The Conformational Characteristics of Ethylene−Norbornene Copolymers and Their Influence on the ¹³C NMR Spectra

Abstract

A rotational-isomeric-state treatment of ethylene−norbornene copolymers has been worked out for the purpose of establishing a correlation between conformation and 13C NMR chemical shifts of this new class of polymers and of clarifying their microstructure. A comparison of conformer populations computed for the stereoregular alternating (N−E) x chains and for copolymers including such defects as N−N, E−E, E−E−E, and N−N−N indicates the need for accounting for the tacticity of the chain, besides its composition, in the interpretation of the spectra. Some stereochemical shifts were suggested by the calculations. In particular, conformational analysis predicted and evidenced the origin of distinct meso / racemic signals for methines C2 and C1 and for the ethylene carbon of the regularly alternating copolymers. This prediction induced a more thorough analysis of the 13C NMR spectra of mainly alternating (N−E) x copolymers, allowing us to recognize and distinguish isotactic and syndiotactic N−E−N sequences. Then, a detailed and unambiguous assignment of the spectra of stereoregular and irregular alternating E−N copolymers was achieved. The strong effect of ring distortions in the presence of N−N dyads and triads is also discussed.

Microstructure of Copolymers of Norbornene Based on Assignments of 13C NMR Spectra: Evolution of a Methodology

“…The thorough microstructure investigation of E-co-N copolymers has taken several years [17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32]. A number of groups dedicated great efforts to assign the 13 C NMR spectra of E-co-N copolymers.…”

“…Assignments indicated in Figure 2 of two methines C2 of the cyclic unit and of the ethylene CH2 signals in regularly alternating isotactic and syndiotactic NEN sequences, as well as of the Sαδ signals and the Sβγ and Sγδ signals shifted lowfield with respect to Sβε e and Sδε, were respectively achieved through comparison of conformer populations [19]. Utilizing the observed peak areas of the assigned 13 C signals and accounting for the stoichiometric requirements, it was possible to compute the molar fractions of the stereosequences, which define the microstructure of an alternating E-co-N copolymer containing meso (m) and racemic (r) alternating sequences [26].…”

“…The elucidation of the conformational structure of the chain of E-co-N copolymers by molecular mechanics calculations and the correlation between conformation and 13 C NMR chemical shifts allowed us to make significant progress [19]. Conformer populations of the stereoregular and stereoirregular polynorbornene and alternating (N-E)x chains and of copolymer chains containing NN, EEE and NNN sequences were computed and allowed us to predict stereochemical shifts.…”

“…Conformer populations of the stereoregular and stereoirregular polynorbornene and alternating (N-E)x chains and of copolymer chains containing NN, EEE and NNN sequences were computed and allowed us to predict stereochemical shifts. For the first time, it was possible to distinguish between meso and racemic NEN sequences and to assign signals of the two methines C2, Cl of the cyclic unit and of the CH 2 of ethylene in regularly alternating isotactic and syndiotactic copolymers, and the signals of the two methines of an N unit in a NEE... sequence [19,20].…”

Phosphine-Thiophenolate Half-Titanocene Chlorides: Synthesis, Structure, and Their Application in Ethylene (Co-)Polymerization

“…[44][45][46][47][48][49][50][51][52][53][54][55][56] Figure 3A showed that the microstructures of the COCs formed using 2a under low NBE concentration of 0.3 mol/L possessed alternating ethylene-NBE sequences and isolated NBE units. In contrast, resonances ascribed to NBE diads or triads were observed for the copolymers prepared at high NBE concentration of 0.5-1.0 mol/L (Figures 3B-C), and the microstructures thus possessed a mixture of NBE repeat units in addition to the alternating, isolated NBE sequences.…”