Polymerization coordination catalysts

Otsuka et al. (110, 112) studied the polymerization of butadiene in the presence of an aged Co2(CO)8/2 MoC15 catalyst. The product obtained was predominantly an atactic poly(l,2-butadiene), the 1,2-structure being favored by low reaction temperature (e.g., at 40° C, 97% 1,2 at 30° C, > 99% 1,2). Similar experiments with a Ni(CO)4/MoCl5 catalyst yielded a polymer with 85% cis- 1,4-structure. The results of Otsuka et al. have been confirmed by Babitski and co-workers (8), who studied the polymerization of butadiene by a large number of binary catalysts, based on transition metal halide, transition metal carbonyl combinations. These systems are of interest as further examples of alkyl-free coordination polymerization catalysts for dienes (9, 15a, 109). Little is known of the origins of stereospecificity of these reactions. 

Radical propagations proceed smoothly in non-polar and also in highly polar media. This is understandable as radical solvation is weaker than ion solvation by one or two orders of magnitude. Propagation on coordination polymerization catalysts is only possible in non-polar media which do not interfere with monomer (usually hydrocarbon) coordination on the transition metal atom. Ionic polymerizations also proceed in non-polar media and they are accelerated with increasing medium polarity. 

Insoluble chiral catalysts bearing stationary supports such as inorganic materials or organic crossUnked polymers, or homochiral organic-inorganic coordination polymeric catalysts without using any external support. 

Coordination polymerization catalysts are complexes of transition metals. The original Ziegler-Natta catalyst, a mixture of titanium tetrachloride and diethylaluminum chloride, has been joined by numerous organometallic complexes such as the widely used bis(cyclopentadienyl)zirconium dichloride. 

Most commercial polyolefins are produced by coordination polymerization catalysts. When compared to free radical processes used to make low-density polyethylene (LDPE), these catalysts work in comparatively gentle conditions, such as lower pressures and temperatures, while providing greater flexibility in controlling the polyolefin molecular structure. An understanding of the polymerization mechanism with coordination catalysts is essential for designing proper systems for the production of polyolefin-clay nanocomposites and wUl be covered in the next section. 

Wang, Q. Quyoum, R. Gillis, D. J. Tudoret, M.-J. Jeremic, D. Hunter, B. K. Baird, M. C. Ethylene, styrene, and a-methylstyrene polymerization by mono(pentamethylcyclopentadienyl) (Cp ) complexes of titanium, zirconium, and hafnium Roles of cationic complexes of the type [Cp MR2] (R = alkyl) as both coordination polymerization catalysts and carbocationic polymerization initiators. Organometallics 1996,15, 693-703. 

One of the main challenges in the use of metal catalysts is the suppression of undesirable termination reactions, which, for reasons described below, tend to be more prevalent for acrylate systems. Most of the recent developments have thus been seen in the polymerization of methacrylate monomers, especially commercially significant methyl methacrylate (MMA), which is polymerized to the commodity material poly(methyl methacrylate) (PMMA), known by its trademark names Perspex , Plexiglass , andLucite (Scheme 23.1). Much of this chapter, therefore, focuses on tacticity control in the production of PMMA using coordination polymerization catalysts. Though little has been reported on stereocontrolled acrylate polymerization, the basic principles established... 

Some of these coordination-polymerization catalysts polymerize up to 20,000 ethylene monomer units per second, a rate otherwise reached only by enzyme-catalyzed biological reactions. Another important characteristic of these catalysts is that they show high reactivity toward 1-alkenes, allowing the formation of copolymers, such as that of ethylene and 1-hexene. 

The mechanism of long-chain branch (LCB) formation with coordination polymerization catalysts is terminal branching. In this mechanism, a dead polymer chain containing a terminal unsaturation - generally a vinyl group - is copolymerized with a growing polymer chain to form an LCB (Figure 8.27) [15]. We have already seen that dead polyethylene chains with terminal unsaturations will be formed by... 

To improve the characteristics of coordination polymerization catalysts, the combinations of different metal derivatives have been investigated. The association of zinc chloride to aluminum alcoholates improves the reactivity significantly and results in higher conversions (see Table 9). However, two polymer fractions are still obtained an isotactic and crystalline polymer of high molar mass and an amorphous and atactic polymer (mole fraction of isotactic dyads is less than 0.6). In contrast to AIR3/H2O or ZnR2/H20, the latter consists of regular head-to-head and tail-to-tail enchainments. ... 

As with iron(II), O Reilly et reported that nickel complexes with an a-diimine ligand (Ni-9), which is an analog of the precursor for a coordination polymerization catalyst, efficiently worked for controlling the radical polymerization of styrene. When coupled with 1-phenylethyl bromide as the initiator, the styrene polymerization with Ni-9 provided well-controlled molecular weights and MWDs (Mw/Mn= 1.15). Neutral Ni(II) acetylides (Ni-10 and Ni-11) were used for the polymerization of DMAEMA and MMA in conjunction with an organic halide as the initiator by Sun et Although judicious conditions, such as concentra-... 

