Polymerization, coordination

This chapter describes the coordination polymerization of acyclic and cyclic vinylic monomers, conjugated dienes, and polar vinylic monomers with the most important catalytic systems known in this area. A chronological classitication for the development of the main coordination catalyst types is outlined, as well as polymerization kinetics and mechanisms and applications of polymers obtained through different metallic complexes. 

The reaction where vinylic monomers polymerize through coordination at the metallic center of some catalytic species is called coordination polymerization. Although the first catalytic system based on this kind of coordination chemistry was reported by Phillips Petroleum Co., most of the literature concerning coordination polymerization refers to the Ziegler-Natta catalysts because of their versatility in controlling chemical composition distribution (CCD) and of the wider variety of monomers they can polymerize [1]. 

The contributions by Ziegler and Natta caused great industrial impact and large advances in research and development of polymer science and engineering, as new kinds of 

Other kinds of coordination catalytic systems developed few years before the Ziegler-Natta catalysts were based on chromium and molybdenum oxides supported on SiOj AI2O3 and other supports. The catalysts were patented by Phillips Petroleum and Standard Oil companies of Indiana for the synthesis of polyolefins. Although Phillips catalysts were the first to produce a fraction of crystalline polypropylene, these systems were more useful for the production of polyethylene. In fact, the Phillips and the Ziegler-Natta catalysts are currently the two commercial systems that dominate the production of HDPE [2]. 

The discovery of Ziegler-Natta catalysts led to many industrial and academic investigations on other kinds of metallic complexes for polymerization of different monomers. Several organometallic and coordination compounds have been synthesized and probed as catalytic systems. They have been classified based on generations or groups, transition-metal type, the chemical structure, the type of activator, and their applications in polymerization processes [2]. Currently, there are different groups of initiator systems based on early and late transition metals or lanthanide complexes, which have been studied in polymerization catalysis [3]. 

Polyethylene and polypropylene are ubiquitous commodity plastics found in applications varying from household items such as grocery bags, containers, toys and appliance housings, to high-tech products such as engineering plastics, automotive parts, medical appliances and even prosthetic implants. They can be either amorphous or highly crystalline, and behave as thermoplastics, thermoplastic elastomers or thermosets. 

Despite their versatility, both polyethylene and polypropylene are made only of carbon and hydrogen atoms. We are so used to these remarkable plastics that we do not often stop to ask how materials made out of such simple building blocks can have this extraordinary range of properties and applications. The answer to this question lies on how the carbon and hydrogen atoms are connected to define the molecular architecture or microstructure of polyolefins. Because microstructure plays such a relevant role in polyolefin properties, several characterization techniques have been specifically developed to measure different aspects of their molecular architectures. Section 2.1 classifies the different types of commercial polyolefins according to their microstructures, discusses several microstructural characterization techniques developed for these polymers, and demonstrates how they are essential to understand polyolefins. 

Even though the catalyst is the crucial component determining polyolefin microstructure, we should not prematurely assume that, once the catalyst is selected, all our problems are 

Section 2.3 describes phenomenological models for polymerization kinetics with coordination catalysts. Molar and population balances will be derived using what we like to call the standard model for olefin polymerization kinetics with coordination catalysts. Also how molecular weight averages can be modeled in batch, semibatch and continuous reactors are shown using the method of moments and the method of instantaneous distributions. Unfortunately, the kinetics of olefin polymerization is complicated by several factors that are not included in the standard model some of these effects and possible solutions and model extensions are mentioned briefly at the end of Section 2.2. 

In summary, the apparent simphdty of your everyday polyethylene and polypropylene consumer goods is deceptive. Few industrial polymers can claim such richness in catalyst types, reactor configurations and microstructural complexity. In this chapter, we will explain how, from such simple monomers, polyolefins have become the dominant commodity plastic in the 21st century. 

It is generally believed that the following oxidation-reduction reaction is responsible for chain growth  

TiCl3 -b (CH3CH2)3A1 TiCl2(CH2CH3) (CH3CH2)2C1A1 

Free-radical polymerization arranges functional groups, such as alkyls, in a random manner, whereas coordination polymerization can exercise stereochemical control over functional groups. With the proper choice of experimental conditions, such as temperature, solvent, and catalyst, monomers can polymerize to any of three arrangements isotactic, syndiotactic, and atactic. (For polyethylene, there are no such stereoisomers, since the monomeric units are identical, —CH2—.) Isotactic and syndiotactic polypropylenes are highly crystaUine atactic polypropylene is a soft, elastic, and rubbery material. Following are the three stereoisomers of polypropylene  

---

Coordination polymerization IS described in more detail in Sections 7 15 and 14 15... 

