A New Insight into the Comonomer Effect through NMR Analysis in Metallocene Catalysed Propene-co-1-Nonene Copolymers.

The "comonomer effect" is an intriguing kinetic phenomenon in olefin copolymerization that still remains without a detailed explanation. It typically relates to the rate of enhancement undergone in ethylene and propene catalytic polymerization just by adding small fractions of an alpha-olefin. The difficulty lies in the fact that changes caused by the presence of the comonomer in reaction parameters are so conspicuous that it is really difficult to pin down which of them is the primary cause and which ones are side factors with marginal contribution to the phenomenon. Recent investigations point to the modification of the catalyst active sites as the main driving factor. In this work, the comonomer effect in the metallocene copolymerization of propene and 1-nonene is analysed and correlated to the comonomer role in the termination of the chain-growing process. The associated termination mechanisms involved furnish most of chain-free active sites, in which the selective interaction of the comonomer was proposed to trigger the insertion of monomers. A thorough characterisation of chain-end groups by means of the 1H NMR technique allows for detailing of specific transfer processes, ascribed to comonomer insertions, as well as evidencing the influence of the growing chain's microstructure over the different termination processes available.

Effect of microstructure on the thermo-oxidation of solid isotactic polypropylene-based polyolefins.

In the present work we aim to clarify the role of the microstructure and the crystalline distribution from the thermo-oxidation of solid isotactic PP (iPP) and ethylene-propylene (EP) copolymers. The effects of the content and quality of the isotacticity interruptions, together with the associated average isotactic length, on the induction time (ti) as well as on the activation energy (Eact) of the thermo-oxidation are analysed. Both parameters have been found to change markedly at an average isotactic length (n1) of 30 propylene units. While ti reaches a minimum when n1 is approximately 30 units, Eact increases quasi-exponentially as the number of units decreases from 30. This variation can be explained in terms of changes induced in the crystalline interphase, i.e. local molecular dynamics, which are closely linked to the initiation of the thermo-oxidation of isotactic PP-based polyolefins.