The predominant approach toward the synthesis of olefin-based BCPs has focused on development of living coordination polymerization systems. Unfortunately, one feature that makes coordination polymerization catalysts so efficient for production of RCPs also limits their use for synthesis of conventional BCPs. These catalysts are susceptible to several chain termination and transfer mechanisms and typically produce many chains during polymerization. Therefore, a sequential monomer addition scheme produces a physical polymer blend with a conventional catalyst (Scheme 1). However, by designing systems that suppress these termination processes, advanced catalysts have been used to make BCPs via sequential monomer addition techniques (Scheme 1). These systems have produced many new BCPs with interesting structures. Unfortunately, the fundamental features that enable precision synthesis also make the processes very inefficient and thus of limited commercial appeal. Conventional catalysts produce hundreds to thousands of chains per metal center, but these living systems produce only one. For these materials to be competitive with other large-volume TPEs, more efficient protocols for BCP synthesis must be developed. 

Coordination polymerization of isoprene using Ziegler-Natta catalyst systems (Section 6 21) gives a material similar in properties to natural rubber as does polymerization of 1 3 butadiene Poly(1 3 buta diene) is produced in about two thirds the quantity of SBR each year It too finds its principal use in tires... 

Section 14 15 Coordination polymerization of ethylene and propene has the biggest eco nomic impact of any organic chemical process Ziegler-Natta polymer ization IS carried out using catalysts derived from transition metals such as titanium and zirconium tt Bonded and ct bonded organometallic com pounds are intermediates m coordination polymerization... 

At present it is not possible to determine which of these mechanisms or their variations most accurately represents the behavior of Ziegler-Natta catalysts. In view of the number of variables in these catalyzed polymerizations, both mechanisms may be valid, each for different specific systems. In the following example the termination step of coordination polymerizations is considered. 

Copolymers of VDC can also be prepared by methods other than conventional free-radical polymerization. Copolymers have been formed by irradiation and with various organometaHic and coordination complex catalysts (28,44,50—53). Graft copolymers have also been described (54—58). 

Polymers with much higher average molecular weights, from 90,000 to 4 x 10 , are formed by a process of coordinate anionic polymerization (43—45). The patent Hterature describes numerous organometaUic compounds, aLkaline-earth compounds, and mixtures as polymerization catalysts. Iron oxides that accumulate in ethylene oxide storage vessels also catalyze polymerization. This leads to the formation of nonvolatile residue (NVR) no inhibitor has been found (46). 

The next major commodity plastic worth discussing is polypropylene. Polypropylene is a thermoplastic, crystalline resin. Its production technology is based on Ziegler s discovery in 1953 of metal alkyl-transition metal halide olefin polymerization catalysts. These are heterogeneous coordination systems that produce resin by stereo specific polymerization of propylene. Stereoregular polymers characteristically have monomeric units arranged in orderly periodic steric configuration. 

In Section 6.21 we listed three main methods for polymerizing alkenes cationic, free-radical, and coordination polymerization. In Section 7.15 we extended our knowledge of polymers to their stereochemical aspects by noting that although free-radical polymerization of propene gives atactic polypropylene, coordination polymerization produces a stereoregulai polymer with superior physical properties. Because the catalysts responsible for coordination polymerization ar e organometallic compounds, we aie now in a position to examine coordination polymerization in more detail, especially with respect to how the catalyst works. 

C—X, Cf, X- and C+ fX (see Fig. 2), the solvation energy increasing the driving force of these dissociations. It is possible that a coordination catalyst is not active in the C—X state but only in one or other of the ionized states. Such behavior blurs the distinction between ionic and coordination polymerization. 

At the present time the concept of catalytic (or ionic-coordination ) polymerization has been developed by investigating polymerization processes in the presence of transition metal compounds. The catalytic polymerization may be defined as a process in which the catalyst takes part in the formation of the transition complexes of elementary acts during the propagation reaction. 

In studying two-component polymerization catalysts, beginning with Feldman and Perry (161), a radioactive label was introduced into the growing polymer chain by quenching the polymerization with tritiated alcohols. The use of these quenching agents is based on the concept of the anionic coordination mechanism of olefin polymerization occurring... 

The specific behavior of surface compounds, being the propagation centers of polymerization catalysts, are mainly determined by two of their features the coordinative insufficiency of the transition metal ion and the presence of the transition metal-carbon bond. 

It should be noted that, similarly to olefin, the insertion of carbon monoxide in the active bond in the propagation centers of polymerization catalysts also follows the coordination mechanism 175). The insertion of carbon monoxide into the active bond was not feasible when a vacant coordination site of the metal ion had been occupied by phosphine. 

Carbon-14 of the alkoxyl group in the polymer was also found during the treatment by other oxide polymerization catalysts containing the oxides Mo, W, V (195). By the character of the polarization of the active bond such systems may be designated coordinated cationic. ... 

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