Chain structure of homogeneous ethylene-propylene copolymers showing inversion, described on the basis of 13C-NMR data. I. Theory

✍ Scribed by V. B. F. Mathot; Ch. C. M. Fabrie

• Publisher: John Wiley and Sons
• Year: 1990
• Tongue: English
• Weight: 764 KB
• Volume: 28
• Category: Article
• ISSN: 0887-6266
• DOI: 10.1002/polb.1990.090281304

✦ Synopsis

Abstract

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, w~1:m~ up to and including w~5:m~, P~11~ and the methylene, ethylene, and propylene sequence length distributions can be calculated, as well as the mole fraction of ethylene (x~1~) and the average sequence lengths s~n~, s~w~, s~z~. 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 = x~2~/(x~2~ + x~3~). 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 w~1:m,2~, and w~1:m,3~. In the constant sum w~1:m~, the ratio of the two is determined by I. The limit values of I (I~min~ and I~max~) are fixed within the model used and are reached for w~1:m,2~ = 0 and w~1:m,3~ = 0, respectively. The values of P~12~ and P~13~ are I‐independent; by any particular choice of I, the other P~ij~'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.