Novel Ethylene-norbornene Copolymers via Chain Shuttling

The insertion of cyclic co-monomers in a polyethylene chain, possible with metallocene catalysts, allowed a modulation of glass transition temperature (Tg) to obtain copolymers with increased Tg and to change mechanical and thermal properties of the material. In particular, ethylene (E) and norbornene (N) copolymers by metallocene catalysis have high thermal resistance, high resistance to polar solvent dissolution, high gas barrier properties and high optical transparency. In order to further widen these material properties we decided to explore a new synthesis strategy, the so-called "Chain Shuttling" polymerization strategy, which was introduced by Dow Chemical Company[1] and that allows to obtain copolymers where different microstructures are bonded together in an alternate sequence in the same polymeric chain (multiblock microstructure). In this work we used two zirconocenes bearing different ligands, with different propensity to incorporate the comonomeric units, one with high and the other with low norbornene incorporation capability, and ZnEt2 as chain shuttling agent, that "shuttles" the growing chain from one catalytic center to another. The activation of the catalyst precursor was performed with dimethylanilinium tetrakis(pentafluorophenyl)borate.[2] All the syntheses were performed at high pressure, at 70 °C and at three different monomer feeds (N/E molar ratio = 1,3; 5,2; 26,0), in order to evaluate the efficiency of the synthesis for producing copolymers with high, medium and low Tg.[3] Copolymers were characterized by NMR, GPC, and DSC techniques. Rheological and mechanical properties have been determined as well. By increasing the amount of ZnEt2 copolymer molar masses prepared with both catalytic systems decrease, indicating that ZnEt2 acts as chain transfer agent. Copolymers synthesized with both catalysts and with shuttling agent show a single Tg and monomodal molar mass distribution. N content, Tg and molar masses are in between those of two references. In our opinion all together results are clues that the polymerization proceeded with chain shuttling mechanism. Acknowledgement. The authors thank CARIPLO Foundation (Crystalline Elastomers Project) for financial support . [1] Arriola D.J., Carnahan E.M., Hustad P.D., Kuhlman R.L., Wenzel T.T., Science, 2006, 312, 714 [2] Bochmann M., Organometallics, 2010, 29, 4711 - 4770. [3] Tritto I., Boggioni L., Ferro D. R., Coordination Chemistry Review, 2006, 250, 212 - 241