Nucleophilic substitution on PVC effect of the tacticity

Summary The substitution reaction with NaSC₆H₅ is studied for two samples of PVC having very different tacticities. From both the kinetic results and the thermal degradation of the modified polymers it is concluded that the observed very fast initial period of the reaction involves selectively a small fraction of isotactic and heterotactic triads, and that such triads are likely to be considered as labile structures in PVC. The results are therefore a contribution to the understanding of the thermal instability of PVC.     

A comprehensive approach to the stereochemical and physical factors in nucleophilic substitution on PVC in the melt

The nucleophilic substitution reactin of poly(vinyl chloride) (PVC) with sodium benzenethiolate (NaBT) has been studied in the melt with the aim of determining whether a conformational mechanism is applicable under conditions that would apply in the polymer processing. The evolution of unreacted syndio, hetero, and isotactic triads content, and of the apparent molecular weight has been followed by ¹³C-NMR and GPC, respectively. The kinetic behavior was defined by choosing appropriate mixing conditions of temperature, shear stress, load, and amount of plasticizer. In all cases the kinetic curves attain a plateau after a rather fast reaction period has elapsed. The conversion at the plateau, that is, the reaction efficiency, proved to depend linearly on the amount of nucleophile and is hardly influenced by the mixing conditions for a given stoichiometric composition. The mixing conditions have been found to affect markedly the kinetics but to be quite inoperative on the evolution of both the content of unreacted tactic triads and the apparent molecular weight with degree of conversion. The results are discussed by taking into consideration those previously obtained in solution.     

A study of the CCl stretching region of the Raman spectrum of PVC

The C-Cl stretching regions of the Raman spectra of a number of samples of PVC prepared by various methods which lead to differences in tacticity, conformational content and crystallinity have been studied. It is shown that the spectra of all samples can be fitted well with a set of nine Lorentzian peaks whose positions and widths are the same for each spectrum. Two of these peaks are attributable to the A_g and B_3g species vibrations of crystalline material and the remaining seven may be assigned with reasonable confidence to specific structures in the amorphous regions, three of which are associated with isotactic placements. The area under the A_g and B_3g peaks may be used to estimate the crystallinity of the samples and the area under the peaks assigned to isotactic material may be used to estimate the tacticity of the polymer.     

Influence of tacticity on the thermal degradation of PVC. VI. New advances in the degradation process through the behavior of modified PVC samples

Three PVC samples having different tacticities, as well as the products resulting from their reaction with sodium thiophenate, which was previously found to occur selectively by some definite isotactic conformations, were degraded at 180°C in solid state up to 0.3%. In agreement with prior works, the stability of the substituted polymers was shown to be higher as the substitution extent increased up to a definite value, which depends on the overall isotactic content of the starting materials. On the other hand, the found changes in polyene distribution for the degraded samples with the substitution extent, as followed by UV-visible spectroscopy, demonstrate that the specific bands at 393, 416, and 437 nm, which are known to be characteristic for the most unstable PVCs, arise from the lability of some chlorine atoms located at GTTG′ (or TTTG) conformations, as prior results suggested. On the basis of the so-obtained correlation between the content of some isotactic triads (especially the GTTG′) and both the thermal instability and the formation of the above-indicated specific polyenes, new advances in the degradation mechanism are proposed.     

Influence of tacticity on thermal degradation of PVC. V. Relation between the nature of labile conformations and the polyene distribution in the degraded polymer

