Stereochemical Pseudohexad ¹³C NMR Resonances and Regioregular Propylene/Ethylene‐1‐¹³C Copolymers

Abstract

a In ref. [1] and ref. [2] the chemical shifts were reported in different scales and the spectra have been obtained under different conditions. In this paper the chemical shifts are always reported in the HMDS scale; the residual difference between the chemical shifts reportedonref. [1] andref. [2] ,incomparisonwiththeonesmeasured on the spectra of our samples, arise from the different conditions. b In this paper the different methylenes are labelled according to Carman and Wilkes, [3] i.e. by reporting, using Greek letters, the number of carbon-carbon bonds bridging the considered methylene with the closer methyne carbons at the right and the left sides.

Bis(phenoxyimine)zirconium and -titanium Catalysts Affording Prevailingly Syndiotactic Polypropylenes via Opposite Modes of Monomer Insertion

“…The 13 C NMR spectrum of the copolymer produced in run 12 is displayed in Figure 5a. According to literature assignments, 22 the ethylene units are actually isolated and are located in two different environments, leading to either a two-methylene sequence or a three-methylene sequence. The three methylene sequences outnum-ber the methylene pairs (79% vs 21%), as deduced from the area of the resonances of the S Rγ carbons and of the S ββ carbons vs the area of the resonances of the S Rβ carbons (corrected for the contribution of unenriched S Rβ carbons arising from head-to-head propene insertion).…”

“…Since the propene insertion is prevailingly 1,2, the S Rγ carbon resonance is diagnostic of the stereochemistry of the two propene units entered into the chain after the enriched ethylene (Scheme 4b). 22 Thus, one can evaluate P′ r (and P′ m ) 1 -P′ r ), the probability of having an r (m) dyad immediately following an ethylene unit: from the areas of the resonances of the r-S Rγ and m-S Rγ enriched carbons, P′ r ) 0.58, which is less than P r in the main chain (0.67), as expected considering that the chiral carbon of the last unit of the growing chain bears substituents of less different ranks in comparison with propene homopolymerization.…”

Direct Probing of Regioregularity for Polycarbodiimide Systems via15N NMR Analysis

“…13 C NMR is a widely used spectroscopic technique for assigning polymer microstructures in multiple systems including poly(2,5-thienylene vinylene)s, polythiophenes, poly( p -alkoxyphenylenes), and simple polyolefins. − Using 13 C NMR analysis, Lu et al assigned regioisomers for a series of polycarbodiimides by building trends between the chemical shifts of diaryl polycarbodiimides, dialkyl polycarbodiimides, and asymmetric alkyl/aryl polycarbodiimides . These chemical shifts were then coupled with FTIR analysis to relate the trends from 13 C NMR with the imine CN stretch of the polymers.…”

Simple Trends in the Methylene Regions of the NMR Spectrum of Poly(propylene) and Ethylene‐Propylene Copolymers

“…The extension of the telephone rule proposed above for S * β (and P β ) resonances finds full confirmation in the spectra of 2,4,6-trimethyloctane and 2,4,6-trimethylnonane, [30] and in those of the S ββ region of regioirregular poly(propylene); [31] in total 2 + 2 + 10 successful applications of the telephone rule and not even one failure. To have a wider experimental basis for the discussion of the chemical shift differences in regioirregular poly(propylenes), mixture of known composition of the diastereomers of 3,6-dimethyloctane (DMO) and of 3,5,8,10-tetramethyldodecane (TMD) have been synthesized and the spectra have been assigned as described in the Experimental Part (Figure 7).…”

“…In that case, at first-order in the perturbation expansion 1 there is no configurational splitting, and computations with the screening parameters adopted here gave results with the correct order of resonances, but with spacings far smaller than the experimental ones. [31] These failures of the computational Table 2. Relative chemical shifts computed ab initio for the rightmost S αβ methylenes of two conformations of the stereoisomers of DMO.…”