Thermal Degradation Of Polyisobutylene Research Articles

Effects of the molecular weight of molecular chains constituting the reaction medium on the thermal degradation of polyisobutylene

Non-volatile oligomers obtained by the thermal degradation of polyisobutylene were finely fractionated and the distribution of the functional groups of each fraction was characterized by a kinetic approach including the intermolecular hydrogen abstraction of primary and tertiary terminal macroradicals (R p· and R t·) and volatile small radicals (S·), followed by β scission. The observed value of the composition ratio [TTD]/[NTTD], which is related to the ratio of the competitive β scission rates of a secondary on-chain macroradical, increases monotonically with decreasing molecular weight ( M) of the fractions. This result shows clearly that the reactivity for β scission depends on the segmental motion of the reacting radicals. With respect to the composition ratio [ t-Bu]/[iPr], which is related to the concentration ratio [R p·]/[R t·], the observed value is roughly constant with M for M > M c, but decreases with decreasing M for M < M c, where M c corresponds approximately to the characteristic molecular weight for the entanglement of molten polymer. This ratio for M > M c decreases with decreasing average molecular weight of the molten polymer matrix, owing to an increase in the rate of diffusion-controlled termination. For M < M c, the value of the exponent a determined from the slope of the double logarithmic plots of [ t-Bu]/[iPr] against M is about 0.5 in all cases. This suggests that the hydrogen abstraction of S· depends on the chain dimension of unentangled polymer. It is confirmed from these results that the degradation reaction of all the polymer molecules in an homogenous reaction medium is entirely affected by the segmental motion and self-diffusional motion of the degrading polymers, and the effect of the chain dimension is observed only for unentangled polymer molecules during the degradation.

Effect of molecular weight on formation of non-volatile oligomers by thermal degradation of polyisobutylene and its kinetic analysis

The formation of functional groups of non-volatile oligomers by the thermal degradation of polyisobutylene is characterized by a kinetic approach including intermolecular hydrogen abstraction of primary (p) and tertiary (t) terminal macroradicals (R p • and R t •) and volatile small radicals (S •), followed by β-scission. By assuming in the kinetic analysis that the reaction occurs competitively under a steady state regarding the on-chain macroradicals, various composition ratios for the functional groups can be represented in terms of the rates of respective hydrogen abstraction. The ratio between the tert-butyl endgroup (t-Bu) and the isopropyl endgroup (i-Pr), which corresponds to that between the abstraction rates of R p • and R t • , is expressed by the product of the rate constant ratio and the integrated macroradical concentration ratio (R p •]/[R t •]). The observed value of the ratio [ t-Bu] [ i-Pr] decreases with reaction time. This is induced by a decrease in the molecular weight ( M) of the reaction medium. The molecular weight dependence ( M a ) of [ t-Bu] [ i-Pr] is expressed by that of R p bull;/[R t •]. The value of exponent a was determined to be about 1.2 and 0.7 at 300 and 320°C, respectively, from the analysis of data at various reaction times. These values are roughly consistent with average values (1.0 and 0.9) of the same power-law exponent for the volatile oligomers. These results support the hypothesis that the concentrations of the respective radicals decrease in different ways.

Thermal degradation of polyisobutylene: effect of end initiation from terminal double bonds

The effect of terminal double bonds on the thermal degradation of polyisobutylene was examined by using a structurally well-defined sample ( M n0 = 7.09 × 10 3) prepared by hydrogenation. The end initiation reactions from a terminal trisubstituted double bond (TTD) and a terminal vinylidene double bond (TVD) lead to the formation of a primary terminal macroradical (R p·) and a tertiary terminal macroradical (R t·), respectively. It is expected from kinetic analysis that the extent of occurrence of R p· is 2–3 times higher than that of R t· depending on the functionalities f TTD and f TVD and M n of the reacting polymer, in addition to the random initiation from the scission of skeletal CC(C) 2, and that the stationary ratio [R p·] [R t·] becomes slightly lower by hydrogenation. The effect of hydrogenation was found to be a decrease in the overall rate of degradation, owing to a marked decrease in the end initiation rate. However, the integrated concentration ratio [R p·] [R t] , which is estimated from the composition ratio of the interesting components, does not decrease, but increases. This result shows that this concentration ratio scarcely depends on the initiation reactions. A similar relationship between the ratio [R p·] [R t] and M n of the molten polymer matrix was found for the degradation of samples with and without TVD and TTD. This result supports the view that [R p·] decreases markedly with decreasing M n during the degradation, owing to a marked depression of the regeneration rate of R p· resulting from an increase in the rate of diffusion-controlled termination.