Four poly(vinyl chloride) (PVC) samples, 1, 2, 3, and 4, were prepared in bulk using 2,2′-azodiisobutyronitrile at 90°C and 60°C and the same initiator together with UV irradiation at 0°C and ?50°C, respectively. Fractions were obtained from samples 2,3, and 4 by extracting with acetone, sample 1 being completely soluble in this solvent. The whole PVC's as well as their acetone soluble and insoluble fractions, were characterized by determining the intrinsic viscosity, the osmometric molecular weight, and the tacticity; then they were thermally degraded up to conversion of 0.3% in powder state. The values of degradation rate for both the insoluble fractions and the whole PVC's were in agreement with some prior results on the influence of syndiotactic sequences on propagation step. The soluble fractions proved to be very unstable in comparison with the insoluble, which, despite the lower molecular weight of the former, seems to obey their higher content of isotactic conformations. The fine polyene distribution in the degraded polymers was carried out by UV–visible spectroscopy. The results reveal the occurrence of two different polyene distributions, and allow for the peculiar one of the soluble fractions to be related to polyenes consisting of two sequences of trans conjugated double bonds separated by a single cis double bond. The results clearly shows that there are two mechanisms for initiation of the PVC degradation, depending on whether it occurs by random unstable structures or by the normal GTTG isotactic or TTTG heterotactic triads. Moreover, these two initiation processes are proved to give rise to different types of polyenes, which accounts for the occurrence of two unlike mechanisms of propagation.     

Propene polymerization with MgCl2-supported TiCl4/dioctylphthalate catalyst. II. Effects of polymerization conditions on the microstructure of isotactic polymer

Propene was polymerized over the MgCl₂-supported TiCl₄/dioctylphthalate catalyst in heptane. Polymer products obtained under different polymerization conditions were separated into isotactic and atactic polypropenes by the extraction of boiling n-heptane. The effects of polymerization time, cocatalyst type, cocatalyst/catalyst ratio, polymerization temperature, and external base/cocatalyst ratio on the isotactic triad of the isotactic portion of polypropene were investigated 2,2,6,6-Tetramethyl piperidine (TMPIP), dimethoxy diphenyl silane (DMDPS), and t-butylmethyl ether (TBME) were employed as the external Lewis base. High concentrations of the first two bases caused a decrease in isotactic triads in the isotactic polymer, while TBME showed no significant effects. The difference can be attributed to the different roles these external bases play in polymerization. © 1995 John Wiley & Sons, Inc.     

Computational simulation of poly(vinyl chloride) reduction with tri‐n‐butyltin hydride. The use of revised rate constants

The computer simulation of the reductive dechlorination of poly(vinyl chloride) (PVC) with Bu₃SnH is reexamined by using a new set of rate constants that has been corrected (a) statistically for the presence of different numbers of chloro substituents and (b) for a triad reactivity factor used to determine the constants for rr‐and mm‐VVV triads from that for mr‐VVV triads. The simulation method is verified by comparison with analytic kinetic results for the reduction of 2,4‐dichloropentane and 2,4,6‐trichloroheptane model compounds. The revised rate constants should and do lead to a smaller ratio of racemic to meso diads in PVC as reaction nears completion, compared to the ratio obtained from the original set of rate constants that had been proposed by previous workers. Experimental data are not sufficiently accurate to distinguish between the two sets of simulation results.     

Formation of anomalous structures in PVC and their influence on the thermal stability: 2. Branch structures and tertiary chlorine

Branch structures were determined in fractions of a commercial suspension of PVC (S-PVC) and experimental PVC samples obtained at subsaturation conditions (U-PVC). The analyses were performed with ¹³C n.m.r. spectroscopy at 50.31 MHz after reductive dehalogenation with tributyltinhydride. With increasing monomer starvation U-PVC was found to have an increasing amount of butyl and long chain branches (LCB). A polymer prepared at 55°C and 59% of the saturation pressure of vinylchloride had 3.4 butyl branches and 2.0 LCB per 1000 monomer units. In the S-PVC series the total content of these two structures varied between 0.5 and 1.0 per 1000 monomer units. By using tributyltindeuteride as reducing agent the structure of the butyl branches could be determined as ～CHClCH₂CCl(CH₂CHClCH₂CH₂Cl)CH₂CHCl～. A major part of the LCB points also contained tertiary chlorine. The formation of LCB is suggested as occurring after abstraction of hydrogen from the polymer chain by macroradicals and chlorine atoms. The latter will lead to LCB points with tertiary hydrogen and internal double bonds. The rate of dehydrochlorination at 190°C in nitrogen could be related to the amount of tertiary chlorine (correction coefficient=0.97). It was assumed that tertiary chlorine is the most important labile structure in PVC.     

