Polymers/Polymerization photo-crosslinkable polymer

Treatment of the monomer with an acidic catalyst leads initially to polymers of low molecular weight and ultimately to crosslinked, black, insoluble, heat-resistant resin (17). Despite their reportedly excellent properties, virtually no commercial use of such resins exists outside the Soviet Union. The structure and polymerization mechanism of these furfural-ketone polymers are described in a recent study (18). An excellent combustion-resistant resin has been reported (19) from the addition of dialkylphosphites to bis(2-furfurylidene) ketone (6). Furfural condensates with other aliphatic and aromatic ketones have been reported (20,21) to provide photo-crosslinkable resins and hypergol components. 

This polymer was synthesized via NMRP (Nitroxide Mediated Radical Polymerization) (Benoit et al. 1999) by sequential polymerization of 2VP and a mixture of NIPAAm and DMIAAm. Using the macroinitiator method, the preparation of well-defined linear block copolymers consisting of a homo polymer block P2VP (pH-sensitive) and a random copolymer block of PNlPAAm (temperamre sensitive) with DMIAAm (photo crosslinker) was possible. 

This was utilized by Tian et al. to prepare a new elass of liquid erystal homopolymers of poly ll-[4- (3-ethoxycarbonyl- eoumarin- 7-oxy)-earbonyl-phenyloxy]-undecyl methacrylate containing a eoumarin moiety as a photo-crosslinkable unit. The preparations included polymers of various chain lengths. Also, liquid crystalline-coil diblock and liquid crystalline-coil-liquid crystalline triblock copolymers with polystyrene as the coil segment were formed. The polymers were reported to have been synthesized with the aid of atom transfer radical polymerization. The dimerization of the eoumarin moieties takes place upon irradiation with light of > 300 nm to yield crosslinked network structures. 

ROMP and PROMP are very useful methods to synthesize a number of novel materials with unique properties. Poly(cyclooctenes), poly(norbornenes) and poly(acetylenes) were discussed in more details and some of their properties like Tg, cristallinity, oxygen permeability, dielectric properties etc. listed. The polymerization of the cycloolefins was done either thermally or photochemically with the "old" Ru(II)-salts and the later developed Ru-phosphines as catalysts, whereas substituted acetylenes were photo-polymerized with W-, Mo- and Ta-catalysts. In addition, polymeranalogous transformations of the double bonds in ROMP polymers, (additions and cyclo-additions, epoxidation, (photo) crosslinking etc.) were discussed. We are convinced that these materials and systems will find useful applications in the near future. 

As already mentioned, it was observed that during photo-oxidative degradation of polymers there occur accumulations of new carbonyl groups with ketonic, carboxylic or ester structure [5]. It was noticed that, whilst in most cases carbonyl structure concentrations from polymer bulk increase with exposure time, concentration of hydroperoxide and peroxide stmctures rapidly reaches a stationary state at relatively low concentrations. An exemplification of such behavior is given in Table 3, in which the variation of hydroperoxide concentration with exposure time (luminous radiation with X > 300 nm) is represented in the case of photo-oxidative degradation of a vinyl-ester based polymeric structure, crosslinked by end double bonds, where BTAC is benzyltributylammonium chloride (Schemes 1, 2—reproduced with kind permission from Elsevier—license no. 3842460646409) [12]. 

For practical use of PDLC, dispersions are mainly prepared by mixing a small amount of misdble monomers with liquid crystals and photo-polymerizing [1], since polymer and liquid crystals tend to be immiscible. As the polymerization evolves, the system undergoes phase separations into a liquid crystalline phase rich in liquid crystals and an isotropic phase rich in polymers. Before the system reaches equilibrium states, however, the polymerization freezes the system into a crosslinked network of polymer-rich domains. Then, the morphologies of the PDLC involve interplay among three kinetic processes polymerization, phase separation, and phase ordering. These phase separation dynamics have been simulated by some aulhors [105-108]. 

The UV cure system contains an epoxy or a vinyl ether functionalized PDMS polymer and a photo catalyst [36]. This latter, a diaryliodonium salt is photolyti-cally decomposed to form an active acid that polymerizes the epoxy or vinyl ether groups and crosslinks the network. 

Photo-initiated polymerization of pendant vinyl ether groups attached to a polystyrene backbone has been used as a convenient method of crosslinking. However, the potential commerical importance of this is marred by the fact that a slow thermal initiation involving the acceptor is also possible and occurs, for example, during processing of the carrier polymer. ... 

More recently, iodonium salts have been widely used as photoinitiators in the polymerization studies of various monomeric precursors, such as copolymerization of butyl vinyl ether and methyl methacrylate by combination of radical and radical promoted cationic mechanisms [22], thermal and photopolymerization of divinyl ethers [23], photopolymerization of vinyl ether networks using an iodonium initiator [24,25], dual photo- and thermally-initiated cationic polymerization of epoxy monomers [26], preparation and properties of elastomers based on a cycloaliphatic diepoxide and poly(tetrahydrofuran) [27], photoinduced crosslinking of divinyl ethers [28], cationic photopolymerization of l,2-epoxy-6-(9-carbazolyl)-4-oxahexane [29], preparation of interpenetrating polymer network hydrogels based on 2-hydroxyethyl methacrylate and N-vinyl-2-pyrrolidone [30], photopolymerization of unsaturated cyclic ethers [31] and many other works. 

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