We extend van Ruymbeke et al.'s time marching algorithm (TMA) ( Macromolecules 2006, 39, 6248) in order to predict the linear viscoelastic properties of comb polymer melts. While former tube models have shown limitations for predicting the relaxation of comb polymer with short side branches, we observe here a good agreement between predictions and the experimental data for both combs with long and short side branches. In order to determine the origin of this improvement, we study the influence of the different elements present in the TMA model. In particular, we show the importance of taking into account the monomeric friction coming from the backbone itself in the total drag of the molecule, considering the modification of early time fluctuations and introducing the tube dilation process as a continuous function evolving through time. Then, based on a wide range of experimental data on different comb structures, we explore the limits of the relaxation behavior that comb polymers can show. If the friction from the relaxed side branches is significant, the backbone segments seem to fluctuate with respect to the closest branching point, just like a Cayley-tree molecule. On the other hand if the extra friction is negligible in comparison to the potential barrier of retraction along the backbone, the segments fluctuate with respect to the middle of the molecule, just like a linear chain.

DFT molecular modeling studies were undertaken to shed light on possible activation and deactivation mechanisms of Ziegler-Natta catalytic systems, as well as on the possible mechanisms for their reactivation by organohalides. We focused our efforts on Ti species attached to the (110) lateral cut of MgCl 2 . First, the possible activation of adsorbed TiCl 4 leading to an adsorbed Ti III species bearing a Ti-alkyl bond and a coordination vacancy, which is a species able to undergo chain-growth, was considered. According to our calculations formation of the first active species can be easily rationalized by cleavage of a Ti-Cl bond of coordinated TiCl 4 by AlEt 3 , followed by transalkylation promoted by another AlEt 3 molecule. Second, we investigated the possible reduction of polymerization active Ti III species leading to polymerization inactive Ti II species, and we found that a Ti-H bond, possibly formed after chain termination, is weaker than the Ti-Et (polymeryl) bond. Third, we investigated the mechanism of reactivation of Ti II species by organohalides, and it was concluded that reoxidation by Cl rich organohalides is thermodynamically more favored.

No abstract

Several organohalide (HC) compounds were tested as promoters for a Ti-based Ziegler-Natta (Z-N) catalyst at different polymerization conditions. Results show that the intensity of the promoting effect depends on the nature and amount of the promoters. A proper amount, especially optimum amount, of aliphatic type organohalides leads to a strong productivity, and aromatic ones leads to a weak productivity improvement; however, 3-chloro-1-propene poison the catalyst even at lower HC/ Ti molar ratios. Among studied compounds, chlorocyclohexane has the best activity promotion effect at HC/Ti molar ratio of 128, and with this as a promoter, the activity increases over 85%. To understand more details about this phenomenon, the rate of polymerization during time and the effect of polymerization conditions (the temperature and hydrogen partial pressure) on the performance of chlorocyclohexane as the most effective promoter were studied. Finally, to explore the mechanism of reactivation of catalyst species by organohalides, molecular modeling was employed and a new oxidation-addition mechanism was proposed, which basically consisting of homolytic breaking of CACl bond in organohalides. It was found that reoxidation of the catalyst, restoring active center, by Cl rich organohalides is energetically more favored.

The slurry homopolymerization of ethylene catalyzed by a Cp2ZrCl2/MAO catalytic system was studied. A simple kinetic model including initiation, propagation, transfer to monomer and cocatalyst, spontaneous transfer and spontaneous deactivation was developed to predict dynamic yield of polymerization and molecular weight of final products. Kinetic constants were estimated by numerical solution of polymerization kinetic model, combined with Nelder‐Mead simplex method. The model predicts that the propagation reaction has the lower activation energy in relation to chain transfer reactions which leads to decrease of molecular weight at elevated temperatures. The initiation reaction has however, the highest activation energy that results in raising the peak of reaction rate at higher temperatures.magnified image