Atactic polymers production

Figure 7.10 shows the 60-MHz spectra of poly (methyl methacrylate) prepared with different catalysts so that predominately isotactic, syndiotactic, and atactic products are formed. The three spectra in Fig. 7.10 are identified in terms of this predominant character. It is apparent that the spectra are quite different, especially in the range of 5 values between about 1 and 2 ppm. Since the atactic polymer has the least regular structure, we concentrate on the other two to make the assignment of the spectral features to the various protons. 

Gas-phase polymerization of propylene was pioneered by BASF, who developed the Novolen process which uses stirred-bed reactors (Fig. 8) (125). Unreacted monomer is condensed and recycled to the polymerizer, providing additional removal of the heat of reaction. As in the early Hquid-phase systems, post-reactor treatment of the polymer is required to remove catalyst residues (126). The high content of atactic polymer in the final product limits its usefiilness in many markets. 

In the 1970s, Solvay iatroduced an advanced TiCl catalyst with high activity and stereoregulahty (6). When this catalyst was utilized ia Hquid monomer processes, the level of atactic polymer was sufftciendy low so that its removal from the product was not required. Catalyst residues were also reduced so that simplified systems for post-reactor treatment were acceptable. Sumitomo has developed a Hquid monomer process, used by Exxon (United States), ia which polymer slurry is washed ia a countercurrent column with fresh monomer and alcohol to provide highly purified polymer (128). 

Erom 1955—1975, the Ziegler-Natta catalyst (91), which is titanium trichloride used in combination with diethylaluminum chloride, was the catalyst system for propylene polymerization. However, its low activity, which is less than 1000 g polymer/g catalyst in most cases, and low selectivity (ca 90% to isotactic polymer) required polypropylene manufacturers to purify the reactor product by washing out spent catalyst residues and removing unwanted atactic polymer by solvent extraction. These operations added significantly to the cost of pre-1980 polypropylene. 

In the slurry process, propylene monomer is dissolved in a hydrocarbon diluent in which the polymerization process occurs. The polymerization products are either soluble (the highly atactic components) or insoluble. Both the insoluble and soluble components are collected and form separate product streams. The insoluble species form a slurry in the solvent, from which they are removed by centrifugation. The soluble, atactic component is removed with the solvent as another product stream. To separate the atactic polymer from the solvent, the solution is heated allowing the solvent to flash off, leaving the atactic polymer behind. Any un reacted monomer is degassed from the solution and recycled to the start of the polymerization process. 

In case of polypropylene some atactic polymer also gets formed in addition to the required isotactic polymer but much of this atactic material is soluble in the diluent so that the product isolated would be largely isotactic polymer. 

After the polymerization step, the reaction mixture is fed to a heated separation tank where the unreacted propylene is flashed off and recycled. The polymer slurry is then washed with alcohol to deactivate and remove the catalyst and the atactic polymer (the bad stuff.) Centrifuging the slurry removes the diluent from the isotactic PP (the good stuff.) The product is washed with acetone, dried, and stabilized with suitable additives. It is sold as a powder or can be pelletized into granules. 

Although exhaustive efforts have been made in the search for biologically acceptable catalysts, there are only a few examples of low toxicity, which mainly lead to atactic polymers of little practical use. Another route to gain control over the tacticity of PHB is the transformation of cheap building blocks to enantiomericaUy pure p-BL, which can be distilled off from the catalyst and polymerized with retention of the stereochemistry by ecofriendly initiators. This route combines many advantages. At first, even toxic metal centers can be chosen since the product can easily be separated from the catalyst and secondly, any tacticity of the polymer will be available by simply mixing enantiopure p-BL with the racemic mixture in the desired ratio. In this manner a fine-tuning of the mechanical properties becomes possible and easily performable (Fig. 36). 

Polymerization with oscillating metallocenes is complicated because solvent fractionation of the polymer product shows separate fractions—highly atactic, mostly isotactic, and isotactic-atactic stereoblock. The mechanism of this phenomenon is not clear. It may result from the initiators not being perfectly single-site initiators. There is some evidence that a metallocene initiator may consist of more than one species, and that each species produces a different stereochemical result (Sec. 8-5g-l, 8-5h-l). 

More sophisticated experimental and theoretical analysis of isoselective polymerizations have been performed by using a two-site model for propagation [Inoue et al., 1984 Wu et al., 1990]. The polymer product is fractionated into the highly isotactic, insoluble and atactic,... 

Polymethyl methacrylate (PMMA) An atactic PMMA with a Ts of HOC and an intrinsic viscosity of 1.4 was obtained from Scientific Polymer Products, Inc. 

At present all commercial polystyrene (with average molecular weights between 100,000 and 400,000) is manufactured by radical polymerization, which yields atactic polymers.476 Peroxides and azo compounds are commonly used initiators. The suspension process (usually as a batch process in water at 80-140°C) produces a product with relatively high residual monomer content.223 More important is the continuous solution process (usually in ethylbenzene solvent at 90-180°C), which yields high-purity product. Styrene can be copolymerized with numerous other monomers.477 One of these copolymers, the styrene-divinylbenzene copolymer produced by free-radical polymerization, has a crosslinked stucture and is used in... 

Current Processes. The development of superactive third-generation supported catalysts enabled the introduction of simplified processes, without sections for catalyst deactivation or removal of atactic polymer. By eliminating the waste streams associated with the neutralization of catalyst residues and purification of the recycled diluent and alcohol, these processes minimize any potential environmental impact. Investment costs arc reduced by approximately one-third over slurry process plants. Energy consumption is minimized by elimination of the distillation of recycled diluent and alcohol. The total plant cost for the production of polymer is less than 130% of the monomer price, when a modem process is used, compared to 175% for a slurry process. 

Metallocenes such as Cp2TiCl2 and Cp2ZrCl2 alone are capable of polymerising styrene to an atactic polymer (involving a free radical propagation mechanism) [97]. The same metallocenes activated with methylaluminoxane form active catalysts for the polymerisation of styrene their productivity and syn-diospecificity, however, are not very high. In contrast, when activated with aluminium alkyls, these metallocenes do not afford catalysts that might be active in the polymerisation of styrene [98,99]. 

Polypropylene owes its current market success to the development of coordination polymerization. Before 1957 it was not produced commercially because radical polymerization gives an atactic polymer that is amorphous and has poor mechanical properties. Using a coordination catalyst, however, enables the production of an isotactic polymer that is semicrystalline. This material is stiff and hard and has a high tensile strength. Among its many useful products are rope, molded objects, and furniture. 

Description The process, with a combination of the most advanced high-yield and high-stereospecificity catalyst, is a nonsolvent, nondeash-ing process. It eliminates atactic polymers and catalyst residue removal. The process can produce various grades of PP with outstanding product quality. Polymer yields of 20,000 to 100,000 kg/kg of supported catalyst are obtained, and the total isotactic index of polymer can reach 98% to 99%. 

The typical process would yield a slurry of 15 to 18% by weight of polymer solid particles in a hexane dispersion. An operating temperature of at least 80° C. would be required to keep the side products of the reaction—atactic polymer, for example—in solution. 

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