Structure and Radical Mechanism of Formation of Copolymers of C60 with Styrene and with Methyl Methacrylate

Abstract

Polymerizations in 1,2-dichlorobenzene solutions containing 0.33 volume fraction of styrene or methyl methacrylate (MMA) and relative weights of monomer/C 60 /azo(bisisobutyronitrile) (AIBN) of 100:1.00:1.12 at 75 °C form high molecular weight materials in which all of the C 60 is incorporated covalently. To understand the structures of the polymers and their mechanism of formation, samples were isolated after low conversion of monomer and analyzed. Molar size exclusion chromatograms from UV detection of fullerenes, differential refractive index detection of the mass of the polymer, and differential viscometry detection of the specific solution viscosity of the polymer show that the fullerene reacts rapidly, and both polystyrene/C 60 and PMMA/C 60 products isolated after low conversion of monomer contain many fullerenes per molecule. Lower intrinsic viscosity and higher absolute molecular weight of the fullerene-containing polymers compared with linear polystyrenes at equal retention time show that the polymer structures are branched. Elemental analyses, NMR spectra, and size exclusion chromatograms show that the C 60 content is higher and the polymer chain lengths are shorter in the low-conversion polystyrene/C 60 than in the low-conversion PMMA/C 60 . C 60 itself polymerizes when initiated by AIBN. NMR analyses of polymers formed by initiation with AIBN-α- 13 C show that in both polystyrene/C 60 and PMMA/C 60 at low conversion 62-72% of the 2-cyano-2-propyl groups are bound to polymer chain ends, and 28-38% are bound to fullerenes. Neither low molar mass AIBN/C 60 adducts nor the polymers at any degree of conversion initiate further polymerization of monomer. Thus, the formation of 2-cyano-2-propyl to fullerene and polymer to fullerene carbon-carbon bonds is irreversible. After high conversion both polystyrene/C 60 and PMMA/C 60 contain much linear polymer. The average number of fullerene units per molecule decreases with increasing reaction time, and after complete reaction of monomer, all polystyrene/ C 60 samples and some PMMA/C 60 samples still have an average of more than one fullerene unit per macromolecule at the high end of the molecular weight distribution. Fullerene radicals were detected by ESR spectroscopy in all of the solid polymers recovered at low and high conversion. Evaluation of a radical chain mechanism for the copolymerizations using estimated rate constants for the microscopic steps shows that the fullerene must exist as clusters early in the polymerization, and that the clusters break down to macromolecules containing smaller numbers of fullerene units as the polymerization continues.