Polymerization mechanism, polypropylene

The isotactic form of propylene has better physical and mechanical properties than the three tactic form mixture (obtained from free radical polymerization). Isotactic polypropylene, in which all of the stereo cen-... 

A special case of the chain back skip polymerization mechanism and therefore an entirely different polymerization behavior was observed for differently substituted asymmetric complexes (for example catalyst 3). Although asymmetric in structure, these catalysts follow the trend observed for C2-symmetric metallocenes [20], Chien et al. [23] reported a similar behavior for rac-[l-(9-r 5-fluorenyl)-2-(2,4,7-trimethyl-l-ri5-indenyl)ethane]zirconium dichloride and attributed this difference in the stereoerror formation to the fact that both sides of the catalyst are stereoselective thus isotactic polypropylene is obtained in the same manner as in the case of C2-symmetric metallocene catalysts. 

High magnetic fields and in particular C-NMR spectroscopy allow the analysis of even longer configurational sequences (tetrads up to nonads). This proved to be important in particular for the analysis of polyolefins like polypropylene or cycloolefin copolymers (COC). These polymers are available via transition-metal mediated (Ziegler-Natta, metallocene) insertion polymerizations, and the configurational analysis provides deep insight into the respective polymerization mechanisms as well as into the structure-property relationships. 

As mentioned in section 4.1, the kinetics of the living polypropylene synthesis have been interpreted in terms of a coordination polymerization mechanism represented by Eq. (22). We discuss here the mechanism of chain propagation on the basis of the structure and stereochemistry of the synthesized polypropylenes. 

The polymerization of propylene using complex 14 activated by MAO (Al Zr ratio=500, solvent toluene, 25 °C) yielded 80 g polymer-mol Zrl-hrl with a molecular weight Mw= 115,000 and polydispersity=2.4 [119]. The reaction was carried out in liquid propylene to avoid, as much as possible, the epimerization of the last inserted monomer unit and to allow rational design of the elastomeric polymer. The formation of elastomeric polypropylene is consistent with the proposed equilibrium between ds-octahedral cationic complexes with C2 symmetry inducing the formation of the isotactic domain, and tetrahedral complexes with C2v symmetry responsible for the formation of the atactic domain (Scheme 7). The narrow polydispersity of the polypropylene obtained supports the polymerization mechanism in which the single-site catalyst is responsible for the formation of the elastomeric polymer. 

Since the discovery of Ziegler-Natta catalyst, isotactic polypropylene has been widely used as a commodity material due to its low cost and excellent physical properties. As the modulus of the resin is closely related to isotacticity, an understanding in polymerization mechanism is important in the chemistry of propylene polymerization as well as in the industry. 

Since 1962 (1), NMR has been one of the most powerful analytical tools for the determination of the microstructure of polypropylene. Many attempts have been made to describe statistical formation of isotactic polymer chains (2), and the two-site model is widely accepted as a desirable stochastic model for olefin polymerizationf3/ This model has been proven to be valid for not only polyolefins but also vinyl-type polymers such as polystyrene. The model is especially useful to describe the polymerization mechanism by using pentad fractions of C-NMR spectra of polyolefins. Indeed, this model is effective toward a better understanding of e nature of the polymerization centers for propylene. 

Currently, there are six review articles published in the literature on metallocene catalysts, covering different aspects of metallocene catalyst synthesis, nature of active sites, polymerization mechanisms, metallocene catalyst patents, and polymerization reaction engineering [1-6]. The present review will highlight some of the particular features of metallocene catalysts that make them attractive for the industrial production of polypropylene (PP). 

To get a closer insight into the polymerization mechanism responsible for the strong dependence of stereoselectivity on monomer concentration observed for this type of asymmetric catalyst, the pentad distributions of the polypropylenes prepared with 9a were investigated using nuclear magnetic resonance (NMR) spectroscopy. The mmmm pentad content was observed to decline with increasing monomer concentration this relationship was attributed to the existence of two coordination sites in these dual-side complexes, which show different stereoselectivities for monomer coordination and insertion. Guerra et al. have supported this hypothesis in a theoretical study. 

To imderstand the functioning mode of this catalyst system and the mechanistic aspects of the stereochemical events involved in the formation of s-PP chains, it is very important to first examine more closely the molecular structure and structural characteristics of the complex 1 (and/or 2). The structural data can be then correlated to the information gleaned from different polymer analyses in order to determine the polymerization mechanism and the elementary steps involved in the formation of s-PP chains on the basis of the well-established principle of catalyst structure-polypropylene chain microstructure interrelationship. 

Various chiral metallocenes with methylaluminox-ane cocatalysts (MAO) are specific for the preparation of isotactic, syndiotactic, atactic and hemiisotactic polypropylenes and higher polyolefins [72]. In hemiisotactic polypropylene (hit-PP) every other methyl is placed isotactically, the remaining methyls randomly. This type of polypropylene has served as a keypoint in the elucidation of the polymerization mechanism with metallocene catalysts [73]. The chiral metallocene catalysts are not as stereorigid as the conventional heterogeneous systems. Consequently under some... 

Polypropylene polymerized with triethyl aluminum and titanium trichloride has been found to contain various kinds of chain ends. Both terminal vinylidene unsaturation and aluminum-bound chain ends have been identified. Propose two termination reactions which can account for these observations. Do the termination reactions allow any discrimination between the monometallic and bimetallic propagation mechanisms ... 

This conceptual link extends to surfaces that are not so obviously similar in stmcture to molecular species. For example, the early Ziegler catalysts for polymerization of propylene were a-TiCl. Today, supported Ti complexes are used instead (26,57). These catalysts are selective for stereospecific polymerization, giving high yields of isotactic polypropylene from propylene. The catalytic sites are beheved to be located at the edges of TiCl crystals. The surface stmctures have been inferred to incorporate anion vacancies that is, sites where CL ions are not present and where TL" ions are exposed (66). These cations exist in octahedral surroundings, The polymerization has been explained by a mechanism whereby the growing polymer chain and an adsorbed propylene bonded cis to it on the surface undergo an insertion reaction (67). In this respect, there is no essential difference between the explanation of the surface catalyzed polymerization and that catalyzed in solution. 

Polyethylene and polypropylene are semitransparent plastics made by polymerization. They are produced from ethylene and propylene in a variety of grades. Their mechanical properties are determined mainly by density (degree of crystallinity) and molecular weight, characterized by the Melt Index (MI). 

Fig. 2. Relationship of conductivity of polypropylene-based polymer composites and filler concentration (natural graphite) 1 — polymerization filling 2 — mechanical mixture [24]...

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