Syndiotactic Triads Research Articles

Stereospecific Polymerization of o-Methoxy-α-methylstyrene

o-Methoxy-α-methylstyrene (o-MeO-α-MeSt) was polymerized under various conditions using BF3OEt2, TiCl4, SnCl2, EtAlCl2, n-butyllithium (n-BuLi), or Na-naphthalene as catalysts, and methylene chloride, n-heptane, toluene, or tetrahydrofuran (THF) as solvents. Generally poly(o-MeO-α-MeSt) was formed below −30°C. The cationic polymerizations proceeded more rapidly than the anionic ones, which gave polymer only in a polar solvent such as THF. α-Methylstyrene (α-MeSt) was also polymerized for comparison with the tacticity of poly(o-MeO-α-MeSt). The stereoregularity of the polymer was investigated by NMR spectroscopy. Assignments of the α-methyl resonances of the polymers were made on the basis of the methylene resonances. In the NMR spectrum of poly(o-MeO-α-MeSt), the methyl groups showed three peaks at 4.45, 4.80, and 5.35 ppm on the high-field side from methylene chloride used as an internal standard; the peaks were assigned to isotactic (I), heterotactic (H), and syndiotactic (S) triad sequences with increasing magnetic field. The polymer obtained at −78°C from the anionic or cationic catalyst showed an S content higher than 74%, and the content decreased with an increasing polymerization temperature. The tacticities of the polymers formed in the cationic polymerizations depended only slightly on the catalyst and the solvent. The stereospecificity in the cationic polymerization was very similar to that of α-methylstyrene (α-MeSt) reported by Ohsumi et al.,1 whereas considerably different results were obtained in the anionic polymerization.

NMR study of poly(vinyl formate)

AbstractThe high resolution nuclear magnetic resonance spectrum of poly(vinyl formate), yielded, upon examination at 100 Mc./sec., and under the conditions of decoupling, information on the three tactic forms present. The normal and decoupled spectra indicate that only the CHO resonance is sensitive to the stereochemical configuration. The three components of the CHO resonance are tentatively assigned to isotactic (i), heterotactic (h), and syndiotactic (s) triads with increasing field strength, respectively. This assignment was made on the basis of poly(vinyl alcohol) and poly(vinyl acetate) derived from poly(vinyl formate). The results show that the tacticity is slightly dependent upon the temperature of free‐radical polymerization.

NMR Studies of Polyaldehydes

Abstract The large differences in chemical shifts that result from oxygen's being incorporated into the polymer backbone yield more accurate qualitative and quantitative analysis through NMR than those polymers that do not have oxygen in the backbone. Configuration effects are observed without interference from methylene groups that frequently interfere in the analysis of polymers. However, the results obtained on polyacetaldehyde with the new 200-MHz Varian Instrument at ambient temperature show a much more complete definition of the configuration than can be obtained with lower-frequency instruments. They show that there are more isotactic triads than syndiotactic triads for the methyl group in “atactic” polyacetaldehyde and that this polyacetaldehyde is nearly a random polymer. By means of the new 220-MHz Varian Instrument and heating of the samples to elevated temperatures for better-resolved lines complete configurational analysis of polyacetaldehyde should become possible.

Configuration of vinyl polymers from high resolution nuclear magnetic resonance spectra. I. Poly(vinyl chloride)

AbstractHigh resolution nuclear magnetic resonance spectra of a number of a number of samples of poly(vinyl chloride) have been obtained. The methinyl proton spectrum consists of three quintets which have been assigned to isotactic, heterotactic and syndiotactic triads. Determinations of the relative numbers of each triad in several samples of polyvinyl chloride indicate that the degree of stereoregularity is not strongly affected by changes in polymerization solvent or temperature.The various conflicting analyses which have been previously suggested for the polyvinyl chloride methylene proton spectrum are also discussed. The 100 Mc.‐spectrum of poly(α‐deuterovinyl chloride) indicates that the methylene proton spectrum consists of two overlapping triplets from isotactic and syndiotactic diads. The possibility that the methylene proton spectrum is complicated by the presence of branching in poly(vinyl chloride) is also suggested.

Configuration of vinyl polymers from high resolution nuclear magnetic resonance spectra. III. Poly(vinyl alcohol)

AbstractHigh resolution NMR‐spectra of poly(vinyl alcohol) samples have been obtained and analyzed. The methinyl proton spectrum consists of three quintets whose intensity can be related to the number of isotactic, heterotactic, and syndiotactic triads. The methylene proton spectrum consists of two triplets from isotactic and syndiotactic diads. The NMR‐spectra indicate that poly(vinyl alcohol) prepared by hydrolysis of poly(vinyl acetate) is essentially atactic, poly(vinyl alcohol) obtained from poly(vinyl t‐butyl ether) is predominately isotactic, and poly(vinyl alcohol) from poly(vinyl trifluoroacetate) is slightly syndiotactic. The influence of conformation on the NMR‐spectrum of poly(vinyl alcohol) is also briefly discussed.

Investigation of polyacetaldehyde structure by high resolution nuclear magnetic resonance

AbstractWe examined the configurations of cationically produced amorphous polyacetaldehyde by high resolution NMR spectroscopy at high temperature in such solvents as dimethylformamide and aniline. Our results show that the polymer backbone is composed of heterotactic and isotactic triad sequences, the former predominating over the latter. Using a 60 Mcycle instrument with or without spin decoupling, we were unable to detect any syndiotactic triad sequences. In addition, the 60 Mcycle NMR spectrum of poly‐α‐deuteroacetaldehyde shows the presence of only the same two stereochemical triads as above. However, examination of polyacetaldehyde with a 100‐Mcycle instrument indicated that a small shoulder exists on the low field side of the heterotactic peak which is probably caused by a small syndiotactic triad content. We used these findings to propose a mechanism for the cationic polymerization of acetaldehyde. We suggest that acetaldehyde may exist in a dimeric form under polymerization conditions leading to a preferred incorporation of mesodimeric units into the propagating chain. Model compound studies involving α,α′‐dimethoxyethyl ether are consistent with this postulate.