In their polymerization, many individual alkene molecules combine to give a high molecular weight product Among the methods for alkene polymerization cationic polymerization coordination polymerization and free radical polymerization are the most important An example of cationic polymerization is... 

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... 

Before coordination polymerization was discovered by Ziegler and applied to... 

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. 

Protonic initiation is also the end result of a large number of other initiating systems. Strong acids are generated in situ by a variety of different chemistries (6). These include initiation by carbenium ions, eg, trityl or diazonium salts (151) by an electric current in the presence of a quartenary ammonium salt (152) by halonium, triaryl sulfonium, and triaryl selenonium salts with uv irradiation (153—155) by mercuric perchlorate, nitrosyl hexafluorophosphate, or nitryl hexafluorophosphate (156) and by interaction of free radicals with certain metal salts (157). Reports of "new" initiating systems are often the result of such secondary reactions. Other reports suggest standard polymerization processes with perhaps novel anions. These latter include (Tf)4Al (158) heteropoly acids, eg, tungstophosphate anion (159,160) transition-metal-based systems, eg, Pt (161) or rare earths (162) and numerous systems based on tri flic acid (158,163—166). Coordination polymerization of THF may be in a different class (167). 

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. 

Before coordination polymerization was discovered by Ziegler and applied to propene by Natta, there was no polypropylene industry. Now, more than 10 ° pounds of it aie prepared each year in the United States. Ziegler and Natta shared the 1963 Nobel Prize in chemistry Ziegler for discovering novel catalytic systems for alkene polymerization and Natta for stereoregular- polymerization. 

The distinction between coordination polymerization and ionic polymerization is not sharp. Let us consider for example a C—X bond, X being a halogen or a metal. Winstein54 and Evans14 have demonstrated that in a compound containing this type of bond an equilibrium may be established in a suitable solvent between... 

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. 

All of these examples explain why such a variety of phenomena are observed in ionic or coordination polymerization. What we need to understand is the cause which gives to a particular center this or other properties, e.g., why dissociation into isolated ions leads to one and not another change in the reactivity and the specificity, how changes in solvation shell change the behavior of the growing center. This whole field is still uncharted, and calls for a thorough academic research. 

In anionic polymerization, as in carbonium ion polymerization, termination does not involve bimolecular reaction between two growing chains. Neither can recombination of ions lead to termination, since a carbon-metal bond is highly polar, in the case of alkali metals frequently completely ionized, and in every case very reactive. The termination step leading to the formation of a terminal C=C double bond is not too probable. This reaction involves the formation of a metal hydride, and this does not contribute greatly to the driving force. Consequently, such a termination is observed at higher temperatures only and it is probably more common in coordination polymerization where the metals involved are less electropositive. 

Configurational energy for clathrates, 12 Configurations, superposition of, 258 Conformal solution theory, 137 Coordination polymerization, 148, 162, 170... 

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. 

From an industrial stand-point, a major virtue of radical polymerizations is that they can often be carried out under relatively undemanding conditions. In marked contrast to ionic or coordination polymerizations, they exhibit a tolerance of trace impurities, A consequence of this is that high molecular weight polymers can often be produced without removal of the stabilizers present in commercial monomers, in the presence of trace amounts of oxygen, or in solvents that have not been rigorously dried or purified, Indeed, radical polymerizations are remarkable amongst chain polymerization processes in that they can be conveniently-conducted in aqueous media. 

In anionic and coordination polymerizations, reaction conditions can be chosen to yield polymers of specific microstructurc. However, in radical polymerization while some sensitivity to reaction conditions has been reported, the product is typically a mixture of microstructures in which 1,4-addition is favored. Substitution at the 2-position (e.g. isoprene or chloroprene - Section 4.3.2.2) favors 1,4-addition and is attributed to the influence of steric factors. The reaction temperature does not affect the ratio of 1,2 1,4-addition but does influence the configuration of the double bond formed in 1,4-addition. Lower reaction temperatures favor tram-I,4-addition (Sections 4.3.2.1 and 4.3.2.2). 

The presence of long chain branches in low density polyethylene (LDPE) accounts for the difference in properties e.g. higher melt strength, greater toughness for the same average molecular weight) between LDPE and linear low density polyethylene (LLDPE, made by coordination polymerization). 

Six-coordinate, polymeric complexes NCCHj- salen NCCHjCo Not givdn Coplanar to with- 1.99 Co—N, 1.89 Not given 2.09 30... 

Coordination polymerization is the most versatile method of preparing PCL and its copolymers, affording high molecular weights and conversions, and either block or random copolymers depending on the conditions. As with the preceding classes of initiators, the product... 

---