Reaction of Chloral with 1,4-Polyisoprene

Abstract The cis-1,4-polyisoprenes are composed of 2-methyl-2-butene units, the double bonds of which must be favorable to the addition of strongly electrophylic reagents such as certain aldehydes. Chloral is of this type and its ionic reactions with various simple olefins has been described. We found it worth while to transpose these results to polyisoprenic macromolecules, either natural or synthetic. The ionic reactions generally create less modifications of the chain, such as fission or crosslinking, than would result from free radical reactions. Also, if the formation of free radicals is avoided, the possible fixation of chloral on the halogen carrying carbon (group —CCl2—CHO) becomes very unlikely. Pure anhydrous chloral is used as the reagent. A solution of 2% cis-1,4-polyisoprene (natural) in cyclohexane or decalin (free of peroxides) is heated under a nitrogen atmosphere with the reagent and in presence of a catalyst such as AlCl3 or BF3 at a concentration of 1–3% relative to the polyisoprene. The macromolecular product obtained is then subjected to a fractionation by the chloroform-methanol combination. After purification and desolvation of the main fraction an elemental analysis is made and the level of fixation n calculated based on the chlorine content. Table 1 relates some of the results obtained by using AlCl3 or BF3. In the absence of catalyst (Figure 1, A) the level of fixation is low: n=4 and therefore the nature of the reaction remains uncertain. In the presence of AlCl3 or of BF3, n increases rapidly in function of the temperature and of the relative molar concentration of the reagents, as shown by the curves B and C of Figure 1. The highest level of fixation obtained in these experiments is n=22. Figure 2 demonstrates this limit which is reached beyond the molar ratio of m=4, in the presence of AlCl3 (3%). Table 2 shows the particular action of AlCl3 which among all the catalysts tested proves to be as effective as BF3 without yielding any notable crosslinking of the macromolecular substance.