Effect of elemental sulfur in the crosslinking reaction of poly(vinyl chloride) with tetramethyl thiuram disulfide

The effect of elemental sulfur (S₈) and tetramethyl thiuram disulfide (TMTD) on the chemical crosslinking of PVC is studied. The tensile strength, elongation at break, and gel content of crosslinked and uncrosslinked PVC molded samples, cured at different times in hot air, are evaluated. It was found that crosslinking PVC with TMTD improves its mechanical properties, and adding S₈ produces a slight improvement. Also, the S₈ presence accelerated the PVC/TMTD crosslinking reaction when the crosslinking reaction was carried out under shear forces, but not when the curing was done in hot air, where the S₈ retards the process.     

On the coupling in molecular motions: A novel molecular micro-structure approach to the physical meaning of the Ngai's coupling parameter

Summary The dielectric behaviour through DETA system has been studied for six PVC samples of distinct molecular microstructure in terms of content and associated surroundings and conformation of the local configurations mmr termini of isotactic sequences. Both the dielectric studies and the microstructure of the samples, as obtained by stereospecific substitution reaction of an additive-free commercial PVC are to be published in detail separately. For purposes of clarity some of the results are mentioned in Experimental section. The phenomenological correlation between the β parameter as defined by Ngai's theory and the microstructure of the samples, quantities both issued from the above quoted work, is considered in the present paper with the objective of providing some novel ideas on the physical meaning, at molecular level, of the Ngai's coupling parameter, in the light of mobility constraints and local free volume associated with mmr structure at its likely conformations. Received: 16 March 2001/Revised version: 18 June 2001/Accepted: 24 July 2001     

The experimental results and simulation of temperature dependence of brittle‐ductile transition in PVC/CPE blends and PVC/CPE/nano‐CaCO3 composites

The effect of chlorinated polyethylene (CPE) content and test temperature on the notched Izod impact strength and brittle‐ductile transition behaviors for polyvinylchloride (PVC)/CPE blends and PVC/CPE/nano‐CaCO₃ ternary composites is studied. The CPE content and the test temperature regions are from 0–50 phr and 243–363 K, respectively. It is found that the optimum nano‐CaCO₃ content is 15 phr for PVC/CPE/nano‐CaCO₃ ternary composites. For both PVC/CPE blends and PVC/CPE/nano‐CaCO₃ ternary composites, the impact strength is improved remarkably when the CPE content or test temperature is higher than the critical value, that is, brittle‐ductile transition content (C_BD) or brittle‐ductile transition temperature (T_BD). The T_BD is closely related to the CPE content, the higher the CPE content, the lower the T_BD. The temperature dependence of impact strength for PVC/CPE blends and PVC/CPE/nano‐CaCO₃ ternary composites can be well simulated with a logistic fitting model, and the simulation results can be illustrated with the percolation model proposed by Wu and Jiang. DMA results reveal that both PVC and CPE can affect the T_BD of PVC/CPE blends and PVC/CPE/nano‐CaCO₃ composites. When the CPE content is enough (20 phr), the CPE is more important than PVC for determining the T_BD of PVC/CPE blends and PVC/CPE/nano‐CaCO₃ composites. Scanning electron microscopy (SEM) observations reveal that the impact fractured mechanism can change from brittle to ductile with increasing test temperature for these PVC systems. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

On the stereochemical composition of poly(vinyl chloride) (PVC) and polypropylene (PP): a phenomenological study

