Table I. Mechanical and Thermal Properties of TPE Polypropylenes sample 3 4 strength, MPa 3.91 12.1 recovery after break, % 86 94 strain to break, % 525 1260 G , , MPa 1.47 0.56 T,," O C 51.2, 66.0 53.1, 63.8 AHf/ cal g-' 3.26 2.93'Samples annealed at 30 O C for 12 h before DSC scan. Table 11. Elastomeric Properties of TPE Polypropylenes sample 3 4 strain, % 100 200 300 100 200 300 stress, MPa 3.13 3.39 3.53 3.07 3.30 3.59 recovery, % 93 91 83 91 96 92h gave 0.74 g of polypropylene (PP) 3, which corresponds to a catalytic activity of 2.5 X IO5 g of PP (mol of Ti)-l h-' atm-I. Polymerization at 25 OC also yielded polypropylene 4. Polymers 3 and 4 were subjected to solvent fractionation.1° Aside from a very small amount of acetone-soluble material, the polymers are completely soluble in refluxing ethyl ether, indicating uniformity of structure. They are also characterized by narrow molecular weight distributions. According to GPC using columns calibrated with polypropylenes of known Mw, 3 has Mn = 66 600, MW = 127 000, and Mw/M,, = 1.9 and 4 has I@,, = 98 400, Mw = 164 000, and MW/Mn = 1.7. The degree of crystallinity was found by Roland analysis of X-ray powder patterns obtained on samples annealed between 30 and 45 OC to be 28.6 f 0.5% and 26.5 f 0.4% for 3 and 4, respectively. The polymers exhibit more than one melting endotherm; the transition temperature (T,) and enthalpy (AHf) obtained by DSC on samples annealed at 30 OC for 12 h are given in Table I. Annealing at lower temperatures resulted in lower T,, which indicates a dependence of the size and order of crystalline domains on crystalline conditions.The dynamic storage modulus (G') measured at 1 rad s-l as a function of temperature showed a 3 orders of magnitude decrease between 60 and 80 O C due to the melting transition. G'increases only slightly with frequency at 50 O C but increases strongly with frequency at 100 O C , indicating an elastomeric state in the former but a single phase melt state in the latter. Stress-strain curves obtained on dogbone samples (molded at 100 OC, annealed at 60 O C for 2 h) showed very strong mechanical properties (Table I).In particular, 4 did not break until stretched beyond 1260%. The polymers exhibit excellent elastic recoveries (Table 11); the strain recovery is better than 90% for 4 after elongation of 100-300%. These properties can be explained only by the presence of physical cross-links due to crystallization of stereoregular segments of the polypropylene chains. The molecular weight between cross-links (nC) was estimated from the equilibrium modulus (Gw), which was measured at 50 OC, 0.5% strain, and stress relaxed for lo4 s ( Table I). The estimates for Mc [=(density)RTlG,] are 2.0 X lo3 and 4.4 X lo3 for 3 and 4, respectively. Since the crystallizable segments in the polymer chains must be quite short as indicated by the low T,, there are many alternating crystallizable and noncrystallizable segments, of the order of 20." These polymers contain low homosteric pentad populations.12 The detailed analysis of 500-MH...