a-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.
Table 12. Physical Properties of a-Methylstyrenea
Property Value molecular weight 118.18
refractive index, n20D 1.53864
viscosity at 20°C, 0.940
surface tension at 20°C, mN/m () 32.40
density at 20°C, g/cm3 0.9106
boiling point, °C 165
freezing point, °C flash point (Cleveland open-cup), °C 57.8
fire point (Cleveland open-cup), °C 57.8
explosive limits, % in air 0.7-3.4
vapor pressure, kPab at 20°C 0.253 60°C 2.400 100°C 13.066 160°C 88.660
critical pressure, Pc, Mpac 4.36
critical temperature, tc, °C 384
critical volume, Vc, mL/g 3.26
critical density, dc, g/mL0.29
specific heat of liquid, J/()d at 40°C 2.0460 100°C 2.1757
specific heat of vapor at 25°C, J/()d 1.2357
latent heat of vaporization, DHv, J/gd at 25°C 404.55
boiling point 326.35
heat of combustion, DHc, gas at constant pressure at 25°C, kJ/mold 4863.73
heat of formation, DHf, liquid at 25°C, kJ/mold 112.97
heat of polymerization, kJ/mold 39.75
Q value 0.76 e value cubical coefficient of expansion at 20°C
solubility in H2O at 25°C 0.056 H2O in monomer at 25°C 0.010 solvent compatibilitye
Production of a-methylstyrene (AMS) from cumene by dehydrogenation was practiced commercially by Dow until 1977. It is now produced as a by-product in the production of phenol and acetone from cumene. Cumene is manufactured by alkylation of benzene with propylene. In the phenol-acetone process, cumene is oxidized in the liquid phase thermally to cumene hydroperoxide. The hydroperoxide is split into phenol and acetone by a cleavage reaction catalyzed by sulfur dioxide. Up to 2% of the cumene is converted to a-methylstyrene. Phenol and acetone are large-volume chemicals and the supply of the by-product a-methylstyrene is well in excess of its demand. Producers are forced to hydrogenate it back to cumene for recycle to the phenol-acetone plant. Estimated plant capacities of the U.S. producers of a-methylstyrene are listed in Table 13 (80).
Table 13. U.S. a-Methylstyrene Producers and Capacities, 1995 (56.000 t/y)