ethers (192), yielding (193) has been described. 9,10-Dicyanoanthracene-sensitized dimerization of cyclohexa-1,3-diene affords the two [4+2] adducts (194) and (195) in a total yield of 60% and in a ratio of 4 1. This is to be contrasted with the previous report of cyclohexadiene dimerization where [2 + 2] dimers were also obtained." A detailed study of the photodimerization of some acenaphthylenes (196) has been reported. " The structures and configurations of the products have been elucidated by spectroscopic methods." ... 

Ring formation takes place in the photodimerization of unsaturated ethers and amines, such as phenyl vinyl ether and vinyl carbazole (Eq. 32). 

Photocross I inking of poly(vinyl-p-azidocinnamate) (XIII) is reported to involve competitive photodimerization of cinnamoyl groups to cyclobutenes along with the dominant nitrene reactions of insertion and radical abstraction. The reaction can be triplet-sensitized (19). 

Novel vinyl liquid crystalline (l.c.) polymers were synthesized with the UV-sensitive p-methoxycinnamate chromophore incorporated into the side chain of the polymers. The objective of this synthesis was to determine if a molecularly organized environment could influence the yield of a chemical reaction in the solid state. The investigation into the photochemical and physical processes of these thin films revealed that the photodimerization of the p-methoxycinnamate moieties was very sensitive to their geometrical arrangement in the polymer matrix. The relative quantum yield of cyclobutane formation increased by a factor of approximately 8 for the l.c. p-methoxycinnamate film compared to its amorphous analog. This quantum yield approaches the theoretical limit for this system. 

A comparison of the photosensitivity of poly(vinyl cinnamate) with the photodimerization of cinnamic acid in poly(vinyl alcohol) coatings was carried out by Nakamura and Kikuchi 44>. Cinnamic acid in polyvinyl alcohol) coatings was found to undergo photoisomerization as well as photodimerization. The ultraviolet absorption spectra used to follow the course of the reaction showed one isosbestic point together with other nonisosbestic curve intersections. Such complex curve changes indicated that photoisomerization and at least one other reaction, photodimerization, was occurring. 

One of the more interesting and successful of recent photopolymers is poly(vinyl 2-fury I aery late) (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 cross I inking is proposed to involve photodimerization of pendant groups to cyclobutanes (12). 

The changes in quantum yield, 0, for the photodimerization of poly(vinyl cinnamate) (PVCm) (1) in films are shown in Table 2.5(3) together with those for the same reaction carried out in solution and in the crystalline phase.7 ... 

Hayashi has investigated in some detail the ionic photopolymerization of styrene monomers. Free ion lifetimes measured by pulse electrical conductivity measurements were found to agree with those calculated from steady-state conductance measurements. Other studies of interest on radical addition polymerization include the photodimerization of polymers containing thymine bases, diene polymerization by terbium complexes, polymerization of vinyl acetate, and preparation of light-sensitive polyacrylates. 

Most of the papers in this section are concerned with photo-oxidation reactions, but a few papers refer to photochemical reactions in polymer matrices. Thus the kinetics of the photo-oxidation of anthracene and naphthacene in solid polystyrene (PS),170 the photoionization of aromatic hydrocarbons dissolved in PMMA and PS,171 the photoreactions of naphthalene in cellulose triacetate,172 the cis-trans isomerization of stilbene residues in the side-chains of polymers,173 intrachain photodimerization in polymers,174 photoisomerization of 1,2-diphenyl-cyclopropane by peptides containing naphthalene in a side-chain,176 and photochemical transformations of poly(vinyl p-azidobenzoate)176 have been reported. 

CdS [Eq. (7)] [107-109]. The ratio of trans- to cis- products is determined by the thermodynamic stability of the two dimers. In another example, CdS-me-diated photodimerization of A/ -vinyl carbazole leads to traris-cyclobutane dimer exclusively [Eq. (8)] [110]. 

The photosensitivity of poly(vinyl cinnamate) derives from the reactivity of the einnamoyl group. Cinnamic acid is a naffirally occurring compound (found in oil of cinnamon, from which it derives its name) and is readily synthesized. Its photosensitivity has long been known, though little use was made of this property until Mink and co-workers, while reviewing the literature in search of a potential cross-linking reaction, found it to be the only solid state photodimerization that was known at the... 

The photodimerization reaetion concept was thus adapted hy Mink and co-workers in the design of a poly(vinyl cinnamate) resist system. Cross-links are formed hy photoaddition between an excited ( ) cinnamoyl group of one chain with a ground state cinnamoyl group belonging to another chain (Scheme 6.3). ... 

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