Monomers styrene

There is considerable research by resin manufacturers to offer resins and processes that minimize emissions of styrene, to conform to increasingly tight legislation at the workplace. Styrene is not known to present any hazard to the general public, during the use of moulded products. 

Crompton and co-authors [76] have described a UV spectroscopy method for the determination of styrene monomer in chloroform. 

In this method the PS sample (0.5 g) is dissolved in 50 ml of chloroform or another suitable spectroscopic solvent and the UV spectrum recorded in the region 280-310 nm against polymer-free solvent in the reference cell. If the polymer is incompletely soluble in the solvent (e.g., due to the presence of gel or pigments), it is dissolved in 30 ml of solvent and the suspension filtered rapidly under light vacuum through an asbestos pad to remove insolubles. 

This baseline correction technique can obviously be applied to the determination of styrene monomer in PS only if any other UV absorbing constituents in the polymer extract (e.g., lubricant, antioxidants) absorb linearly in the wavelength range 288-300 nm. If the polymer extract contains polymer constituents other than styrene with non-linear absorptions in this region, then incorrect styrene monomer contents will be obtained. An obvious technique for removing such non-volatile UV absorbing compounds is by distillation of the extract followed by UV spectroscopic analysis of the distillate for styrene monomer as described next. 

Method Solvent Styrene monomer (% w/w) Polystyrene sample  

Reproduced with permission from T.R. Crompton, L.W. Myers and D. Blair, British Plastics, 1965, 38, 12, 740 [76]  

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FIGURE 27 14 A section of polystyrene showing one of the benzene rings modified by chloromethylation Indi vidual polystyrene chains in the resin used in solid phase peptide synthesis are con nected to one another at various points (cross linked) by adding a small amount of p divinylbenzene to the styrene monomer The chloromethylation step is carried out under conditions such that only about 10% of the benzene rings bear —CH2CI groups... 

Compositional control ia suspension systems can be achieved with a corrected batch process. A suspension process has been described where styrene monomer is continuously added until 75—85% conversion, and then the excess acrylonittile monomer is removed by stripping with an iaert gas... 

Ketone Peroxides. These materials are mixtures of compounds with hydroperoxy groups and are composed primarily of the two stmctures shown in Table 2. Ketone peroxides are marketed as solutions in inert solvents such as dimethyl phthalate. They are primarily employed in room-temperature-initiated curing of unsaturated polyester resin compositions (usually containing styrene monomer) using transition-metal promoters such as cobalt naphthenate. Ketone peroxides contain the hydroperoxy (—OOH) group and thus are susceptible to the same ha2ards as hydroperoxides. 

The kinetics of initiation reactions of alkyllithium compounds often exhibit fractional kinetic order dependence on the total concentration of initiator as shown in Table 2. For example, the kinetics of the initiation reaction of //-butyUithium with styrene monomer in benzene exhibit a first-order dependence on styrene concentration and a one-sixth order dependence on //-butyUithium concentration as shown in equation 13, where is the rate constant for... 

The reaction rate of fumarate polyester polymers with styrene is 20 times that of similar maleate polymers. Commercial phthaHc and isophthaHc resins usually have fumarate levels in excess of 95% and demonstrate full hardness and property development when catalyzed and cured. The addition polymerization reaction between the fumarate polyester polymer and styrene monomer is initiated by free-radical catalysts, commercially usually benzoyl peroxide (BPO) and methyl ethyl ketone peroxide (MEKP), which can be dissociated by heat or redox metal activators into peroxy and hydroperoxy free radicals. 

The free styrene monomer is restrained within the gel and further reaction with fumarate groups is determined by the spacial arrangement the styrene polymerizes in homopolymer blocks as it intercepts fumarate reaction sites. As individual micelles expand and deplete available fumarate sites in the short polymer chains, the remaining styrene forms homopolymer blocks that terminate at the boundaries between overlapping micelles (Fig. 4). 

Methacrylate monomers are most effective with derivatives of bisphenol A epoxy dimethacrylates, in which the methacrylate—methacrylate cross-linking reaction proceeds at a much faster pace than with styrene monomer. This proves beneficial in some fabrication processes requiring faster cure, such as pultmsion and resin-transfer mol ding (RTM). 

Ethylbenzene. This alkylben2ene is almost exclusively used as an intermediate for the manufacture of styrene monomer [100-42-5]. A small amount (<1%) is used as a solvent and as an intermediate in dye manufacture (1,39,40). The current ethylben2ene growth rate projections for 1990—1995 range from 3.0 to 3.5%/yr (39). 

Vinylpyridine (23) came into prominence around 1950 as a component of latex. Butadiene and styrene monomers were used with (23) to make a terpolymer that bonded fabric cords to the mbber matrix of automobile tires (25). More recendy, the abiUty of (23) to act as a Michael acceptor has been exploited in a synthesis of 4-dimethylaminopyridine (DMAP) (24) (26). The sequence consists of a Michael addition of (23) to 4-cyanopyridine (15), replacement of the 4-cyano substituent by dimethylamine (taking advantage of the activation of the cyano group by quatemization of the pyridine ring), and base-cataly2ed dequatemization (retro Michael addition). 4-r)imethyl aminopyri dine is one of the most effective acylation catalysts known (27). 

