Kinetic Heterogeneity and Distribution of the Active Sites of the TiCl4-Al(i-C4H9)3 Catalytic System by Stereoregulating Ability in Butadiene Polymerization

Abstract

A main problem in the theory and practice of ioniccoordination polymerization is the structure of active sites [1][2][3] and their stereospecific and kinetic heterogeneity. Recent publications [4][5][6] have confirmed the earlier assumptions [7,8] on the multicenter structure of the ionic-coordination catalytic systems in the polymerization of dienes. Solution of the inverse problems of molecular weight distributions (MWDs) exemplified by lanthanide-and vanadium-containing catalytic systems gave distributions of active sites by kinetic activity and demonstrated the presence of several types of active sites differing in the kinetic parameters of the processes involving these sites. No data on the distribution of active sites by stereoregulating ability in diene polymerization are available. These data would enable substantial progress in the study of mechanisms of ionic-coordination polymerization and the structure of active sites. Therefore, the purpose of this study was to obtain the distributions of the active sites of the ionic-coordination titanium-containing catalytic systems of butadiene polymerization by stereoregulating ability and kinetic activity.Butadiene polymerization was studied in the presence of the TiCl 4 -Al( i -C 4 H 9 ) 3 catalytic system, which allows one to prepare polybutadienes with microstructures varying over a broad range by varying the polymerization conditions [9].The synthesis of polybutadiene was conducted in toluene at 25 ± 1°ë under conditions excluding the presence of impurities, moisture, or air in the reaction vessels. First, an organic derivative of a main-group metal, Al( i -C 4 H 9 ) 3 , was introduced into a butadiene solution, and, after that, a transition metal compound, TiCl 4 , was added. The monomer concentration C m was 2 mol/L and the catalyst concentration C Ti was 0.01-0.005 mol/L for the molar ratio Al/Ti = 2, which corresponds to the highest activity of the catalytic system.The polymer microstructure (the content of 1,4-cis , 1,4-trans , and 1,2 units) was calculated from the relative intensity of the IR absorption bands at 740, 970, and 910 cm -1 . The spectra were recorded on a Specord M-80 spectrophotometer.The molecular weights ( M w and M n ) and the MWD of polybutadiene were determined on a Waters GPS 2000 gel chromatograph at 80°ë using toluene as the eluent. A three-column system packed with styrogel was calibrated against polystyrene standards with a narrow MWD ( M w / M n ≤ 1.2 ) using the universal Benoit dependence and an equation relating the MW of 1,4-trans -polybutadiene to the intrinsic viscosity [10]. The MWD curves were corrected with application of the instrument broadening correction as described previously [11].The polymer was fractionated by fractional precipitation with methanol from a 1% toluene solution followed by reprecipitation.The kinetic heterogeneity of the catalytic system was estimated by solving the inverse problem of MWD formation by the Tikhonov regularization method [3][4][5][12][13][14].In all cases, the polymers had bimodal ...

Influence of the structure of the organoaluminum compound on the stereoregulating heterogeneity of catalytic systems based on TiCl4

“…To compare the obtained distributions with respect to the kinetic activity of the sites with their stereoregulating ability, the polybutadiene samples synthesized using various organoaluminum compounds were fractionated. As in the case of the previously studied polybutadiene synthesized using the catalytic system prepared in situ [4,5], in the case of preliminary formation of the catalyst the molecular weights of the polydiene fractions also correlate with their stereoregularity (Fig. 1, curves 3-6).…”

“…When the catalytic complex is formed in advance, the reactions between the components of the catalytic system, apparently, are more complete, the catalytic surface is formed, and the active sites get chemically unifi ed. As a result, in the case of AlCl(i- C 4 H 9 ) 2 , compared to the results obtained in situ [12,13], the sites forming the lowest-molecular-weight fractions are absent, and in the case of Al(i-C 4 H 9 ) 3 [5] the molecular weights are shifted toward higher values.…”

“…Recently much attention has been given to the multiplesite nature of ion-coordination catalytic systems and to its dependence on such factors as catalytic system composition and conditions of the polymerization and catalyst preparation [2,3]. Studies in the fi eld of the multiple-site nature [4,5] showed that the polymerization sites are heterogeneous not only in the kinetic activity, but also in stereospecifi city of the action.…”

“…Sigaeva et al [4,5] showed previously that, in polymerization of butadiene in situ under the action of the catalytic system TiCl 4 -Al(i-C 4 H 9 ) 3 under the conditions corresponding to the maximal activity of the catalyst, formation of polybutadiene of mixed microstructure is possible as a result of formation of three types of active sites, differing in the stereoregulating power. Sigaeva et al note the relationship between the molecular weight and microstructure for all the isolated polybutadiene fractions.…”