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synopsisThe crystallization kinetics and resultant morphologies of thin films of cis-polyisoprenes from guayule rubber have been studied using osmium tetroxide staining and transmission electron microscopy. Polymers used were extracted from the plant either on a laboratory scale or in a pilot plant operation. The polymers show significantly different behavior with regard to both morphological features and to lamellar growth rates. Freshly extracted cis-polyisoprene as well as freshly prepared specimens of pilot plant rubber show only a growth, as in cispolyisoprene from hevea, but a slightly faster lamellar growth rate. Aged pilot plant rubber shows both a and p growth, both at higher rates than in natsyn, with the rate of a growth being lower than in hevea. Freshly extracted and aged specimens show a maximum lamellar growth rate at -2O"C, whereas the maximum for pilot plant rubber occurs at -24°C. The data are interpreted in terms of oxidation of double bonds during aging to produce pendant ester groups. Lamellar growth rates and infrared analyses are both consistent with 1.2-1.5% of the double bonds being affected. Data from guayule and natayn are now consistent in showing that the effeds of microstructural impurities on the crystallization of cis-polyisoprene are to reduce the rates of a-lamellar growth and to induce the fi growth, which would not otherwise exist. Secondary nucleation analyses show the regime II-In transition, as in hevea; however, the temperature of the transition, as well as the ratio of the slopes, is different for this linear polymer.