Coordination Polymerization of Olefins

The catalysts for these polymerizations can be separated into two groups. To the first one belong the so-called Ziegler-Natta catalysts. To the second otic, transition metal oxides on special supports, like carbon black or silica-alumina, etc. Besides the two, there are related catalysts, like transition metal alkyls or metal halides that also catalyze some coordinated anionic polymerization. This group also includes transition metal-7t-allylic compounds and transition metal hydrides. The mechanism of polymerization is generally coordinated anionic, based on all the evidence to date. 

The two disclosures stimulated intensive research into the mechanism of catalysis. Much knowledge was gained to date. Some uncertainties about the exact mechanisms of the reactions still persist. 

The Ziegler-Natta catalysts can be sub-divided into two groups (1) heterogeneous insoluble catalysts, and (2) homogeneous, or soluble ones. At times, however, it is difficult to distinguish between the two. For instance, it may be hard to determine whether a particular catalyst is truly in solution or merely in a form of a very fine colloidal suspension (and in fact heterogeneous). 

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The role of [Cp2TiX]+ ions (X = C1, Me) in coordination polymerization of olefins has been studied by means of electrodialysis, mass spectrometric, and quantum chemical investigations.1215... 

Dyachkovskii, F. S. Role of Ions in Coordination Polymerization of Olefins-, Elsevier Science Amsterdam, 1994 Vol. 89, pp 201-208. 

In general, a polymerization process model consists of material balances (component rate equations), energy balances, and additional set of equations to calculate polymer properties (e.g., molecular weight moment equations). The kinetic equations for a typical linear addition polymerization process include initiation or catalytic site activation, chain propagation, chain termination, and chain transfer reactions. The typical reactions that occur in a homogeneous free radical polymerization of vinyl monomers and coordination polymerization of olefins are illustrated in Table 2. 

Precipitation polymerization, also known as slurry polymerization, involves solution systems in which the monomer is soluble but the polymer is not. It is probably the most important process for the coordination polymerization of olefins. The process involves, essentially, a catalyst preparation step and polymerization at pressures usually less than 50 atm and low temperatures (less than 100°C). The resultant polymer, which is precipitated as fine floes, forms a slurry consisting of about 20% polymer strspended in the liquid hydrocarbon employed as solvent. The polymer is recovered by stripping off the solvent, washing off the catalyst, and if necessary, extracting any undesirable polymer components. Finally, the polymer is compounded with additives and stabilizers and then granulated. 

Coordination polymerization of olefins was first proposed in 1956 for the unusual, at that time, low-pressure polymerization of ethylene and polymerization of propylene with the transition metal catalysts discovered by Ziegler in 1953, and for the ferric chloride catalyzed ring-opening polymerization of propylene oxide to crystalline polymer reported by Pruitt et al. in a Dow patent. Polymerization carried out in the presence of a coordination catalyst is referred to as coordination polymerization . This term is used when each polymerization step involves the complexation of the monomer before its enchainment at the active site of the catalyst [9]. 

In conclusion, it must be emphasized that coordination polymerization of olefins is now considered among the most important areas in polymer research. This area occupies the most prominent place in polymer science and technology that chemical industries intend to cope with. 

From Heterogeneous Ziegler-Natta to Homogeneous Single-Center Croup 4 Organometallic Catalysts A Primer on the Coordination Polymerization of Olefins... 

The coordination polymerization of olefins mediated by ZN catalysts have characteristics of living polymerizations with respect to preservation of the number of active sites, which can have lifetimes of hours or days, but not in regards to individual propagating chains that have lifetimes on the order of only seconds, or minutes at most, under typical reactor conditions. Four primary irreversible chain-termination pathways have been elucidated and these consist of ... 

Post Ziegler and Natta Coordination Polymerization of Olefins... 

In chain reactions the different types of monomers can be added subsequently to an active chain end. The most important techniques here are sequential living polymerization techniques, such as anionic or cationic polymerization. Certain metallocenes can be used in coordination polymerization of olefins leading to stereo block copolymers, like polypropylene where crystalline and amorphous blocks alternate with each other due to the change of tacticity along the chain [34]. In comparison to living polymerization techniques, free radical and coordination polymerization lead to rather polydisperse materials in terms of the number of blocks and their degree of polymerization. 

The coordinative polymerization of olefins at a transition-metal center M is characterized by the presence of a metal—carbon bond able to insert a monomer molecule previously coordinated to the same metal center (chain-propagation step) ... 

In general, the compounds of the Group 4 metals, such as halides and alkoxides, are well known as Lewis acids to catalyze two-electron electrophilic reactions, and their metallocenes coupled with alkylation and/or reduction agents were effective catalysts for the coordination polymerization of olefins. For the transition metal-catalyzed radical polymerization, their alkoxides, such as Ti(Oi-Pr)4, have also been employed as an additive for a better control of the products. Contrary to the common belief that the Group 4 metals rarely undergo a one-electron redox reaction under mild conditions, there have been some reports on the controlled radical polymerization catalyzed or mediated by titanium complexes, although the conflict in the mechanism between the (reverse) ATRP and OMRP is also the case with the Group 4 metal complexes. 

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