Structural Defects In Poly Research Articles

Overview and assessment of recent research on the structural defects in poly(vinyl chloride)

New mechanistic studies of structural defect formation in ordinary poly(vinyl chloride) (PVC) are summarized and critiqued. Evidence relating to the creation of some of the internal double bonds is tentatively suggested to imply the occurrence of hydrogen transfer via one or more cyclic transition states having ≥9 ring atoms. The absence of free chlorine atoms from polymerizations of vinyl chloride (VC) is reaffirmed, and a new mechanism, not yet verified, is identified as a possible explanation for a reported enhancement of the chloromethyl branch concentration at high conversions of monomer. This mechanism involves an intramolecular 1,5 hydrogen shift in a 1,3,5,6-tetrachlorohexyl radical. Copolymerization of VC with the chloroallylic chain ends of PVC is argued to be insignificant, and the new information in the literature is shown not to invalidate the currently accepted chemistry for the polymerization of vinyl chloride by standard free-radical methods.

Structural Defects in Poly(vinyl chloride) and the Mechanism of Vinyl Chloride Polymerization: Comments on Recent Studies

Investigations in the title areas within the past ten years are summarized and critiqued. The polymerizations studied were performed by conventional free-radical methods. A new mechanism, not yet confirmed, is suggested to explain a reported enhancement in the chloromethyl branch concentration of poly(vinyl chloride) (PVC) prepared at high conversions of monomer. This mechanism involves an intramolecular 1,5 hydrogen shift in a 1,3,5,6-tetrachlorohexyl radical. Evidence showing that most of the internal double bonds in PVC are not formed via intermolecular H abstraction from internal monomer units is tentatively rationalized, in part, by hydrogen transfer via at least one cyclic transition state containing more than eight members. The absence of free chlorine atoms from polymerizations of vinyl chloride (VC) is reaffirmed, and the copolymerization of VC with the chloroallylic chain ends of PVC is argued to be insignificant. New information in the literature does not invalidate the currently accepted mechanism of vinyl chloride polymerization.

New Insight into the Formation of Structural Defects in Poly(Vinyl Chloride)

The monomer conversion dependence of the formation of the various types of defect structures in radical suspension polymerization of vinyl chloride was examined via both 1H and 13C NMR spectrometry. The rate coefficients for model propagation and intra- and intermolecular hydrogen abstraction reactions were obtained via high-level ab initio molecular orbital calculations. An enormous increase in the formation of both branched and internal unsaturated structures was observed at conversions above 85%, and this is mirrored by a sudden decrease in stability of the resulting PVC polymer. Above this threshold-conversion, the monomer is depleted from the polymer-rich phase, and the propagation rate is thus substantially reduced, thereby allowing the chain-transfer processes to compete more effectively. In contrast to the other defects, the chloroallylic end groups were found to decrease at high conversions. On the basis of the theoretical and experimental data obtained in this study, this decrease was attributed to copolymerization and abstraction reactions that are expected to be favored at high monomer conversions. Finally, a surprising increase in the concentration of the methyl branches was reported. Although a definitive explanation for this behavior is yet to be obtained, the involvement of transfer reactions of an intra- or intermolecular nature seems likely, and (in the latter case) these could lead to the presence of tertiary chlorine in these defects.

Structural defects in poly(vinyl chloride)

AbstractThis article describes, in narrative style, the research of the author and his associates, performed over a period of 30 years, that led to the identification and quantification of the anomalous structures in poly(vinyl chloride) (PVC) and to detailed descriptions of their mechanisms of formation. Also examined here are the implications of this work for the thermal stability of PVC, for the overall chemical mechanism of vinyl chloride polymerization, and for other relevant aspects of the chemistry and technology of vinyl chloride homopolymers and copolymers. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 2451–2467, 2005

Structural defects in poly(vinylene disulfide)

We have investigated the structural defects in poly (vinylene disulfide), -(S-CH=CH-S) n -, using optical, magnetic and surface spectroscopic techniques. This has been done by comparing the analysis results obtained on pristine samples with those on protic-acid-doped samples. In the undoped sample, the main type of defect is found on carbon broken bonds which gives rise to carbon-oxygen bonding due to oxidation of the sample. In doped samples, the sulfur sites are greatly affected by doping and appear as the main protonation sites. A careful analysis of all the results has enabled the role of the defects in the electrical conduction mechanism to be clarified.

