Propene−Norbornene Copolymers: Synthesis and Analysis of Polymer Structure by 13C NMR Spectroscopy and ab Initio Chemical Shift Computations

Abstract

A series of propene−norbornene (P−N) copolymers were synthesized in the presence of rac-Et(indeny)2ZrCl2/MAO in toluene at 30 °C. P−N copolymers were characterized by 13C NMR spectroscopy, SEC, and DSC. Results were compared with those of a series of P−N copolymers synthesized in the presence of rac-Me2Si(Indenyl)2ZrCl2/MAO under the same experimental conditions. Polymerization activity appears to be quite low in comparison with ethene−norbornene (E−N) copolymerization. A first assignment of the main 13C NMR signals of P−N copolymers containing isolated N units was obtained on the basis of distortionless enhancement by polarization transfer (DEPT) 13C spectra and by comparison with isotactic polypropene (i-PP) and E−N copolymer spectra. Ab initio theoretical 13C NMR chemical shifts, computed for the most relevant conformers of a rather simple model compound and averaged using the RIS conformer populations estimated for an isotactic chain (P4−N)x, gave important detailed indications for the assignment of the complex 13C spectra of P−N copolymers with N content up to 35 mol %. Such assignments were used to estimate the N copolymer content. The discrepancy between the values obtained from the areas of methyl signals and from the areas of the signals assigned to norbornene carbons was found to be partially due to the presence of 1,3-propene misinsertions, which are formed in significant amount when increasing the [N]/[P] ratio of the feed. Our results confirm that, despite the relatively lower polymerization activity, at low norbornene/olefin ratio it is possible to obtain P−N copolymers that are relatively richer in norbornene than the E−N copolymers prepared in similar conditions. However, at higher norbornene/olefin feed ratios the great amount of 1,3-propene misinsertions clearly reveals that the steric hindrance of the Mt−tertiary carbon bond when N is the last inserted unit makes difficult the next propene insertion, causing low polymerization activities, molecular masses, and Tg.