Commercial products, controlled radical polymerization

We have demonstrated a new class of effective, recoverable thermormorphic CCT catalysts capable of producing colorless methacrylate oligomers with narrow polydispersity and low molecular weight. For controlled radical polymerization of simple alkyl methacrylates, the use of multiple polyethylene tails of moderate molecular weight (700 Da) gave the best balance of color control and catalyst activity. Porphyrin-derived thermomorphic catalysts met the criteria of easy separation from product resin and low catalyst loss per batch, but were too expensive for commercial implementation. However, the polyethylene-supported cobalt phthalocyanine complex is more economically viable due to its greater ease of synthesis. 

Copolymer composition has a direct effect on the Tg of the polymer, which determines the minimum film forming temperature (MFFT) of the latex and the application. Thus, a 95/5 wt/wt butyl acrylate/methyl methacrylate is an adhesive, whereas a 50/50 copolymer of the same monomers is a binder for paints. Copolymer composition affects properties such as resistance to hydrolysis [4] and weatherability. In situ formed blends of random copolymers of different compositions may be beneficial for application properties [5]. Conventional free-radical polymerization, which is the process used to manufacture almost all commercial emulsion polymers, does not allow the production of block and gradient copolymers (accessible by means of controlled radical polymerization [6], Section 3.3). Nevertheless, graft copolymers are frequently formed, and the extension of grafting largely determines the application properties. Thus, grafting determines the size of the rubber domains in ABS polymers, and the toughness of these polymers increases with rubber size. 

Radical polymerization is often the preferred mechanism for forming polymers and most commercial polymer materials involve radical chemistry at some stage of their production cycle. From both economic and practical viewpoints, the advantages of radical over other forms of polymerization arc many (Chapter 1). However, one of the often-cited "problems" with radical polymerization is a perceived lack of control over the process the inability to precisely control molecular weight and distribution, limited capacity to make complex architectures and the range of undefined defect structures and other forms of "structure irregularity" that may be present in polymers prepared by this mechanism. Much research has been directed at providing answers for problems of this nature. In this, and in the subsequent chapter, we detail the current status of the efforts to redress these issues. In this chapter, wc focus on how to achieve control by appropriate selection of the reaction conditions in conventional radical polymerization. 

The new knowledge and understanding of radical processes has resulted in new polymer structures and in new routes to established materials many with commercial significance. For example, radical polymerization is now used in the production of block copolymers, narrow polydispersity homopolymers, and other materials of controlled architecture that were previously available only by more demanding routes. These commercial developments have added to the resurgence of studies on radical polymerization. 

Coordination copolymerization of ethylene with small amounts of an a-olefin such as 1-butene, 1-hexene, or 1-octene results in the equivalent of the branched, low-density polyethylene produced by radical polymerization. The polyethylene, referred to as linear low-density polyethylene (LLDPE), has controlled amounts of ethyl, n-butyl, and n-hexyl branches, respectively. Copolymerization with propene, 4-methyl-1-pentene, and cycloalk-enes is also practiced. There was little effort to commercialize linear low-density polyethylene (LLDPE) until 1978, when gas-phase technology made the economics of the process very competitive with the high-pressure radical polymerization process [James, 1986]. The expansion of this technology was rapid. The utility of the LLDPE process Emits the need to build new high-pressure plants. New capacity for LDPE has usually involved new plants for the low-pressure gas-phase process, which allows the production of HDPE and LLDPE as well as polypropene. The production of LLDPE in the United States in 2001 was about 8 billion pounds, the same as the production of LDPE. Overall, HDPE and LLDPE, produced by coordination polymerization, comprise two-thirds of all polyethylenes. 

Many industrial free-radical polymerization processes have been developed and commercialized for a variety of monomer and polymer types. A variety of monomers and polymerization processes have been commercially exploited. Further, there have been significant developments in synthesizing polymers with controlled architectures using free-radical polymerization during the last two decades. Materials synthesized include block copolymers, graft copolymers, and radial polymers. These materials find use in many common industrial and household applications such as adhesives, paints and coatings, textiles, nonwoven fabrics, personal care products, wallpaper, construction materials, specialty additives, and many other areas. 

The grafting from procedure requires the generation of active sites on the main polymer chain which are capable of initiating the polymerization of a second monomer. Free radicals can be created by several methods such as irradiation of a polymer in the presence of oxygen [31,32], chain transfer to the backbone [33,34] or redox reaction [35]. Several commercial products have been produced by these methods because they are simple and rather easy to perform. Nevertheless, significant amounts of homopolymers are produced, and, in combination with the poor control of radical polymerization, the final products are characterized by chemical heterogeneity. 

Controlled/living radical polymerization (CRP) is among the most rapidly developing areas of chemistry and polymer science. " The research on CRP encompasses very fundamental mechanistic studies, synthetic polymer chemistry, many detailed physical chemistry studies to evalrrate properties of prepared materials and also search for new appheations to commercialize CRP products. 

Commercial production of PVA from PVAc is carried out via a continuous process. PVAc is polymerized with a free radical initiator in methanol, usually between 55 and 85°C. Molecular weight is controlled by the residence time in the reactors, monomer feed rate, solvent concentration, initiator concentration, and polymerization temperature. Direct hydrolysis or catalyzed alcoholysis converts the PVAc into the corresponding PVA, a water-soluble polymer (1) The degree of hydrolysis can be controlled to yield various grades of PVA super-hydrolyzed (>99 mol%), fully hydrolyzed (98 mol%), and partially hydrolyzed (88 mol%). There are also specialty grades less than 80 mol%. The annual worldwide capacity of PVA is about 750 million pounds. 

Poly(acrylates) and poly(methacrylates) are commercially important polymers with a myriad of uses, including paper and textile coatings, adhesives, caulks and sealants, plasticizers, paint and ink additives, and optical components for computer displays. Since they are derived from monosubsti-tuted and unsymmetrical 1,1-disubstituted vinyl monomers, poly(acrylate) and poly(methacrylate) products with a spectrum of tacticities and thereby mechanical properties are potentially accessible. To date, however, industrially produced materials are generated using free-radical polymerization technology, which offers limited scope for tacticity control. Therefore, there has been much interest in the development of metal-catalyzed routes to these polymers where the coordination environment of the metal offers the potential to influence tacticity. 

The batch emulsion polymerization is commonly used in the laboratory to study the reaction mechanisms, to develop new latex products and to obtain kinetic data for the process development and the reactor scale-up. Most of the commercial latex products are manufactured by semibatch or continuous reaction systems due to the very exothermic nature of the free radical polymerization and the rather limited heat transfer capacity in large-scale reactors. One major difference among the above reported polymerization processes is the residence time distribution of the growing particles within the reactor. The broadness of the residence time distribution in decreasing order is continuous>semibatch>batch. As a consequence, the broadness of the resultant particle size distribution in decreasing order is continuous>semibatch>batch, and the rate of polymerization generally follows the trend batch>semibatch>continuous. Furthermore, the versatile semibatch and continuous emulsion polymerization processes offer the operational flexibility to produce latex products with controlled polymer composition and particle morphology. This may have an important influence on the application properties of latex products [270]. 

Reaction of two or more monomers by free radical polymerization is an effective way of altering the balance of properties of commercial products. Addition of the polar monomer acrylonitrile to styrene (or methyl acrylate to ethylene) produces a polymer that combines the strength of the base homopolymer with much improved oil and grease resistance. The adhesive and cohesive properties of coatings resins are balanced by controlling the mix and relative proportions of monomers in the recipe. 

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