Incorporation of C60 into Poly(methyl methacrylate) and Polystyrene by Radical Chain Polymerization Produces Branched Structures

Abstract

Polymerizations of styrene and methyl methacrylate (MMA) containing 1 wt % C60 initiated by 5 or 10 mol of azobis(isobutyronitrile)/mol of C60 in 1,2-dichlorobenzene solution produce brown polymers in 53−97% yield with all of the C60 incorporated, linear polymer equivalent molecular weights of Pn = 19 000−31 000, and Pw/Pn < 2. There are short induction periods before polymerization begins. All of the C60 is incorporated into the polymer after low conversion of the monomer. Multidetector size exclusion chromatography analyses measured polymer mass by differential refractive index, Mw by two-angle laser light scattering, intrinsic viscosity by differential viscometry, and mass of only C60 derivatives by UV. Molar chromatograms show that all of the polymer at the high end of the molecular weight distributions contains C60, and there are sizeable amounts of a lower molecular weight linear polymer. The high molecular weight polystyrene contains as many as 10−100 C60 units, but the high molecular weight PMMA contains an average of one C60 unit per macromolecule. All of the polymers have lower intrinsic viscosities and higher Mw than linear standards of the same retention volume due to branched or star structures. Calculations from a random branching model of Zimm and Stockmayer indicate that the PMMAs have an average branch number of five over the entire molecular weight distribution and systematically increasing average branch lengths with an increasing degree of conversion.