Thermal degradation of polyisobutylene: effect of rotational motion around CC· bond on the β scission leading to monomer formation

The formation of isobutylene monomer and volatile oligomers by the thermal degradation of polyisobutylene at 300 °C is simulated by using assigned values of the rate constants ( k td and k pd) of the depolymerization of primary and tertiary terminal macroradicals (R p· and R t·), based on a radical chain reaction model including diffusion-controlled termination reactions. With increasing reaction time, the composition of isobutylene monomer in the volatiles increases and that of the volatile oligomers decreases. This change in composition is well matched by the simulation using the value of k td larger than k td. This result is caused by a marked decrease in [R p·] and a gradual decrease in [R t·] with reaction time. The difference in the rate constants of the radicals R p· and R t· is estimated to be k td k pd = 50 − 100 and the ratio of rate constants corresponds to the values 18.8–21.7 kJ/mol of the difference of activation energy ΔE a. This difference is reasonably explained by assuming that the β scission depends on the energy barrier of rotation of the CC· bond of R p· and R t· whose value is estimated from an extended Hückel molecular orbital calculation on the molecular model of R p· and R t·.

Preparation and characterization of end-reactive oligomers by thermal degradation of polyisobutylene

The chemical structure of the non-volatile oligomers ( M n = 2600–9000) isolated from the polymer residues obtained by the thermal degradation of polyisobutylene at 300 and 320°C was determined by 400 MHz 1H n.m.r. spectroscopy with regard to the reactive end groups. The functional groups formed in the degradation process are as follows: tert-butyl end group (t-Bu), isopropyl end group (i-Pr), terminal trisubstituted double bond (TTD), terminal vinylidene double bond (TVD), and non-terminal trisubstituted double bond (NTTD). The average number of TTD and TVD per molecule ( f t) is in a range from 1.46 to 1.64. This value suggests that 53–67 mol% of the non-volatile oligomers are telechelic oligomers having both TTD and/or TVD, and 39-29 mol% of the oligomers are macromonomer-like oligomers having a double bond at one chain end. Both t-Bu and i-Pr are produced by the intermolecular hydrogen abstraction of primary and tertiary terminal macroradicals, and the subsequent β scission of the resulting on-chain macroradicals at the skeletal CC bond of the main chain yields TTD and TVD. The formation of telechelic oligomer is interpreted by a major contribution of the hydrogen abstraction of volatile radicals, in addition to a minor contribution of the hydrogen abstraction of terminal macroradicals.

Thermal degradation of polymers in the melt, 2. Kinetic approach to the formation of volatile oligomers by thermal degradation of polyisobutylene

AbstractThe thermal degradation of polyisobutylene is characterized by kinetics consisting of four types of intramolecular hydrogen abstraction (back‐biting) of primary (p) and tertiary (t) terminal macroradicals (R and R) and the successive β scission at the inner position of the main chain. This reaction affords four types of terminal monoolefins in the volatile oligomers. Assuming the reaction occurs competitively under a steady state regarding the on‐chain macroradicals, the composition of the monoolefins are represented as the rate ratios of the respective back‐biting processes. The rate ratio between the abstractions of different types of hydrogens (CH2 and CH3) from the same type of macroradicals is expressed only by the rate constant ratio. The (TTD)p/(TVD)p and (TTD)t/(TVD)t ratios remain constant during degradation, independently of the decreases in volume and molecular weight of the reacting polymers and this tendency agrees fairly well with the kinetic expectation. This result suggests that back‐biting depends only on the local motion of the reacting chain ends. The ratios between the abstractions of the same type of hydrogen from different macroradicals are expressed by the product of the rate constant ratio and the integrated macroradical concentration ratio ([R]/[R]). The observed values of (TTD)p/(TTD)t and (TVD)p/(TVD)t decrease with reaction time. This decrease results from the decrease in macroradical concentration.

