Polymerization photoactivated

The processes by which unsaturated monomers are converted to polymers of high molecular weight exhibit the characteristics of typical chain reactions. They are readily susceptible to catalysis, photoactivation, and inhibition. The quantum yield in a photoactivated polymerization in the liquid phase may be of the order of 10 or more, expressed as the number of monomer molecules polymerized per quantum absorbed. The efficiency of certain inhibitors is of a similar magnitude, thousands of monomer molecules being prevented from polymerizing by a single molecule of the inhibitor. ... 

Table IV. Photoactivated Polymerization of S-AN...EASC Charge Copolymer...

Polymers produced by photoactivated polymerizations are called photo-... 

Polymerization of the gel may be achieved either by ultraviolet photoactivation with riboflavin, or, preferably, using ammonium persulphate as a catalyst. It is necessary to include an initiator for the reaction, TEMED (tetramethylethylene diamine) being commonly used. 

Ruthenium catalysts found many applications in C-C bond formation reactions (selected reviews [157-161]). Ruthenium occurs mostly in oxidation states +2 and +3, but lower as well as higher oxidation states can easily be reached. Thus ruthenium compounds are frequently used in oxidative transformations proceeding by either single or two electron transfer pathways (selected reviews [162-164]). It has long been known that ruthenium complexes can be used for the photoactivation of organic molecules (selected reviews [165, 166]). Ruthenium complexes are applied as catalysts in controlled or living radical polymerizations [167-169]. 

Since the excited charge-transfer complexes are photoactivated, weaker donor-acceptor combinations can be used than those in thermal polymerizations, including not only strong D/strong A, but also strong D/weak A and weak D/strong A. 

The system benzophenone—triethylamine produces ketyl and amine radicals by electron transfer between the photoactivated benzophenone and the amine in the ground state. The generated radicals initiate the polymerization of methyl methacrylate [74]. 

Charge transfer (CT) complexes, different from aromatic ketone/amine systems, such as quinoline-bromine, pyridine-bromine, tetrahydrofuran-bromine etc. have also been reported to behave as initiators of vinyl polymerization, particularly under photoactivation [65-68]. 

Although the result of photoactivation of a monomer is generally the generation of a radical species, ionic polymerization has been reported where a cationically polymerizable monomer, i.e. isobutylene, underwent photoionization in the vapor state and the ionized fragments were separated from their electrons by an electric field (l). 

Photoactivated Copolymerization. Although polymerization and copolymerization generally involve the addition of a monomer to a reactive chain end, the ground state charge transfer complex generated by the interaction of an electron donor monomer and a strong electron acceptor monomer, acts as a single unit and, upon excitation of the complex, both monomers enter the chain. 

Figure 2. Photopolymerization exotherm trace. Rate of sample heat change vs. time at 40° C. Photoactivated Trigonal 14 initiation of lauryl acrylate polymerization.

Peroxide initiated polymerizations (scan 30°-250°C) of lauryl acrylate (LA), I,6-hexanediol diacrylate (HDDA), neopentyl glycol diacrylate (NPGDA), and trimethylol propane triacrylate (TMPTA) revealed total polymerization heats per mole of C=C of 19.2, 17.6, 16,7, and 15.4 kcal, respectively. PhotoactIvated Trigonal 14 initiated polymerizations at 40° yielded total polymerization heats per mole of C=C of 17.4, 11.7, 8.2, and 9.1 kcal for LA, HDDA, NPGDA, and TMPTA, respectively. At 60°C the photopolymerization heats for the latter three monomers increased to 16.6, 9.7, and 10.2 kcal per mole of C=C, respectively. 

Ethynytation. On photoactivation in the presence of hexabutylditin the reaction of the triflone with various alkyl iodides leads to the silylated alkynes. To minimize polymerization of the reagent reactions are conducted at low concentration. 

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