Polystyrene chemical initiators

Commercial polystyrenes are normally rather pure polymers. The amount of styrene, ethylbenzene, styrene dimers and trimers, and other hydrocarbons is minimized by effective devolatilization or by the use of chemical initiators (33). Polystyrenes with low overall volatiles content have relatively high heat-deformation temperatures. The very low content of monomer and other solvents, eg, ethylbenzene, in PS is desirable in the packaging of food. The negligible level of extraction of organic materials from PS is of cmcial importance in this appHcation. 

Styrene and its derivatives can be polymerized by all possible propagation mechanisms. Free-radical polymerization, however, is the primary process for industrial production of polystyrene. Free-radical polymerization of styrene can be carried out without chemical initiators simply by heating the monomer.223,224 On heating, isomeric 1-phenyltetralins are formed in the Diels-Alder... 

Many chemical compounds that react with polystyrene can initiate crazing if the material is subjected to high stress. On this basis several series of chemicals have been used. Both photomicrographs and their corresponding laser diffraction patterns are recorded side by side for clarity. Table I also lists the chemical formulations and an indication of the intensity of crazing for each of the compounds used. 

SPEEDING UP THE RATE OF POLYSTYRENE PRODUCTION USING CHEMICAL INITIATORS... 

Over 200 references describing spontaneous, and chemically initiated styrene polymerization chemistry are reviewed with special emphasis on advances taking place in the past decade. The review is limited to chemistry useful for making amorphous high molecular weight polystyrene in solution polymerization processes. Chemical initiators have been categorized into three basic groups as follws 1) anionic 2) mono-radical and 3) diradical. Analytical techniques used for determination of free radical polymerization kinetics and mechanisms are also discussed. 

Styrene is one of the oldest and most studied monomers. It spontaneously generates free radials upon heating above 100 °C and polymerizes yielding amorphous polystyrene (PS). Styrene can also be polymerized by other mechanisms (anionic, cationic, or Zeigler-Natta) with the aid of chemical initiators. Commercially, over twenty billion pounds of PS are produced annually worldwide. All of this polystyrene is produced via free radical (FR) chemistry, and mostly via continuous solution polymerization processes. The commercial preference for the continuous solution process is due mainly to economic factors. Non-solution polymerization processes (suspension and emulsion) have lower reactor efficiency (product/reactor volume) due to reactor volume occupied by the water which adds to the manufacturing cost. 

Polystyrene was first manufactured commercially (1938) by The Dow Chemical Company. Styrene was non-continuously bulk polymerized, without the aid of a chemical initiator, to high conversion by heating it in metals cans. The cans were opened and the solid PS ground into small pieces. Over the next 35 years, most of the research focused on understanding the mechanism of self-initiated (spontaneous) polymerization of styrene and developing continuous solution polymerization processes. In recent years, solution polymerization research emphasis has focused upon understanding the chemistry of chemical initiators. Today, most PS is produced via continuous solution polymerization with the aid of peroxide initiation. 

High Impact Polystyrene (HIPS) HIPS is a heterogeneous material produced by continuous bulk or bulk-suspension processes, in which a butadiene-based elastomer (polybutadiene (PB), or a block copolymer of styrene-butadiene) is first dissolved in styrene monomer (St) and the resulting mixture is then heated so that the polymerization proceeds either thermally or with the aid of a chemical initiator. At the molecular level, the product is a mixture of free polystyrene (PSt) chains and elastomer chains grafted with PSt side chains. The process yields a continuous (free) PSt matrix containing... 

Since the introduetion of polystyrene, many modification techniques have been developed to enhanee the properties of polystyrene. Because of the breadth of publications on this topic, this chapter only diseusses eurrent trends in development of styiene-based polymers by highlighting some of the significant processes. Basically, chemical modifications are defined as any process that involves changes in chemical bonding. Thus, copolymerization is considered a kind of chemical modification. Other ways to modify polystyrenes chemically include altering the polymerization conditions with different initiators, chain transfer agents, and comonomers. 

In contrast to a living polymer, the lifetime of a growing chain in free radical polymerizations is very short, typically on the order of milliseconds. Free radical concentrations - which include all the growing chain - are very low, typically 10" to 10" mole m"To a reasonable approximation, the system consists of unreacted monomer, unreacted initiator, and dead polymer. The free radicals are formed spontaneously in some systems such as polystyrene, but chemical initiation is more common ... 

The cation is more reactive than the free radical, thereby dominating the initiation reaction. Radical cations can also he formed by high energy electrons, vacuum UV radiation, high intensity electric fields, or anodic oxidation of the monomer. Nuclear chemical initiation involves the use of tritium this method produced very high molecular weight polyisobutylenes and polystyrenes (65). 

A wide variety of polystyrene-like polymers and copolymers (crystal. Impact modified, ABPMS, PMS-AN, PMS-BR, PMS-MA and PMS-MMA)(28) have been prepared from PMS using bulk, solvent and suspension polymerization techniques in our laboratories and pilot plants using thermal, anionic and chemical initiation. From a resin manufacturing point of view, PMS monomer can be processed in existing styrene polymerization equipment to produce poly-PMS analogues. However, process development must be done to optimize conditions for each resin type. 

These results demonstrate some interesting chemical principles of the use of acrylic adhesives. They stick to a broad range of substrates, with some notable exceptions. One of these is galvanized steel, a chemically active substrate which can interact with the adhesive and inhibit cure. Another is Noryl , a blend of polystyrene and polyphenylene oxide. It contains phenol groups that are known polymerization inhibitors. Highly non-polar substrates such as polyolefins and silicones are difficult to bond with any technology, but as we shall see, the initiator can play a big role in acrylic adhesion to polyolefins. 

An effective method of NVF chemical modification is graft copolymerization [34,35]. This reaction is initiated by free radicals of the cellulose molecule. The cellulose is treated with an aqueous solution with selected ions and is exposed to a high-energy radiation. Then, the cellulose molecule cracks and radicals are formed. Afterwards, the radical sites of the cellulose are treated with a suitable solution (compatible with the polymer matrix), for example vinyl monomer [35] acrylonitrile [34], methyl methacrylate [47], polystyrene [41]. The resulting copolymer possesses properties characteristic of both fibrous cellulose and grafted polymer. 

The presence of two hydroxyl groups per molecule in poly-(methyl methacrylate) and in polystyrene, each polymerized in aqueous media using the hydrogen peroxide-ferrous ion initiation system, has been established " by chemical analysis and determination of the average molecular weight. Poly-(methyl methacrylate) polymerized by azo-bis-isobutyronitrile labeled with radioactive has been shown to... 

We have considerable latitude when it comes to choosing the chemical composition of rubber toughened polystyrene. Suitable unsaturated rubbers include styrene-butadiene copolymers, cis 1,4 polybutadiene, and ethylene-propylene-diene copolymers. Acrylonitrile-butadiene-styrene is a more complex type of block copolymer. It is made by swelling polybutadiene with styrene and acrylonitrile, then initiating copolymerization. This typically takes place in an emulsion polymerization process. 

P. Zhou, G. Q. Chen, C. Z. Li, F. S. Du, Z. C. Li, F. M. Li, Synthesis of hammerlike macromolecules of C60 with well-defined polystyrene chains via atom transfer radical polymerization (ATRP) using a C60-monoadduct initiator, Chemical Communications, pp. 797-798, 2000. 

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