Thermal degradation of polymers in the melt, 1. Characterization of volatile oligomers formed by thermal degradation of polyisobutylene

AbstractChemical structures of a number of components of volatile oligomers including (n+2)‐mers (n ≧ 0) produced by thermal degradation of polyisobutylene were systematically determined by high‐resolution capillary gas chromatogaphy/mass spectrometry (GC/MS). The total ion current (TIC) chromatogram consists of about 100 peaks ranging from dimers (2‐mers, n = 0) to dodecamers (12‐mers, n = 10) and most of the main peaks are classified into four types of terminal monoolefins: a trisubstituted olefin with a tert‐butyl end, a vinylidene olefin with a tert‐butyl end, a trisubstituted olefin with an isopropyl end, and a vinylidene olefin with an isopropyl end. The formation of these monoolefins is reasonably interpreted by intramolecular hydrogen abstractions (back‐biting) of primary and tertiary terminal macroradicals and subsequent β; scissions at the inner position of the main chain. In all the chromatograms of each (n+2)‐mer (n ≧ 1), the retention times of terminal trisubstituted types of monoolefins were shorter than those of terminal vinylidene types of monoolefins, in contrast to the elution order of dimers. The relative intensities between the interesting peaks of each (n+2)‐mer (n ≧ 1) clearly represent that back‐biting more predominantly occurs at the methylene hydrogen rather than at the methyl group consistent with the difference in the bond dissociation energy of the HC bond of interest, as opposed to the steric hindrance mechanism5,7.

The degradation mechanism of polyisobutylene: Part 2. Characterisation of the products and the dependence of their yields on sample thickness provides detailed mechanistic information

The thermal degradation of polyisobutylene has been studied by pyrolysis gas chromatography, using a microthermocouple-controlled resistive filament. All samples were pyrolysed at 400 °C for 10 s. The overall pyrolysis mechanism, an array of both parallel and consecutive processes, has been elucidated by performing a sample size study. This involves the measurement of product yields as a function of sample thickness, for samples in the microgram range. The approach provides an indirect method of varying the residence time of primary products in the polymer melt, and thereby influencing the opportunities for secondary reactions. Results from this work have shown that monomer, dimers and trimers are produced in excess of those yields expected on the basis of primary reactions, whilst oligomers in the range of tetramer to octamer are produced in lower than expected yields. This leads to the conclusion that these higher oligomers are undergoing secondary reactions within the polymer melt to produce lower oligomers. Analysis of the corresponding bonus and deficit yields shows that monomer is the principal secondary degradation product and that oligomer depropagation is consequently a dominant secondary process. Previous work by the authors on the statistical analysis of the yields of decomposition products showed that the degradation of polyisobutylene proceeds essentially by parallel depropagation and random scission mechanisms. The present work has yielded further mechanistic information, and it may now be proposed that polyisobutylene thermally degrades by parallel depropagation with random scission, plus secondary depropagation of oligomers formed by random scission, with only small contributions from transfer processes. This mechanism is contrary to that put forward by Kiran and Gillham and Tsuchiya and Sumi, who proposed that, in addition to depropagation, intramolecular transfer with associated scission was the principal decomposition mechanism.

Thermocatalytic degradation of isobutylene polymers in presence of magnesium chlorides

The thermal degradation of polyisobutylene and butyl rubber in presence of magnesium chloride is activated by chemically bound water as is confirmed by the experimental results and quantochemical calculations. A feature of the thermocatalytic degradation is the combination of the processes of initiation by the “law of chance” and the “law of end groups”.

Thermal Degradation of Polyisobutylene. Effect of Sample Size on Weight Loss

Thermal Degradation of Polyisobutylene. Effect of Sample Size on Weight Loss

The cationic degradation of polyisobutylene

The thermal degradation of polyisobutylene at temperatures above 560°K proceeds as a random free radical degradation of the macrochains. In the presence of aluminium chloride complexes, the decomposition is effected by cationic depolymerization of the macrochains.