Poly , photocrosslinking

Bryant, S.J. and Anseth, K.A., The effects of scaffold thickness on tissue engineered cartilage in photocrosslinked poly(ethylene oxide) hydrogels. Biomaterials, 22, 619-626, 2001. 

Photocrosslinking. The second class of photopolymer chemistry that is used in some commercial products is based on the reaction of unsaturated moieties attached to an organic polymer. These photopolymer materials include the [2+2] cycloaddition of the ethylenic groups in poly(vinyl cinnamate) polymers and in the newer styryl pyridinium (10) and thiazolium (77) derivatives of poly(vinyl alcohol). The main advantage of this chemistry is that, unlike free-radical photopolymerization, they are insensitive to the presence of oxygen. This photopolymer mechanism is principally used in applications employing a washout development process (e.g. resists). 

The use of phenolic polymers in photocrosslinkable systems usually involves multicomponent systems which incorporate polyfunctional low molecular weight crosslinkers. For example, Feely et al. [9] have used hydroxymethyl melamine in combination with a photoactive diazonaphthoquinone which produces an indene carboxylic acid upon irradiation to crosslink a novolac resin. Similarly, Iwayanagi et al. [10] have used photoactive bisazides in combination with poly(p-hydroxy-sty-rene) to afford a negative-tone resist material which does not swell upon development in aqueous base. 

Reaction of poly(vinyl alcohol) with cinnamoyl chloride yields polymers that produce photocrosslinked resists for microelectronic applications. 

EDTA in water. Ru(bpy)2 + and its dicinnamate were immobilized in a membrane prepared from cinnamate of poly(vinyl alcohol) by photocrosslinking 51), and the immobilized complex sensitized the photoreduction of MV2 +. ... 

It has been found [105,106] in particular that the reaction product [poly (BMEGMA-co-MMA)] derived from glycidyl methacrylate/methyl methacrylate copolymers [poly(GMA-co-MMA)] and a-(2-carboxyethyl)benzoin methyl ether (CEBME) (Scheme 26), although less efficient than CEBME alone in the UV initiated polymerization of styrene, exhibits a markedly enhanced photocrosslinking activity. 

Introduction of these photocrosslinkable structures in macro-molecular chains can be performed by esterification of hydroxyla-ted polymers with cinnamoyl chloride. Cellulose Q).condensation products (4, ) and mainly poly(vinyl alcohol) have Been treated( by this method. Other chemical modifications have been studied as ester interchange of poly(vinyl acetate) 7) and Knoevenagel reaction on polyesters (8). Very few results on the synthesis of such photocrosslinkable polymers by polymerization have been reported. Therefore free radical polymerization of cinnamic acid vinyl derivatives did not lead to the expected polymers, but to insolubilization reactions. Howewer cationic procedure can be a good way in some cases since Kato et al. could polymerize by this way with high yields p-vinyl phenylcinnamate (9) and B-vinyloxyethyl cinnamate (10). 

The increasing order of photocrosslinking efficienties of polymers is nearly the same for each type of irradiation. Poly 2 is the most sensitive polymer with a low presure lamp since in THF solution, 92% of polymer is precipitated in one minute. To obtain the same result with Poly 1, it is necessary to wait for five times more, while this result is not reached after 7 minutes with Poly 3 solution. These results are in agreement with the substitution degrees of the cinnamic double bond on one hand and with absorption characteristics of Poly-2 X max = 312 nm), Poly-1 (290 nm) and Poly-3 (288,5 nm) on the other hand. 

By photocrosslinkable polymers are meant macromolecules which bear photoreactive groups capable of forming crosslinks between their respective chains. A typical photocrosslinkable polymer is poly(vinyl cinnamate) 6>. The photocrosslinking occurs when a network of crosslinks is built up between adjacent chains of poly (vinyl cinnamate). The mechanism of crosslinking will be described in detail in a later section. 

Robertson, VanDeusen and Minsk 33> prepared poly(vinyl cinnamate) and found that incorporation of sensitizers such as Michler s ketone into its coatings provided a means of optical sensitization. Optical sensitization decreased the required exposure time to give imagewise photocrosslinking of poly(vinyl cinnamate). 

Extension of the use of the cinnamoyl unit to photocrosslinkable polymers is found in the numerous chalcone-type polymers prepared by Unruh and Smith 34>. Typical is a group of chalcone polymers which were prepared by reaction of poly(4-vinylacetophenone) with aromatic aldehydes such as benzaldehyde. 

Another quite different group of photocrosslinkable polymers synthesized by Merrill and Unruh 35> was based on the azide group. In this case, an azide group usually attached to an aromatic nucleus is appended to, or incorporated within, a polymer chain. Typical is poly(4-azidosty-rene). 

Minsk, et al. 6-33i, discovered the feasibility of extension of the spectral response of insolubilization of poly (vinyl acetate cinnamate). Several other investigations have extended the group of addenda which provide spectral extension of the photocrosslinking of poIy(vinyl acetate cinnamate) 40,4i,42,43,44 Most striking is the spectral extension obtained when poly(vinyl acetate benzoate cinnamylidenacetate) 45> is sensitized with various pyrylium salts as shown in Fig. 7. 

The photocrosslinking of poly(vinyl cinnamate) was assumed by Minsk to involve reactions which led to the disappearance of the double bond. A number of studies have confirmed that ultraviolet irradiation of poly(vinyl cinnamate) causes diminution of the value of absorbance in the region characteristic of the cinnamate ester group. Typical experiments involved the irradiation of coatings of poly(vinyl cinnamate) on quartz plates. It was then possible to follow the disappearance of the cinnamate ester absorption spectra with irradiation time. The photoreaction taking place has been presumed by Tsuda 41> to be analogous to dimerization of low molecular weight cinnamate esters. 

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