Coordination polymerization thermoplastics

Very highly branched polymers, like polyethylene made by free-radical, high-pressure processes, will have Mw/Mn ratios of 20 and more. Most polymers made by free-radical or coordination polymerization of vinyl monomers have ratios of from 2 to about 10. The M /M ratios of condensation polymers like nylons and thermoplastic polyesters tend to be about 2, and this is generally about the narrowest distribution found in commercial thermoplastics. 

Table 5.1 shows the main families of polymers obtained by coordination polymerization (most of them commercial polymers), which were grouped according to their thermomechanical behavior, such as polymer and copolymers, thermoplastics, elastomers, and plastomers. Most of the polymers synthesized by coordination mechanisms correspond to different grades of polyolefins and polydienes, made with Ziegler-Natta or Phillips catalyst [31]. 

Polyolefins, which are normally defined as polymers based on alkene-1 monomers or a-olefins, are the most widely used group of thermoplastic polymers today. The use of many different coordination catalysts has been reported for the production of polyolefin/clay nanocomposites. The methods of in situ synthesis of polyolefin/clay nanocomposite by coordination catalysts mostly depends on the role of clay and can be divided into three categories (1) clay as pol5nner filler, (2) clay as catalyst or cocatalyst support, and (3) Clay acts as a cocatalyst for coordination polymerization. 

Polymerization of vinyl monomers is of enormous industrial importance. These vinyl polymers are mostly thermoplastics and they are used in a wide variety of end-use applications. Many vinyl monomers are polymerized by free-radical, ionic, and coordination polymerization mechanisms. Among these, free-radical polymerization is the most widely used in industrial production of vinyl polymers. Ionic polymerization is generally used to manufacture specialty polymers. Free-radical polymerization is advantageous over other processes in that it is less sensitive to impurities in the raw materials, and the rate of polymerization as well as polymer properties can be controlled by the choice of initiator and polymerization conditions. 

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. 

Ethylene reacts by addition to many inexpensive reagents such as water, chlorine, hydrogen chloride, and oxygen to produce valuable chemicals. It can be initiated by free radicals or by coordination catalysts to produce polyethylene, the largest-volume thermoplastic polymer. It can also be copolymerized with other olefins producing polymers with improved properties. Eor example, when ethylene is polymerized with propylene, a thermoplastic elastomer is obtained. Eigure 7-1 illustrates the most important chemicals based on ethylene. 

Polystyrene (PS) is the fourth big-volume thermoplastic. Styrene can be polymerized alone or copolymerized with other monomers. It can be polymerized by free radical initiators or using coordination catalysts. Recent work using group 4 metallocene combined with methylalumi-noxane produce stereoregular polymer. When homogeneous titanium catalyst is used, the polymer was predominantly syndiotactic. The heterogeneous titanium catalyst gave predominantly the isotactic. Copolymers with butadiene in a ratio of approximately 1 3 produces SBR, the most important synthetic rubber. 

Polypropylene (PP) is a semicrystalline commodity thermoplastic produced by coordination addition polymerization of propylene monomer [197]. Most frequently, stereospecific Ziegler-Natta catalysts are used in industrial processes to produce highly stereospecific crystalline isotactic (iPP) and syndiotactic (sPP) polymer with a small portion of amorphous atactic PP as a side product. Polymerization of non-symmetrical propylene monomer yields three possible sequences however, the steric effect related to the methyl side group highly favors the head-to-tail sequence. The occurence of head-to-head and tail-to-tail sequences produces defects along the PP chain [198]. Presence of such defects affects the overall degree of crystallinity of PP. 

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