Cyclolefin Copolymers via Chain Shuttling

The insertion of co- and ter-monomers in a polyethylene chain synthesized with Ziegler-Natta catalysts allows tailoring physical and chemical properties in a wide range. The insertion of cyclic co-monomers, 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-norbornene copolymers by metallocene catalysis have high thermal resistance, high resistance to polar solvent dissolution, high gas barrier properties and high optical transparency. However, at high norbornene content they become brittle. To overcome this limitation, we decided to explore a new synthesis strategy, the so-called "Chain Shuttling" polymerization strategy, which was introduced by Dow Chemical Company[1] in 2006. It allows to obtain ethylene-octene copolymers where different microstructures are bonded together in an alternate sequence in the same polymeric chain (multiblock microstructure): the physical and mechanical properties are maintained, while the elastic component and the optical transparency of the material are increased. The catalytic system of "chain shuttling" copolymerization is composed by three fundamental components: two catalysts with different propensity to incorporate the comonomeric units and one chain shuttling agent that "shuttles" the growing chain from one catalytic center to another catalytic center. In this case: oOne catalyst with high norbornene incorporation capability; oOne catalyst with low norbornene incorporation capability; oA chain shuttling agent. In this work two couples of catalytic systems are used: the first catalytic system is composed by two zirconocenes to evaluate the influence of the ligand on the final polymer microstructure; the second catalytic system is composed by two metallocenes with the same ligand but different metal atoms. In both systems, the activation of the catalyst precursor (alkylation of the metallocene dichloride and subsequent ion pair formation) was performed with dimethylanilinium tetrakis(pentafluorophenyl)borate.[2] The chain shuttling agent is ZnEt2. All the syntheses were performed at high pressure (in autoclave) at three different monomer feeds (norbornene/ethylene molar ratio = 1,3; 5,2; 26,0), in order to evaluate the efficiency of the synthesis for producing ethylene-norbornene copolymers with high, medium and low Tg.[3] All the polymers were characterized with NMR spectroscopy, GPC exclusion chromatography and thermal analysis by DSC. Results obtained suggest the possibility that application of the concept of "chain shuttling" is possible to ethylene-norbornene copolymerization. Acknowledgement. The authors thank CARIPLO Foundation (Crystalline Elastomers Project) for financial support . Reference [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. Keywords: Chain Shuttling polymerization, norbornene, copolymers