The number of mmr‐ and rrm‐based structures which occur necessarily whenever an isotactic or a syndiotactic sequence breaks off respectively; the extent to which they are isolated or are extended to atactic sequences, and the fact that the mmr repeating sequence, especially when it takes the GTTG^?TT conformation, is shorter and exhibits greater local free volume than rrm, mmm and rrr sequences, are shown to be the stereochemical composition determining structures in poly(vinyl chloride) (PVC) and polypropylene (PP) polymers. These structures, as analyzed by ¹³C NMR spectroscopy and probability calculations, have been determined as a function of the overall tacticity for one series of samples prepared by bulk polymerization at temperatures varying from ?50 °C to 70 °C, and one series of samples prepared by polarity‐based fractionation of a commercial polymer. Using this approach, the stereochemical composition of the samples could be identified. The results are of interest to understand the changes in the physical properties as shown in earlier and current work for both series of polymers. © 2003 Society of Chemical Industry     

Crystallization properties of elastomeric polypropylene from alumina-supported tetraalkyl zirconium catalysts

An analysis of the crystallization properties of fractions of elastomeric polypropylene (ELPP) prepared with Al₂O₃-supported tetraalkyl zirconium catalyst is presented. A comparison with the polymorphic behavior of isotactic polypropylene (iPP) samples prepared with a single center homogeneous metallocene catalyst is also shown. The ELPP sample has been fractionated by extraction with boiling solvents. The irregular fraction insoluble in pentane and soluble in hexane crystallizes from the melt almost totally in the γ form, whereas the more stereoregular fraction, insoluble in n-heptane, crystallizes mainly in the α form. The relative amount of γ form crystallized from the melt is much lower than the one formed in samples of metallocene-made iPP samples with a similar average content of isotactic stereosequences. Since the γ form crystallizes in chains having short regular isotactic sequences, these data indicate that in the fractions of the ELPP sample the regular isotactic sequences are longer than those present in chains of metallocene-made iPP. In particular, in the more irregular crystalline fractions of the ELPP sample the chains are characterized by a stereo-block microstructure, consisting in the presence of quite regular isotactic sequences alternating with irregular sequences, the latter containing most of the defects. The presence of stereoblocks allows crystallization of these highly irregular fractions, even in the presence of a very high content of defects, and accounts for the elastomeric properties shown by this sample. The whole ELPP sample is constituted mostly by amorphous ether soluble (40%) and pentane soluble (26%) fractions and shows elastic properties due to the high molecular weight of chains in all the fractions and the crystallization of isotactic sequences present in the stereoblocks.     

Dielectric and mechanical properties of poly(vinyl chloride)–dioctylphthalate systems

The mechanical properties, tensile strength, and elongation were investigated for poly(vinyl chloride) (PVC) samples mixed with dioctylphthalate (DOP) at concentrations from 0 to 100 parts per hundred parts PVC at 23°C. It was found that the tensile strength decreased with the increase of concentration, and the elongation was increased until a concentration of 30 DOP content, and then decreased. This leads to the suggestion that intermolecular plasticization is dominant until 30 DOP content, while interstructural plasticization is prevailing for higher concentrations. The permittivity ε′ and the dielectric loss factor ε″ of the same samples have been measured in the frequency range 10²–10⁵ Hz at temperatures from 3 to 96°C. Results show that as the DOP content increases in PVC, the dielectric absorption becomes broader, and the glass transition temperature T_g is lowered. The magnitude of the loss peak decreases with an increase of DOP content to a minimum at concentrations from 40 to 60 DOP content. At higher concentrations the loss peak is increased and T_g is unaltered. Another absorption was observed at 100 Hz and at high temperatures, which was attributed to Maxwell–Wagner effect or direct current conductivity or both of them. It was found that the sample containing 40 parts DOP in 100 parts PVC possesses the best mechanical and electrical properties.     