NMR study of the structural defects in poly(3-alkylthiophene)s: influence of the polymerization method

We present an NMR study of structural defects in poly(3-decylthiophene)s synthesized with different methods. The polymers obtained with FeCl 3 oxidation and by electropolymerization show mainly correlated defects in the enchainment. A lower number of defects, which are uncorrelated, is introduced with the Grignard reaction yielding a polymer with the highest mean conjugation length. The extension of the defect free polymer segments is, however, not very different for the different polymers studied.

Spectroscopic Analysis of Structural Defects in Poly(3-decylthiophene)s: Influence of the Polymerization Method

Spectroscopic Analysis of Structural Defects in Poly(3-decylthiophene)s: Influence of the Polymerization Method

Structural defects in poly (methylphenylsilylene)

The Si-based structural defects in poly(methylphenylsilylene) backbones were detected as two new 29 Si NMR signals and as two new Si-Si vibrational IR bands. The defects, consisting of an organosilyne unit and about three methylphenylsilylene monomer units near the branch, were presumed to be in a fairly lengthened Si-Si bonding state. The empirical linear relationship between the relative intensity of broad photoluminescence and the defect density predicts the disappearance of the broad luminescence for a defect density of less than 1%.

Structural defects in poly(vinyl chloride)—V. Thermal and photodegradation of copolymers of vinyl chloride with various acetylene derivatives

Vinyl chloride has been copolymerized with various acetylene derivatives in bulk at 50. It has been shown by ozonolysis that these copolymers contain a significantly increased amount of internal double bonds. A comparison of the thermal and photodegradation behaviours of the copolymers has been made. The built-in double bonds show an effect on the reactivity of the adjacent allylic chlorine depending on the electron-withdrawing or repelling nature of the substituent. Among the comonomers investigated, copolymers containing 3-chloropropine have the least reactive defect sites, because of the electron-withdrawing chloromethyl substituent. On the contrary, the electron-repelling n-butyl group, built-in by using hexine-1 as comonomer, results in enhanced reactivity of the allylic chlorines.

Structural defects in poly(vinyl chloride)—IV. Thermal degradation of vinyl chloride/acetylene copolymers

Vinyl chloride/acetylene copolymers have been prepared under subsaturation conditions. Copolymerization rates and molecular weights of the copolymers decrease with increasing concentration of acetylene in the monomer feed, indicating that acetylene is a retarder in vinyl chloride polymerization. The concentration of internal double bonds in the copolymers determined by ozonolysis increases with increasing amount of acetylene in the feed. Thermal degradation has been performed at 110 with solid samples and at 170° in solution under inert atmosphere. The extent of HCl loss as a function of time shows a rapid initial phase followed by a slower steady rate The initial dehydrochlorination rates are higher for copolymer samples containing higher concentrations of internal double bonds. Quantitative analysis of the u.v. and visible spectra of degraded copolymers shows that the sum of the concentration of polyenes with 4–12 conjugated double bonds increases rapidly in the first phase of degradation, but then decreases slowly, due to secondary reactions of polyene sequences.

Structural defects in poly(vinylchloride)—III. Degradation of vinyl chloride-phenylacetylene copolymers in solution

Vinylchloride-phenylacetylene copolymers have been prepared and characterized. A known amount of defined defect sites, viz. double bonds, has been introduced into the main chain of the polymer. Thermal degradation behaviour of the copolymers has been studied in trichlorobenzene solution. A strong dependence of the kinetics of dehydrochlorination and polyene formation on the defect concentration has been found. Rate constants and activation parameters have been determined. The effects of different structures have been compared.