Photoinitiated polymerization of di- and tetrafunctional methacrylic monomers in a SBS matrix. Kinetic, mechanistic, and structural aspects

The kinetics and mechanism of the photoinitiated polymerization of tetrafun ctional and difunctional methacrylate monomers (di-, hexa-, and decamethyle ne dimethacrylates; and ethylhexyl and dodecyl methacrylates) in a styrene- butadiene-styrene (SBS) block copolymer matrix have been studied. Reaction diffusion was found to be the only termination mechanism for tetrafunctiona l monomers when the monomer concentration in the matrix is below 30-40%; fo r higher monomer concentrations, reaction diffusion controls the terminatio n process only after approximately 10% conversion was reached. The values o f both the propagation kinetic constant and the overall double bond convers ion for the three tetrafunctional monomers studied showed the following ord er: deca- > hexa- > dimethylene dimethacrylate. The termination process in the photoinitiated polymerization of difunctional methacrylate monomers is clearly controlled by reaction diffusion right from the beginning of the po lymerization reaction only at a very low monomer concentration in the matri x (10-15%); for medium monomer concentrations (20-40%), a combination of bo th mechanisms, segmental diffusion-controlled (autoaccelerated kinetics) an d reaction diffusion, was observed until reaching a double bond conversion of 20%, from which point reaction diffusion predominated; for higher monome r concentrations (60-90%), the termination kinetic constant values at low c onversions (<30%) were close to those corresponding with standard polymeriz ations, observing the Trommsdorff effect (autoacceleration) at higher doubl e bond conversions. The SBS matrix participates appreciably in the polymeri zation process through the direct addition of the macroradical or the prima ry radical to the double bond of the polybutadiene moiety and through hydro gen abstraction from the matrix with the formation of benzylic and allylic radicals.