Preparation and characterization of poly(vinyl chloride)–continuous carbon fiber composites

In this article, we report on the preparation and characterization of novel poly(vinyl chloride) (PVC)–carbon fiber (CF) composites. We achieved the reinforcement of PVC matrices with different plasticizer contents using unidirectional continuous CFs by applying a warm press and a cylinder press for the preparation of the PVC–CF composites. We achieved considerable reinforcement of PVC even at a relatively low CF content; for example, the maximum stress (σ_max) of the PVC–CF composite at a 3% CF content was found to be 1.5–2 times higher than that of the PVC matrix. There were great differences among the Young's modulus values of the pure PVC and PVC–CF composites matrices. The absolute Young's modulus values were in the range 1100–1300 MPa at a 3% CF content; these values were almost independent of the plasticizer content. In addition, we found a linear relationship between σ_max and the CF content and also recognized a linear variation of the Young's modulus with the CF content. The adhesion of CF to the PVC matrix was strong in each case, as concluded from the strain–stress curves and the light microscopy and scanning electron microscopy investigations. The mechanical properties of the PVC–CF composites with randomly oriented short (10 mm) fibers were also investigated. At low deformations, the stiffness of the composites improved with increasing CF content. Dynamic mechanical analysis (DMA) was used to determine the glass‐transition temperature (T_g) of the PVC–CF composites. The high increase in the Young's modulus entailed only a mild T_g increase. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Influence of the tacticity of poly(methyl methacrylate) on the miscibility with poly(vinyl chloride)

It can be concluded from the work of Schurer et al.¹⁰ that poly(vinyl chloride) (PVC) is more miscible with syndiotactic than with isotactic poly(methyl methacrylate) (PMMA). By choosing different molar masses for the various tactic forms of PMMA it is possible to obtain blends with PVC with similar phase behaviour, i.e. in all cases a cloud-point curve with a minimum in the vicinity of 190°C. In this way a more quantitative statement about the influence of the tacticity of PMMA on its miscibility with PVC can be made. One of the principal differences between syndiotactic or atactic PMMA and isotactic PMMA is the higher flexibility of the latter. Using Flory's equation of state theory it will be shown that the effect of this difference is large enough to explain the difference in phase behaviour observed. Heats of mixing of low molar mass analogues were also measured and found to be negative.     

Effect of metal stearate stabilizers on the thermal degradation of PVC in solution: The reversible blocking mechanism of stabilization

Systematic thermal degradation studies of PVC solutions have been carried out in the presence of different metal carboxylates (Pb-, Cd-, Ba-, Ca- and Zn- stearates). ZnSt₂ markedly accelerates degradation of the polymer. Significant induction periods (t_i) for the appearance of free HCI are obtained in the presence of the other salts. CaSt₂ acts mainly as an HCI-scavenger and has no direct influence on the elimination process. Mainly short polyenes are formed during the induction period when PbSt₂, CdSt₂, and BaSt₂ are used. After consumption of stabilizers rapid HCI loss occurs, and suddenly longer polyenes are formed, i.e., the amount of double bonds and the average length of polyenes sharply increases. At the same time, the rate of initiation (i.e., the rate of the polyene sequence formation) does not change at all around t_i. None of the investigated stabilizers retard effectively the random initiation of HCI loss. In the presence of PbSt₂, BaSt₂, and CaSt₂, and CaSt₂, the initiation rate is nearly identical to that of the unstabilized PVC. The concentration of labile sites (h₀) in PVC at the end of the induction period has been estimated by kinetic analysis. It has been found that in some cases h₀ is higher than the concentration of labile structures in the virgin PVC. These results indicate that the main role of metal soap stabilizers is the blocking of the fast zip-elimination of HCI from the PVC chain. It is likely that blocking occurs by attachment of a carboxylate group at the end of a propagating zip. This is a reversible process: the blocked structures become active again mainly after the consumption of stabilizers presumably by HCI-catalyzed hydrolysis. In contrast to other stabilization mechanisms by Frye and Horst, Minsker and coworkers, and Michell the reversible blocking mechanism of PVC stabilization is able to explain the experimental findings presented in this study. It also explains such facts of practical importance as color stability of the resin during the induction period and fast blackening after the consumption of stabilizers.     

