Polymerization of Styrene

Polystyrene (PS) is one of the largest volume vinyl polymers, used in countless products from food-packing and plastic cutlery to house insulation. The primary reasons for its great popularity are its low cost, high transparency, good mechanical properties and ease of coloring, foaming, and processing.

Commercial polystyrene is mostly synthesized by bulk, suspension or solution polymerization of ethylbenzene (styrene). The most common method is free radical polymerization, using benzoyl peroxide as initiator. However, other initiators such as redox systems and azo compounds can be used as well to start the polymerization. The reaction is exothermic, and thus the monomer-polymer mixture has to be cooled. In the case of bulk or mass polymerization, the reaction exotherm is controlled by using a two-stage polymerization process. In the first stage, the styrene is polymerized in a stirred tank reactor, the so-called pre-polymerizer. Only a low conversion is achieved. The mixture of dispersed polymer in monomer is then transferred to a tubular thin-film reactor. The pure molten polystyrene that emerges from the reactor is pumped through spinnerets or through an extruder to produce the desired finished granulate (2-5 mm pellets).¹
The polystyrene produced by mass polymerization is a fully transparent, atactic, low cost thermoplastic known as general purpose polystyrene (GPPS) or crystal-clear polystyrene. PS can also be prepared by solution and suspension techniques. Both processes can be carried out in batch or continuously. In the case of solution polymerization, styrene is dissolved in a suitable solvent such as ethylbenzene, which makes temperature control much easier. However, the presence of solvent reduces the molecular weight and lowers the transparency due to a higher degree of impurities. The suspension process is also very common, especially in the production of expandable polystyrene (EPS) and high impact polystyrene (HIPS). The later contains 5 to 10% butadiene rubber and has much improved impact resistance.

PS is one of the few vinyl monomers that can be polymerized at moderate temperatures without the addition of free radical initiators. The process is known as spontaneous thermal polymerization and has been extensively studied for more than half a century.^2,3 However, until recently, there was no consensus about the correct mechanism. The first plausible mechanism was suggested by Flory.² He postulated that two styrene monomers (M) combine to form a 1,4-diradical (M₂··), which either ring-closes to form 1,2-diphenylcyclobutane (DCB) or reacts with a third styrene monomer to produce two monoradicals (M·, M₂·) via hydrogen transfer or abstraction.

Another mechanism of self-initiation was suggested by Mayo.³ He postulated that two monomers combine to form a Diels-Alder styrene dimer (AH), which then transfers a hydrogen to a third monomer (molecular assisted homolysis) to form two monoradicals. This mechanism was confirmed by Khuong et al. who showed that AH, the Diels-Alder dimer, is indeed the key intermediate for self-initiation of styrene polymerization.⁴

Styrene is one of the most versatile monomers. Besides (thermal or radiation induced) free radical polymerization, it can be polymerized by practically any other method of chain polymerization including cationic and anionic polymerization. The relative ease of polymerization of styrene can be explained by resonance stabilization of the growing polystyrene in its transition state, that is the aromatic ring of the growth center delocalizes and stabilizes positive and negative charges as well as radicals as shown below.

The styrene polymerization has been studied more extensively than any other mainly because of its relatively reproducible and simple kinetic. The free radical mechanism for styrene is shown below. It involves (a) the formation of (heat induced) radicals followed by the radical's reaction with a styrene monomer (Initiation)⁶, (b) the progressive addition of monomers to the growing polymer chain (propagation), and (c) a termination step, which is the destruction of the growth active center by combination or coupling of two radicals.

 

In the case of heat-induced initiation, the initiator concentration in the steady-state is second-order in styrene concentration:

[I] = k_i [M]²

The rate of propagation is proportional to both the concentration of monomer and free radical initiator:

R_p = -d[M] / dt = k_p [M] ([I] / k_t)^1/2

Combining the two expressions for rate we have⁷

R_p = -d[M] / dt = k_p [M]² (k_i / k_t)^1/2 = k_M [M]²

where [M] and [I] are the concentration of monomer and initiator and k_p, k_i and k_t are the reaction constants of propagation, initiation and termination, respectively.
The equation above states that the rate of polymerization is second order in respect to styrene concentration. However, above relationships do not necessary apply to all free-radical polymerization techniques. For example, if a solvent is present, the solvent can act as a regulator (free-radical transfer to solvent) which will change the kinetic model. As Mayo has shown for styrene in solution, a third-order initiation kinetic is more consistent with experimental results:⁸

R_p = -d[M] / dt = k_p [M]^5/2 (k_i / k_t)^1/2 = k_M [M]^5/2

References and Notes

1. The polymerization can also be completed under aqueous suspension conditions.

2. P. J. Flory, J. Am. Chem. Soc., 59, 241-253 (1937)

3. F. R. Mayo, J. Am. Chem. Soc., 90, 1289-1295 (1968) & 75, 6133-6141 (1953)

4. K.S. Khuong, W.H. Jones, W.A. Pryor & K.N. Houk; J. Am. Chem. Soc.; 127; 1265-1277 (2005)

5. W. Trakarnpruk, N. Apipanyasopon, J. Metals, Materials and Minerals, 12, 2 19-26 (2003)

6. Not all radicals form polymer radicals; some react with each other to form inactive species.

7. C. Walling, E.R. Briggs, and F.R. Mayo, J. Am. Chem. Soc., 68 (7), 1145-1149 (1946)

8. F.R. Mayo, J. Amer. Chem. Soc., 75, 6133 (1953)