THEPOLYMERIZATION OF BROMOPRENE
Feb., 1933
THE EXPERIMENTAL STATION O F DE NEMOURS & CO.]
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Acetylene Polymers and their Derivatives. V. The Polymerization of Bromoprene (Third Paper on New Synthetic Rubbers) BY WALLACE H. CAROTHERS, JAMES E. KIRBYAND ARNOLD M. COLLIXS
Chloroprene (I, X = C1) polymerizes spontaneously to yield a rubber-like product,' and the speed of this transformation is roughly 700 times greater than the analogous transformation of isoprene. The present paper deals with the behavior of bromoprene2 (I, X = Br), and the results may be summarized in the statement that i t shows no significant qualitative differences from chloroprene, although its speed of polymerization under most conditions appears to be somewhat greater. CHz=C-CH=CHz I
I
X
I
CH=C-H I
I
X I1
In connection with the behavior of these materials the following analogies are of interest. Vinyl chloride and vinyl bromide (11, X = C1 and Br) polymerize spontaneously yielding products of very high molecular weight,3 but this behavior is not shown by propylene, which polymerizes only in the presence of special catalysts or under drastic conditions and then yields products having only moderately high molecular weights. The haloprenes (I) bear the same structural relationship to isoprene that the vinyl halides bear to propylene. Thus the very powerful activating effect of a single halogen atom on ethylene is also manifested in butadiene (when the halogen atom is on the @-carbon). The rate of polymerization is of great importance in studying the behavior of dienes since a high rate not only makes it possible to obtain experimental results in a reasonable length of time, but it permits one to extend the observations over a wide range of conditions. It becomes possible then to recognize the different types of reactions involved in the spontaneous polymerization and to obtain data on the way in which these different types of reactions are affected by changes in the conditions. Data of this ty'pe on chloroprene have already been presented, and it will be useful to review them briefly (with some extensions) and to make comparisons with those now available for bromoprene and for other dienes. The polymerization of chloroprene leads to the four well defined and qualitatively distinct types of polymers shown in the chart. (1) Carothers, Williams, Collins a n d Kirby, THISJOURNAL, 53, 4203 (1931). (2) Carothers, Collins and Kirby, rbid , 16, 786 (1933). (3) Staudinger, Brunner, and Feisst, HeLv Chim Icfa, 13, 805 (1930).
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WALLACE H. CAROTHERS, JMES E. KIRBYAND ARNOLD M. COLLINS Vol. 55
Volatile (8) polymer
Cmonoprene
J
granular ( 0 )polymer
+plastic (4 polymer
J
non-plastic ( p ) polymer
The influence of various conditions on each of these reactions is indicated in Table I. TARLE I
INPLURNCR OF CONDITIOAS ON Condition
THE POLYMERIZATION OF CHLOROPR~NR Manoprene to Manoprcae to e-Palypreae to Moooprroe to 8-polyrner =.po1yprene "-DoIyprc.c w-po1ypreae
+++
+
+
Temperature Aut-talytic. iniPressure +? tiated by strong n n? ultraviolet light Light Oxygen 0 ++? and by metal 0 *a surfaces Antioxidants " Certain substances cammonly classified as antioxidant5 (e. g., phenyl-8-naphthylamine) act BS inhibitors; others (e. g.. benzidine) act as acceleratorr: accelerates. 0 no eflect, - inhibits.
+
++ ++ +++ ___
+
Of these different types of polymers the m a t important are the a- and the p-polyprenes since the former corresponds to unvulcanized rubber and the latter corresponds t o vulcanized rubber. The p-polyprene is the final product of the spontaneous polymerization of chloroprene; the a-polyprene is an intermediate step in the formation of the p-product. Isolation of the apolymer free of p-polymer is possible only when the reaction is conducted under &ain conditions: the temperature must not be too high, - since elevated temperature accelerates t h e I! transformation of a-polymer to p-polymer more than i t accelerates the formation of a-polymer from monomer; the reaction must not be too slow, since most inhibiting influences h a v e a greater decelerating &ect on the formation of the a- than on the pproduct; moreover, very slow reaction frequently Fig. I.--s~l