Effect of copolymers modifying PVC on its physical and mechanical properties and its UV-radiation resistance. III. Changes in thermal stability of PVC films modified by MMA/MA,VC/VAC,MBS or ABS induced by UV-irradiation

The influence of UV-radiation on thermal stability of PVC films, depending on the type and quantity of modifying copolymers, was studied. It was found that UV radiation can induce various changes in thermal stability of PVC decreasing its dehydrochlorination temperature T⁰₁ while increasing temperature of decomposition T⁰₂. Particularly distinct changes in sampels containing VC/VAC indicate that even 1% of this copolymer accelerates the photodehydrochlorination and the processes which decrease the temperature of decomposition of PVC. Smaller changes than in pure PVC in thermal parameters in samples containing ABS after irradiation show that this copolymer increases the PVC resistance to UV radiation. Films containing MBS, ABS, MMA/MA reveal the largest changes in T⁰₁ after irradiation in the case of a content of 1% of these copolymers in the samples. In the case of bigger quantity (3 to 10%) the changes are smaller and may even diversify their direction. These copolymers, contrary to VC/VAC, make PVC films a little turbid indicating the limited compatibility of both components. Hence, the conclution is that the structure of a film, depending on compatibility of components and changing with their differing concentration, has decisive influence on the thermal and photochemical processes.     

Dynamics of macromolecular chains—ultrasonic study of polystyrene in solution

Ultrasonic relaxation measurements on 2,4,6,8-tetraphenylnonane and a series of narrow molecular weight samples of polystyrene dissolved in ethyl benzene are reported as a function of temperature and frequency. These data are compared with previous ultrasonic studies on the configurational isomers of 2,4-diphenylpentane and 2,4,6-triphenylheptane which have shown that the relaxation frequency of the isotactic triads or meso diads is always much lower than that determined for the syndiotactic triads or racemic diads. The ultrasonic spectra are thus resolved to give two relaxation processes. The origin of the highest frequency relaxation, which needs up to ten monomer units, is thought to be an intramolecular process of the syndiotactic sequences. In this model, several segments of the polymer chain independently undergo the conformation change:

$\text{gg}\text{tt gg tt}\text{gg}\text{?}\text{tt}\text{tt gg}\text{tt}$

The absorption of energy in the low frequency range may be associated with a crankshaft motion.     

Microwave plasma reactions of imidazole on poly(vinyl chloride) surfaces: A spectroscopic study

Although there are various forms of energy available for conducting surface and interfacial reactions, microwave plasma energy is an attractive means for surface modifications because it is fast and it usually does not alter bulk properties. In this study, a closed‐system microwave plasma reactor was used to react imidazole molecules to poly(vinyl chloride) (PVC) surfaces. Newly created surfaces were analyzed using attenuated total reflectance (ATR) Fourier‐transformed infrared (FTIR) spectroscopy. These studies show that surface reactions on PVC are heavily dependent upon a prior thermal history of the PVC substrate. It appears that the plasma reactions on hot‐pressed PVC not only result in the development of CH₂ linkages, but a significant increase of crystallinity in the hot‐pressed PVC inhibits the reactivity of imidazole to the PVC surface. On the other hand, for a solvent‐cast PVC with a significantly lower surface crystalline phase content, imidazole reacts to the PVC surface through CC bond opening. The amount of imidazole reacted to the PVC surface changes with the depth from the surface. Using quantitative ATR FTIR spectroscopy, imidazole content can be quantified, and its concentrations are in the 10⁻⁶ mol/cm² range at about 0.8–1.2 μm for the PVC surface. A mechanism of the PVC–imidazole reactions is also proposed. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 1–6, 1999