Polynorbornene microstructure

To conclude this section, an example is given of the application of statistical models to examine double bond sequences in polymers with unsaturated backbones. The presence of backbone unsaturation leads to the occurrence of cisitrans isomerism and this can be treated entirely analogously to comonomer sequence. Ivin [42] and others have used NMR spectroscopy extensively to study the microstructure of polymers prepared by transition metal catalysed ring-opening metathesis polymerisation (ROMP) such as polynorbornene (PNB). 

Nickel-0- and palladium-O-complexes are very active catalysts for the polymerization of norbornene and also for cyclopentene [552-554], Nickel catalysts produce soluble polymers with a molecular weight of over one million while polymers obtained with palladium or metallocene complexes are insoluble. The soluble polymers have an atactic structure. The microstructure of the polynorbornene depends on the catalyst used and is isotactic by synthesis with chiral metallocenes. 

Notably, copolymers obtained with various catalysts substantially differed in properties. For example, copolymer of NBSi(OFt)3 with norbornene obtained on [(fl -toluene)Ni(C6F5)2] catalyst, exhibited suitable mechanical properties (a tensile modulus of 1.4 GPa, an elongation-to-break of 15%) while catalyst [(fl -crotyl)Ni(l,4-COD)]PFe gave completely brittle polymers, despite the fact that they had equal molecular weights [231]. Ludovice et al. suggested that these differences could be connected with polymers microstructure a more atactic polymer was brittle, and a more stereoregular one had better mechanical properties [232]. However, determination of microstructure of substituted polynorbornenes is a very intricate... 

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