Abstract
The influence of silver nitrate on the crystallization of poly(isoprene)s has been demonstrated as well as the synergistic effect of natural rubber on the crystallization of lithium catalyzed poly(isoprene). This synergism does not appear to be a “seeding” effect in the strictly definitive sense, since no seed crystals are actually added. In this case a better description of this effect might be cocrystallization where nucleation is promoted by the addition of a quantity of a highly regular similar species. Actually there is a good probability that the nucleating crystals themselves contain segments of the synthetic poly(isoprene). This conclusion is supported by the data shown in Figure 6 for a 50/50 mixture of natural rubber and a noncrystallizable Alfin polymer. The dilato meter curve for this mixture indicates that the 50 per cent non-crystallizable polymer has a slight retarding effect on the induction period for the crystallization of the natural rubber but the crystallization rate of the natural rubber in the 50/50 mixture is quite rapid up to the total volume change expected for natural rubber portion of the mixture. However, in the presence of 50 per cent of a crystallizable lithium poly(isoprene) (also shown in Figure 6), the crystallization rate of this mixture is much slower due to co crystallization. The proposed change from head-tail cis-1,4 addition to tail-head cis-4,1 addition whenever a 3,4 unit is introduced into the polymer chain seems to explain the experimental crystallinity data obtained in these studies. In addition, it raises some questions about the polymerization mechanisms involved and resulting from the orientation of a 3,4 unit being added to the growing end of a polydienyl chain.
