Chlorination Of Natural Rubber Research Articles

On the structure of chlorinated natural rubber. 1H- and 13C-NMR data

1H- and 13C-NMR spectra of compositionally homogeneous samples of chlorinated natural rubber containing 0.4-3.0 Cl/C 5 have been investigated. In the samples of composition Cl/C 5 ⩽ 1 there are no units containing > 1 chlorine atom, meaning that chlorine docs not add to CC double bond. Simultaneously, unsaturation of the polymer decreases by approx. 10%. These data support a hypothesis about cyclization of the chain during the fast stage of chlorination (up to Cl/C 5 = 1). However, quaternary C atoms typical of 6-membered cyclic structures, as postulated in the literature, have not been observed in spectra. Weak signals in the region 75–76 ppm indicate the existence of ▪ fragments. The data obtained suggest that 5- not 6-membered cycles are formed during chlorination of natural rubber. With an increase of the chlorination degree, signals in the olefinic region of spectra appear, indicating formation of exo- and internal CC double bonds differing from analogous fragments forming in the fast stage of the reaction.

Structure of products of the initial stage of chlorination of natural rubber

The structure of the products of the initial stage of chlorination of natural rubber was studied by ozonolysis and IR spectroscopy. Specimens of uniform composition containing up to one atom of chlorine per monomer unit were obtained in a stop-flow setup. On the basis of data on unsaturation and the contents of methyl and vinylidene groups in the specimens under study it was concluded that cyclization occurs during the initial stage of chlorination and the probable paths involving the formation of cyclic structures are considered.

Chlorination of Natural Rubber in Solution with Gaseous Chlorine

Abstract The chlorination of rubber solutions by gaseous chlorine was followed by isolating the partially chlorinated products and preparing their ozonides. The ozonides were hydrolyzed, and the acids and aldehydes formed on hydrolysis were determined. By a comparison with the amounts of acids and aldehydes obtained from ozonides of unreacted rubber, the amount of residual isoprenic double bonds present was found. The loss of double bonds attending the introduction of chlorine atoms into the molecule of rubber indicates four definite stages in chlorination : (1) initial substitutive attack by chlorine, with concomitant cyclization, resulting in a loss of one double bond between two isoprenic units, (2) substitution, (3) additive reaction, and (4) essentially substitution. Chlorination of aged rubber solutions differs from the above in that the cyclization reaction (stage 1) seems to be absent.

Chlorination of Natural Rubber. II. Preparation and Properties of Rubber Dichloride

Abstract As we have seen in Part I, the chlorination of rubber with chlorine under the most varied conditions leads to products in which the chlorine is initially attached fairly loosely to an allylic carbon atom. Consequently, chlorinated rubbers of low chlorine content have rather poor properties. We shall now proceed to describe two methods by means of which, with saturation of the double bond, chlorine is introduced into rubber and the formation of products with allylic chlorine is avoided. In this way chlorinated rubbers of comparatively low chlorine content can be obtained, and these are considerably more stable than the products of the same chlorine content resulting from the direct chlorination of rubber. One of these methods depends on a catalytic additive chlorination of rubber with sulfuryl chloride; the principle of the other is the addition of HC1 onto rubber followed by normal chlorination. The aniline test described in Part I (in which the chlorinated rubber is heated with aniline for 17 hours to 100° C) was again employed to establish the nature of the chlorine bond in chlorinated rubber. Under the conditions of this test, allylic and tertiary combined chlorine reacts quantitatively. Chlorine attached by addition to the double bond of the rubber reacts to the extent of only about 4 per cent. We are therefore in a position to mark off clearly allylic and tertiary chlorine from addition chlorine. This reactive tertiary chlorine is introduced into rubber by the addition of HC1, with the formation of rubber hydrochloride. In the chlorination of rubber we are mainly concerned with allylic and addition chlorine, between which the aniline test distinguishes unmistakably. Moreover, the splitting off of the chlorine from chlorinated rubber with aniline gives us a good idea of its chemical stability.

Chlorination of Natural Rubber. I. Preparation and Properties of Chlorinated Rubber

Abstract The chlorination of rubber has been the subject of extensive investigations, the rubber being commonly treated in solution with gaseous chlorine at a given temperature and pressure. The properties of the products of rubber thus treated vary in close dependence on the chlorine content, which may be as much as 70 per cent. The drawback to a low chlorine content is that, as a rule, the chlorine is loosely bound, with the result that, when exposed to heat or light, it splits off as HC1, with formation of discolored and cyclized products and serious deterioration of the mechanical properties. But the higher the chlorine content, the more stable are these chlorinated rubbers. The stability of chlorinated products of natural rubber containing approximately 65 per cent of chlorine is such that they are used commercially as a component of anticorrosive paint. The instability of chlorinated rubber of low chlorine content is closely connected with its chemical constitution and the mechanism of reaction. The current opinion ten years ago was that chlorine first adds to the double bond of the rubber, and that this primary product, while splitting off HC1, enters into further reaction with chlorine ; but reexamination of this reaction in the laboratory of our affiliated English organization and independently in our own laboratory has led to very different views. It was realized that rubber dichloride (the addition product of chlorine and rubber) is far too stable to act as an unstable intermediate product during the chlorinating reaction. Bloomfield argued that, during the primary reaction, the chlorine is attached by substitution to an α-methylenic carbon atom of the rubber molecule and that there is a considerable amount of cyclization. It was not our sole concern to study the mechanism of reaction; we were also bent on following the chlorination of rubber under the most diverse conditions, starting from dry rubber, rubber in solution, and rubber in latex, and with highly diversified sources of chlorine such as from liquid chlorine, gaseous chlorine, hypochlorites, and sulfuryl chloride. The normal course of reaction, involving the formation of unstable intermediate products, is discussed in Part I of this series; methods of producing comparatively stable rubber chlorides by adding chlorine onto the double bond are considered in Part II.