A first‐order Markovian terpolymer model is used to describe the arrangement of ethylene (1), inverted propylene (2), and normal propylene (3) units in an ethylene‐propylene chain by means of chain propagation probabilities (P set) and corresponding reactivity ratios (r set). From nuclear magnetic resonance data on the weight fractions of methylene sequences, w1:m up to and including w5:m, P11 and the methylene, ethylene, and propylene sequence length distributions can be calculated, as well as the mole fraction of ethylene (x1) and the average sequence lengths sn, sw, sz. With the usual fitting procedures, these results can be obtained for quite different P sets and r sets. This unsatisfactory situation is characterized by the assignment of different values to the degree of inversion I = x2/(x2 + x3). Insight into this indeterminacy was obtained by distinguishing the methylene sequences (s : m) ending with 2 and sequences ending with 3 (s : m, 2 and s : m, 3, respectively), the weight fractions of which appear to be independent of I except for w1:m,2, and w1:m,3. In the constant sum w1:m, the ratio of the two is determined by I. The limit values of I (Imin and Imax) are fixed within the model used and are reached for w1:m,2 = 0 and w1:m,3 = 0, respectively. The values of P12 and P13 are I‐independent; by any particular choice of I, the other Pij's and hence a P set is fixed. For every P set there is a P″ set symmetrical with it, with I′ = 1 − I, which describes an identical chain formed in the opposite direction.
For a series of homogeneous ethylene‐propylene copolymers with mole fraction ethylene (x1) ranging between 0.40 and 0.85, the inversion (I) independent propagation probabilities (Pijs) and reactivity ratios ( rijs) and the methylene, ethylene, and propylene sequence length distributions have been determined from 13C nuclear magnetic resonance (NMR) data, using a first‐order Markovian terpolymer [ethylene (1), inverted propylene (2), and normal propylene (3)] model. For each sample, the limiting values of I (Imin, Imax) are given. Calculations of the common parameters for 19 samples show that the polymerization direction characterized by the r set and I is statistically more probable than the opposite direction, which is characterized by the r′ set and I′. Further, I = Imin appears to be close to the most probable value of I. The resulting r set is r12 = 119, r13 = 19.7 and, for I = Imin, r21 = 0, r31 = 0.034, r32 = 2.98. In the limited range 0.60 < x1 < 0.85, there appears to be no preference for either polymerization direction, so that the solutions characterized by the r set and I = Imin and by the r′‐set and I = I′max, respectively, are about equally probable. If the copolymer reactivity ratios re and rp are defined in the usual manner, it follows that for the limited range re = 17. In the model used here, rp is found to be dependent on the ethylene‐propylene ratio in the reactor. Nevertheless, it can be stated that rp ≅ 0.029 and re rp ≅ 0.50.
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