Ziegler-Natta polymerization productivity

Cyclic Polyolefins (GPO) and Gycloolefin Copolymers (GOG). Japanese and European companies are developing amorphous cycHc polyolefins as substrate materials for optical data storage (213—217). The materials are based on dicyclopentadiene and/or tetracyclododecene (10), where R = H, alkyl, or COOCH. Products are formed by Ziegler-Natta polymerization with addition of ethylene or propylene (11) or so-called metathesis polymerization and hydrogenation (12), (101,216). These products may stiU contain about 10% of the dicycHc stmcture (216). 

When ethylene reacts with triethyl- or tripropylaluminum, multiple carbometa-lation takes place, resulting in the formation of oligomers.509 Oxidation of the products followed by hydrolysis yields alcohols, whereas displacement reaction produces terminal alkenes that are of commercial importance.510 Transition-metal compounds promote the addition to form polymers (Ziegler-Natta polymerization see Section 13.2.4). 

Theoretically, it is possible for the process of olefin coordination and insertion to continue as in Ziegler-Natta polymerization (Chapter 52) but with palladium the metal is expelled from the molecule by a p-hydride elimination reaction and the product is an alkene. For the whole process to be catalytic, a palladium(O) complex must be regenerated from the palladium(ll) product of P-hydride elimination. This occurs in the presence of base which removes HX from the palladium(II) species. 

By far the most important industrial coordination polymerization processes are Ziegler-Natta polymerizations of 1-olefins [107-110], most notably the production of high-density polyethene [111] and stereo-specific olefin polymers and copolymers [108], However, these processes employ solid catalysts, and the complex kinetics on their surfaces have no place in a book on homogeneous reactions. 

Solution polymerization. Solution polymerization involves polymerization of a monomer in a solvent in which both the monomer (reactant) and polymer (product) are soluble. Monomers are polymerized in a solution that can be homogeneous or heterogeneous. Many free radical polymerizations are conducted in solution. Ionic polymerizations are almost exclusively solution processes along with many Ziegler-Natta polymerizations. Important water-soluble polymers that can be prepared in aqueous solution include poly(acrylic acid), polyacrylamide, poly(vinyl alcohol), and poly(iV-vinylpyrrolidinone). Poly(methyl methacrylate), polystyrene, polybutadiene, poly(vinyl chloride), and poly(vinylidene fluoride) can be polymerized in organic solvents. 

Olefin metathesis was first discovered during research stemming from Ziegler-Natta polymerization catalysis in the late 1950s [13-15]. The term olefin metathesis was not coined until 1967 [16]. Olefin metathesis is the apparent exchange of the carbons of olefins to produce new olefins. Empirically, this process can swap the substituents of olefins to give aU possible products (Scheme 6.1). 

The utilization of electrochemical syntheses for the production of Ziegler-Natta polymerization catalysts has received some consideration in view of possible industrial applications. 

In 1957, at the Montecatini Laboratories in Italy, Giulio Natta continued the work of Ziegler and used what is now termed Ziegler-Natta polymerization to create polypropylene. When Natta reported the polymerization of ethylene with a titanocene catalyst, it became clear that polymer chains with specific tacticities, or specific ordered structures, were possible. Polypropylene rose to become a substitute for polyethylene in products in which slightly higher temperature stability was necessary, for example, dishwasher-safe cups and plates. 

Freqnently, the prodnct of Ziegler-Natta polymerization is sterically impure and can be preferentially extracted to give two products a highly crystalline stereoregular fraction and an amorphous atactic one. This may be attributed to the size of the catalyst particles as stereoregularity is enhanced by having large particles, whereas a finely divided catalyst tends to produce an amorphous polymer. 

Ziegler-Natta polymerization is well known to involve a two-stage process [148, 149]. In the first stage, an aluminum alkyl (such as trialkyl aluminum) is reacted with TiCU in order to produce active jS-TiCls. The alkyl radicals, which are also produced in this reaction, are terminated by coupling and create inert products. Subsequent alkylation of -TiCb then occurs to generate the titanium species that is capable of initiating the polymerization of olefins such as ethylene (Scheme 11.37). 

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