A Kinetic Study of the Hydrochlorination of Natural Rubber and Synthetic Polyisoprene Latices

Taylor, James S (1954) A Kinetic Study of the Hydrochlorination of Natural Rubber and Synthetic Polyisoprene Latices. PhD thesis, University of Glasgow.

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Abstract

The kinetics of the hydrochlorination of natural and synthetic polyisoprene are studied in order to amplify the very limited information previously recorded in the literature regarding the nature of the reaction. The polymers are reacted in a latex process, and the accumulation of precision measurements permits a much clearer understanding of the mechanism, rate control, and loci of the reaction. The reaction progress is followed kinetically by the use of a convenient density-gradient technique for measuring accurately the increase in density of the polymer during its hydrochlorination. Extent of reaction (% hydrochlorination) is evaluated from the density change by means of a calibration provided by semi-micro potentiometric titration chlorine analyses. A chain mechanism is postulated for the hydrochlorination of Hevea rubber latex on the basis of Bunn and Garner's accepted X-ray structure of rubber hydrochloride crystals (1). This mechanism proposes the coiling of a rubber chain segment into a trans-decalin-shaped transition state which is necessary and sufficient to explain the alternation along the rubbbr hydrochloride polymer chain of D- and L-asymmetric carbon atoms each separated from the next by three CH2groups. It is proved that the hydrochlorination of polyisoprene latices occurs at two different reaction loci. The main reaction takes place within the bulk of the polymer particles. Initially superimposed on this however, there is a reaction at the interphase between the particle surface and the aqueous acid. The two reaction loci react at different rates and by different mechanisms. Despite the fact that natural and synthetic polyisoprenes have different detailed chemical structures, practically identical zero-order rate constants applyto both substrates for the main (unretarded) hydrochlorination. . It follows that the substrates themselves do not participate in the rate control, since the rate of chain reactions which might give zero-order constants are very sensitive to detailed polymer structure. Energetic and concentration factors support the deduction that the rate is controlled by a purely inorganic step involving the formation of an ion pair (H+,Cl-) from solvated precursors in the rubber medium (H2Cl+ + Cl-). The surface reaction affects only the isoprene units lying within 15+/-2.5A of the particle surface, i. e. about 2.5% of the rubber in Hevea latex and up to over 20% in synthetic polyisoprene latex of sufficiently small particle size. The increased extent of surface hydrochlorination in the synthetic substrate permits kinetic measurements on it. The surface effect can be isolated at low pressures of hydrogen chloride at which the bulk rate becomes negligible. The high order surface reaction rate is controlled by the diffusion into the polymer of a reactive hydrogen chloride species of very limited stability in rubber. The retarded zero-order bulk reaction, evident only in Hevea latex hydro chlorination, is proved to be due to the presence of a basic retarder in trace amounts within the rubber particles. Although 100% hydrochlorination of synthetic polyisoprene latex is possible, the reaction with Hevea rubber latex comes to a standstill well before stoichiometric completion - a fact probably primarily due to the onset of crystallinity in natural rubber hydrochloride. The influence, on both the surface and the bulk hydrochlorination rates, of prior sulphur vulcanization of synthetic polyisoprene latex is examined. The resulting rate curves suggest that vulcanization itself has a surface locus also. The reaction kinetics are shown to be reproducible and free from uncontrolled effects despite the complexity of the latex medium ( especially Hevea latex).

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Keywords: Polymer chemistry
Date of Award: 1954
Depositing User: Enlighten Team
Unique ID: glathesis:1954-79111
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 05 Mar 2020 11:42
Last Modified: 05 Mar 2020 11:42
URI: http://theses.gla.ac.uk/id/eprint/79111

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