Magnesium chloride supported high mileage catalyst for olefin polymerization. VIII. Decay and transformation of active sites

Abstract

AbstractMore than a hundred propylene polymerizations were carried out with the CW catalyst (our particular MgCl2/ethylbenzoate/p‐cresol/AlEt3/TiCl4 supported high mileage catalyst). Highest I.I. index (% yield of boiling heptane insoluble product) of 96.2 ± 0.9 was obtained at [Ti] = 2.4 × 10−4 M, A/T (amount of AlEt3 with 0.33 equivalent of methyl‐p‐toluate to amount of Ti in the catalyst) = 167 at 50°C. The I.I. values became lower when any one of these variables was changed. The I.I. values did not change with time of polymerization, indicating that both stereospecific and nonstereospecific sites were produced at the same time and polymerized monomers during the course of a polymerization. Estimates of maximum active site concentrations, [Ti*]0,Because of the complexity of the catalyst system, the active sites are designated as follows: [Ti*], active sites of all kinds at a given time; [Ti*]0 active sites of all kinds at time zero; [Ti1*], active sites of the first kind formed initially upon activation; [Ti2], active sites of the second kind, which were transformed from the former, and are responsible for olefin polymerization after the initial phase of rapid decay of activity; [Ti*]i, stereospecific active sites; [Ti*]a, nonstereospecific active sites, and [Ti*]t = [Ti*]i + [Ti*]a Similarly, the subscripts 1 and 2 for the various rate constants refer to active sites Ti1* and Ti2*, respectively. Finally [Ti] is the concentration of total titanium in the amount of catalyst used. [Ti–P] is the titanium polymer bond concentration as determined by 14CO tagging; [Ti–P] (1 h) and [Ti–P] (48 h) are the values obtained with indicated time of contact of 14CO with the polymerization mixture in the obsence of monomer. were obtained from a variation of vn−1 versus t−1. The values of [Ti*]0.i and [Ti*]0,a for the stereospecific and nonstereospecific sites, respectively, are in excellent agreement with those values of [MPB]0 (metal polymer bond concentrations at t = 0) determined earlier by radiotagging with tritiated methanol. The rate of formation of [Ti*]1 (the initial active site) is first order with respect to [Ti] and [A] with an activation energy of 12 kcal mol−1 where [A] is the AlEt3 concentration. The rate constants of propagation at 50°C are kp,i ∼ 160M−1s−1 and kp,a ∼ 11M−1s−1. The activation energy for the stereospecific propagation is about 4.1 kcal mol−1. At 50°C the rate of polymerization decreases according to second order kinetics suggesting bimolecular processes which transform one‐half to one‐fourth of the Ti1* site to Ti2* types depending upon experimental conditions, while the remainder decay to inactive species. The values of kt1 lie between 19 and 61M−1s−1. These processes are more complicated at 70°C involving two consecutive reactions; at low [A], the data fits better with first order decay kinetics. Comparison of the [Ti2*] values and the values of [Ti–P] obtained by 14CO tagging suggests that CO reacts primarily with the Ti2* sites and very little with the initially formed Ti1* sites. The Ti2* sites are slightly less active than the Ti1* sites having kp2,i ∼ 86M−1s−1 and kp2,a ∼ 7M−1s−1 at 50°C.