Donna J. Crowther

Stereoregular, alternating ethylene–norbornene copolymers from monocyclopentadienyl catalysts activated with non-coordinating discrete anions

Abstract Cyclic olefins copolymerize efficiently with ethylene using metallocene catalysts to make rigid engineering polyolefins with high chemical homogeneity. Materials with a high chemical homogeneity have value because they show reduced light scattering resulting in excellent clarity. The ultimate chemical homogeneity in a two component copolymer occurs when an alternating sequence distribution is achieved. Low symmetry bis-metallocenes have been shown to make alternating, stereoregular cyclic olefin copolymers under the right process conditions. These copolymers are semicrystalline and have high melt temperatures. Herein we report two group 4 monocyclopentadienyl catalyst systems that yield semicrystalline ethylene/norbornene copolymers over a range of reactor conditions. The clear copolymers have glass transition temperatures over 110°C and melt temperatures over 240°C. Further characterization data is presented and a short discussion of a plausible polymerization mechanism is given.

Linear rheology and structure of molecular bottlebrushes with short side chains

We investigate the microstructure and linear viscoelasticity of model molecular bottlebrushes (BBs) using rheological and small-angle X-ray and neutron scattering measurements. Our polymers have short atactic polypropylene (aPP) side chains of molecular weight ranging from 119 g/mol to 259 g/mol and narrow molecular weight distribution (Mw/Mn 1.02–1.05). The side chain molecular weights are a small fraction of the entanglement molecular weight of the corresponding linear polymer ( Me,aPP= 7.05 kg/mol), and as such, they are unentangled. The morphology of the aPP BBs is characterized as semiflexible thick chains with small side chain interdigitation. Their dynamic master curves, obtained by time-temperature superposition, reveal two sequential relaxation processes corresponding to the segmental relaxation and the relaxation of the BB backbone. Due to the short length of the side chains, their fast relaxation could not be distinguished from the glassy relaxation. The fractional free volume is an increasing ...