Carbon-13 NMR chemical shifts and the microstructure of propylene-vinyl chloride copolymers with low propylene content

Abstract

13C NMR chemical shifts expected for the carbon atoms in propylene-vinyl chloride (P-VC) copolymers of low propylene content are calculated as a function of copolymer stereosequence. Mark's conformational model of P-VC copolymers is coupled with the -gauche effect, which results in upfield chemical shifts for those carbon atoms in a gauche arrangement with carbon or chlorine substituents in the position, to calculate the 13C NMR chemical shifts of the carbon atoms in the vicinity of a propylene unit surrounded by vinyl chloride units. Agreement between the calculated chemical shifts and those we observe in the 50-MHz 13C NMR spectrum recorded for a P-VC copolymer containing 5 mol % propylene units is excellent. As an example, the chemical shifts calculated for the CH3 carbons are sensitive to pentad but not to heptad or longer stereosequences. This is confirmed in the 13C NMR spectrum of our P-VC copolymer with low P content and contrasts with the predicted and observed heptad sensitivity of the CH3 resonances in the 13C NMR spectra of atactic polypropylene. The agreement achieved between calculated and observed chemical shifts for our P-VC copolymer enables us to assign its 13C NMR spectrum, to evaluate the statistics of stereochemical placements during the copolymerization of P with VC monomers, and to compare these stereochemical statistics to those resulting from the homopolymerization of P and VC.

Kinetic Control of Chlorine Packing in Crystals of a Precisely Substituted Polyethylene. Toward Advanced Polyolefin Materials

“…The major differences between the spectra of forms I and II are a 2–3 ppm upfield shift of the crystalline CH (from ∼68 to 66 ppm) and crystalline CH 2 adjacent to the methine (from ∼42 to 39 ppm). Following prior work, , the upfield shifts conform to a γ-gauche effect or shielding of CH and α CH 2 by the CH 2 that are gamma (or three bonds apart) to them. Shielding takes place only when the CH–C bond is in a gauche conformation. , As it is clear from the spectra of Figure that the βCH 2 (27.7 ppm) and other CH 2 further from the CH unit have equivalent resonances for forms I and II, the 13 C NMR spectra provide further evidence of the gauche conformation around the crystalline CH–Cl unit.…”

“…14 The major differences between the spectra of forms I and II are a 2−3 ppm upfield shift of the crystalline CH (from ∼68 to 66 ppm) and crystalline CH 2 adjacent to the methine (from ∼42 to 39 ppm). Following prior work, 33,34 the upfield shifts conform to a γ-gauche effect or shielding of CH and α CH 2 by the CH 2 that are gamma (or three bonds apart) to them. Shielding takes place only when the CH−C bond is in a gauche conformation.…”

“…Shielding takes place only when the CH−C bond is in a gauche conformation. 33,34 As it is clear from the spectra of Figure 8 that the βCH 2 (27.7 ppm) and other CH 2 further from the CH unit have equivalent resonances for forms I and II, the 13 C NMR spectra provide further evidence of the gauche conformation around the crystalline CH−Cl unit. Moreover, even if cp MAS spectra are not quantitative, they also provide useful qualitative informa- higher scattering vector (lower periodicity) when form II is enabled at T c > 53 °C is intriguing.…”

Crystallization of Polyethylenes Containing Chlorines: Precise vs Random Placement

“…These shifts were first estimated following the work of Tonelli et al 29 According to this work, shielding of a CH by a substituent that is in a carbon three bonds apart (γ position) from the observed carbon results in an upfield shift of ∼-2.5 ppm only when the CH-C bond is in a gauche conformation. In addition, the R-CH 2 vicinal and in a gauche conformation to the substituted carbon presents a similar upfield chemical shift.…”

Formamido-Diterpenes from the South China Sea Sponge Acanthella cavernosa

“…The relative configuration of the decalin ring in 5 was the same as 12 , inferred from the chemical shifts of C-1 to C-10 and NOESY correlations of H-1/H-7, H 3 -20/H-6, and NH/H-1. A significant difference between 5 and 12 was found in the chemical shift of C-12 ( δ C 31.0 in 5 instead of δ C 38.2 in 12 ), which was caused by the γ-gauche effect [ 20 , 21 , 22 , 23 , 24 ], indicating an axial orientation of Cl–14. The relative configurations of C-7 and C-11 in 5 were determined as S * and R *, respectively, from NOESY correlations of H 3 -18/H-6, H 3 -18/H-8b, and H-8a/H-12, as shown in the Newman projection ( Figure 4 ).…”