Ethylene Polymerization over Homogeneous and Supported Catalysts Based on Bis(imino)pyridine Co(II) Complex: Data on the Number of Active Centers and Propagation Rate Constant

Abstract

The number of active centers (CP) and propagation rate constant (kP) for ethylene polymerization with homogeneous catalyst LCoCl2 + MAO and supported catalyst LCoCl2/SiO2 + Al(i‐Bu)3, where L is 2,6‐(2,6‐(Me)2C6H3N = CMe)2C5H3N, have been determined using the method of polymerization quenching by radioactive carbon monoxide (14CO). The unstable rate profile of the reaction was attributed to a decrease in the number of active centers from 0.23 to 0.14 mol · mol−1(Co) corresponding to an increase in the reaction time from 5 to 15 min, whereas the kP value remained constant, amounting to 3.5 × 103 L · mol−1 · s at 35 °C. A narrow molecular weight distribution of the obtained polyethylene (PE) samples ($\overline M _{\rm w} /\overline M _{\rm n}$ = 1.9) testifies that the homogeneous catalyst LCoCl2 + MAO can be regarded as a single‐site system. The activity of the supported catalyst was stable and noticeably lower than that of the homogeneous catalyst due to the low concentration of the active centers (0.02–0.03 mol · mol−1(Co)). PE with a broad molecular weight distribution ($\overline M _{\rm w} /\overline M _{\rm n}$ = 36) and noticeably higher molecular weight is formed in the presence of the supported catalysts. The activity of the supported catalyst increases sharply at polymerization in the presence of hydrogen. The data obtained on the CP and kP values allow suggesting the formation of the “dormant” centers at polymerization without hydrogen and regeneration of the active centers in the presence of hydrogen. The average kP values for the supported catalyst containing multiple active centers were determined to be 5.9 × 103 and 10.5 × 103 L · mol−1 · s, respectively, at 35 and 50 °C.magnified image

Kinetic study of ethylene polymerization over 2,6-bis(imino)pyridine cobalt complexes with bulky substituents in ligand

“…The k P value is virtually independent of the polymerization time (2.4 × 10 3 and 2.1 × 10 3 L mol −1 s −1 ). A similar result was obtained in our earlier study of the 2,6-Me 2 LCoCl 2 + MAO catalyst [19]. It is worth noting that, in contrast to the above results, in the case of the multiple-site catalyst 2,6-Me 2 LFeCl 2 + MAO, the calculated average value of the propagation rate constant decreases with the increase in polymerization time [18].…”

“…results are collected in Table 1 along with the data for the catalyst 2,6-Me 2 LCoCl 2 + MAO (exp. 3) studied earlier [19]. The data in Fig.…”

“…The structures of cobalt 2,6-bis(imino)pyridine complexes used in the study. the single-site catalyst 2,6-Me 2 LCoCl 2 + MAO [19]. In the present study, using the same method, the data on the effect of the alkyl substituents in the aryl ring of the ligand of Co bis(imino)pyridine complexes on the number of active centers and the values of propagation rate and chain transfer (k m tr ) constants were obtained for the first time.…”

“…To determine the C P and k P values, the method of polymerization quenching by 14 CO, applied earlier in the studies of homogeneous catalysts based on 2,6-Me 2 LFeCl 2 [18] and 2,6-Me 2 LCoCl 2 [19], was used. In course of these studies, the conditions of polymerization quenching (the amount of 14 CO and duration of its contact with the reaction mixture) were selected.…”

“…We had earlier studied the kinetics of ethylene polymerization over homogeneous bis(imino)pyridine complexes of Fe and Co [18,19]. Using polymerization quenching with radioactive carbon monoxide ( 14 CO), we have determined the number of active centers (C P ) and the value of propagation rate constant (k P ) at ethylene polymerization over Scheme 1.…”