Coisotactic shift contributions in ¹³C NMR spectra of propylene/butene‐1 copolymers

Abstract

Coisotactic shift contributions caused by sterically crowded isotactic side‐chain alkyl groups are proposed for peak assignments of isotactic propylene/butene‐1 copolymers. Previously disputed analyses of methine triads and PB tetrads of backbone methylene carbons (α,α′‐CH2) have been verified using first‐order Markovian distribution theory. Coisotactic shift contributions also account for the reverse order of the propylene‐centered triads from that predicted by the Grant‐Paul equation.

Monomer Sequence Distributions in Propylene−1-Butene Random Copolymers

“…Figure 4 shows the extended carbon-13 NMR peaks of the methyl carbon in the propylene unit. According to the previous assignments [1][2][3][4][5][6][7][8][9]11 and this expanded experimental result, it can be seen that methyl carbons have lower comonomeric sequence sensitivity than configurational sequence sensitivity. For example, differences among the triad comonomeric sequence distributions, PPP, PPB, and BPB, are less than 0.15 ppm; however, differences among the triad configurational sequence distributions, PPP (mm), PPP (mr) and PPP (rr), are over 0.80 ppm, that is to say, all the comonomeric sequences are compressed only in the region of the triad configurational sequence PPP (mm).…”

“…Although the aim of this study is to recommend a new method for quantitatively determining monomer sequence distributions by minimizing the effect of configurarional distributions of propylene sequence, it is necessary to first discuss the carbon-13 NMR chemical shift assignments in propylene−1-butene copolymers. Figure shows the carbon-13 NMR spectrum of a selected propylene copolymer with 8.17 mol % 1-butene comonomer, most peaks of which can been identified and assigned in terms of the previous assignments. − However, a few peaks in the experiment remain unclear. It was natural to link these new peaks to the configurational structure of propylene sequence, since the samples used in the study contained small amounts of 1-butene, and from comparison of the chemical shifts of these peaks with those of a propylene homopolymer 11 it was concluded that such a link was reasonable.…”

“…According to the previous assignment [1][2][3][4][5][6][7][8][9]11 and this expanded spectrum, it is clear that the methylene carbon in the branch of the 1-butene unit has higher monomer sequence sensitivity, (making the peaks relating to the B-centered triad comonomeric sequence distributions even longer n-ads sequence distributions), is comparatively isolated and is easily identified, and accurate content of B-centered triad sequences can be directly obtained from it. However, the triad comonomeric sequence sensitivity of P-centered methine is comparatively much lower, even though 400 MHz high-field NMR cannot resolve it well.…”

“…Propylene−1-butene copolymers, unlike ethylene−propylene copolymers and ethylene−1-butene copolymers, have received relatively less attention in carbon-13 NMR studies, even though over 20 years ago the carbon-13 NMR technique was utilized to characterize their molecular microstructure and to determine the monomer sequence distributions. − The studies of Randall, Cheng, and Aoki are considered the most significant in this field: 3,6-8 they determined the chemical shift assignments of triad and tetrad sequences from chemical shift calculations using empirical rules and comparison of the spectra of copolymers with different compositions. , In addition, the pentad and hexad sequence assignments were proposed from quantitative analysis of the spectrum by a reaction probability model . Aoki et al advanced the study in this field: 7,8 use of the powerful 2D-INADEQUATE technique determined the carbon-13 NMR chemical shift assignments in propylene−1-butene copolymers and confirmed the validity of previous assignments of triad and tetrad sequences .…”

13C NMR analysis of propylene‐butylene copolymers by a reaction probability model

“…Bunn and Cudby: Randall,5 and Usami et a1. 6 Despite an earlier uncertainty in the assignments of two lines,6p7 the 13C NMR spectra of propylene-butylene copolymers are now well understood.…”