Styrene [100-42-5] (phenylethene, viaylben2ene, phenylethylene, styrol, cinnamene), CgH5CH=CH2, is the simplest and by far the most important member of a series of aromatic monomers. Also known commercially as styrene monomer (SM), styrene is produced in large quantities for polymerization. It is a versatile monomer extensively used for the manufacture of plastics, including crystalline polystyrene, mbber-modifted impact polystyrene, expandable polystyrene, acrylonitrile—butadiene—styrene copolymer (ABS), styrene—acrylonitrile resins (SAN), styrene—butadiene latex, styrene—butadiene mbber (qv) (SBR), and unsaturated polyester resins (see Acrylonithile polya rs Styrene plastics). 

Table 5. Specifications for Typical Polymerization-Grade Styrene Monomer Product...

Fig. 6. Approximate explosive limits of styrene monomer vapor in equiUbrium with Hquid styrene in air, where represents the explosive region. To...

OC-Methylstyrene. This compound is not a styrenic monomer in the strict sense. The methyl substitution on the side chain, rather than the aromatic ring, moderates its reactivity in polymerization. It is used as a specialty monomer in ABS resins, coatings, polyester resins, and hot-melt adhesives. As a copolymer in ABS and polystyrene, it increases the heat-distortion resistance of the product. In coatings and resins, it moderates reaction rates and improves clarity. Physical properties of a-methylstyrene [98-83-9] are shown in Table 12. 

Styrene. Commercial manufacture of this commodity monomer depends on ethylbenzene, which is converted by several means to a low purity styrene, subsequendy distilled to the pure form. A small percentage of styrene is made from the oxidative process, whereby ethylbenzene is oxidized to a hydroperoxide or alcohol and then dehydrated to styrene. A popular commercial route has been the alkylation of benzene to ethylbenzene, with ethylene, after which the cmde ethylbenzene is distilled to give high purity ethylbenzene. The ethylbenzene is direcdy dehydrogenated to styrene monomer in the vapor phase with steam and appropriate catalysts. Most styrene is manufactured by variations of this process. A variety of catalyst systems are used, based on ferric oxide with other components, including potassium salts, which improve the catalytic activity (10). 

In 1990, the annual U.S. capacity to manufacture styrene monomer was 4,273,000 t/yr, and production was 3,636,000 t/yr (11). Polystyrene resin is the dominant user of styrene monomer. SBR use is about 7% of U.S. domestic styrene monomer production. Woddwide production in 1995 was projected to be 77% of capacity as demand increased just under 5% per year, from 1990 consumption of 13,771,000 to 17,000,000 metric tons in 1995. 

One of the key benefits of anionic PS is that it contains much lower levels of residual styrene monomer than free-radical PS (167). This is because free-radical polymerization processes only operate at 60—80% styrene conversion, whereas anionic processes operate at >99% styrene conversion. Removal of unreacted styrene monomer from free-radical PS is accompHshed using continuous devolatilization at high temperature (220—260°C) and vacuum. This process leaves about 200—800 ppm of styrene monomer in the product. Taking the styrene to a lower level requires special devolatilization procedures such as steam stripping (168). 

Most of the styrene monomer manufactured globally goes into the manufacture of PS and its copolymers, thus the price of the two tend to parallel each other (Fig. 32). 

Fig. 32. Historical comparison of styrene monomer and general-purpose PS pricing.

The performance of many metal-ion catalysts can be enhanced by doping with cesium compounds. This is a result both of the low ionization potential of cesium and its abiUty to stabilize high oxidation states of transition-metal oxo anions (50). Catalyst doping is one of the principal commercial uses of cesium. Cesium is a more powerflil oxidant than potassium, which it can replace. The amount of replacement is often a matter of economic benefit. Cesium-doped catalysts are used for the production of styrene monomer from ethyl benzene at metal oxide contacts or from toluene and methanol as Cs-exchanged zeofltes ethylene oxide ammonoxidation, acrolein (methacrolein) acryflc acid (methacrylic acid) methyl methacrylate monomer methanol phthahc anhydride anthraquinone various olefins chlorinations in low pressure ammonia synthesis and in the conversion of SO2 to SO in sulfuric acid production. 

In these mbber-modified polystyrene polymers, the mbbers should have low T, large particle sizes (0.5—5 J.m), graftable and cross-linkable sites, and should be compatible with styrene monomer (93). Polybutadiene, with a T of —SS C, meets all of these requirements and is used most frequently. These mbber-modified systems exhibit excellent low temperature impact strength, a required attribute for use in refrigerators. 

The product of this reaction (S—B LC) may initiate a further reaction with styrene monomer to give S—B—S LC. This, in turn, can react with an alcohol, ROH, to give S—B—SH + LiOR. Alternatively, S—B LC may react with a coupling agent such as an organohalogen (45) ... 

This product is a difunctional initiator and can polymerize styrene monomer ... 

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