Styrene, polymerization, anionic thermal

Figure 1.40. Chain scission (given as number of scissions, S, per number-average molecule) during thermal degradation of poly(styrene) polymerized anionically (a), M = 2.3x10, or by free-radical initiation (b), M — 1.5x 10. Adapted from McNeill (1989).

Since polystyrene is one of the oldest commercial polymers with over 9 million tonnes/yr of sales, there have been thousands of patents issued covering all aspects of its manufacture and property enhancement. The styrene monomer readily polymerizes to polystyrene either thermally or with free-radical initiators (see Chapter 6 on free-radical polymerization and Chapter 8 on nitroxide-mediated polymerization). Commercial processes for the manufacture of polystyrene are described in Chapter 3 while process modelling and optimization of styrene polymerization is examined in Chapter 5. Styrene also can be polymerized via anionic and Ziegler-Natta chemistries using organometallic initiators. Using free radical and anionic polymerization chemistries, the... 

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. 

If styrene is polymerized at SO C in emulsion, the initiating radicals are essentially produced by the decomposition of the persulfate to radical anions. In case of monomers like 1,4-DVB, however, additional radicals in substantial amounts are formed by thermal initiation. Therefore the radical formation is significantly higher, more micelles are initiated and consequently more but smaller particles are formed. 

Anionic polymerization not only allows the production of low residual PS (i.e. typically PS produced using continuous anionic polymerization contains <20 ppm residual styrene) but also anionically produced PS is more thermally... 

Polymerization of styrene in microemulsions has produced porous solid materials with interesting morphology and thermal properties. The morphology, porosity and thermal properties are affected by the type and concentration of surfactant and cosurfactant. The polymers obtained from anionic microemulsions exhibit Tg higher than normal polystyrene, whereas the polymers from nonionic microemulsions exhibit a lower Tg. This is due to the role of electrostatic interactions between the SDS ions and polystyrene. Transport properties of the polymers obtained from microemulsions were also determine. Gas phase permeability and diffusion coefficients of different gases in the polymers are reported. The polymers exhibit some ionic conductivity. 

Most Grignard reagents are inert toward styrene (up to the temperature of spontaneous thermal polymerization). This is a significant difference from lithium alkyls, which are readily able to initiate styrenic monomers [123]. The only reported exception is p-vinylbenzyl magnesium chloride, which polymerized styrene in THF at O C, but not at — 78X [50,51]. Substitution at the puru-position of a phenyl ring may stabilize the benzyl anion, owing to the delocatlization of electrons, and favor ionic dissociation of... 

Anionic-Radical Combinations. Radical grafting of one monomer on the backbone of another polymer is well known and is the basis of an important commercial process for making high impact polystyrene. Styrene is thermally bulk polymerized in the presence of 5 to 10% (by weight) polybutadiene, the polymerization proceeding by a free-radical grafting path (70). 

Styrene can be polymerized radically either thermally or by using free-radical initiators, anionically or cationical ly. Thermally the reaction is initiated by a Diels Alder adduct in the following manner (2) ... 

Evidence clearly shows that the thermal stability of PS is dependent on the polymerization mechanism. Upon heating at 285 C for 2.5 h under different vacuum levels, anionic PS loses less molecular weight (Fig. 14) and generates l s styrene monomer (Fig, 15) than FR PS both produced using continuous solution polymerization processes [73]. 

Qutubuddin and coworkers [43,44] were the first to report on the preparation of solid porous materials by polymerization of styrene in Winsor I, II, and III microemulsions stabilized by an anionic surfactant (SDS) and 2-pentanol or by nonionic surfactants. The porosity of materials obtained in the middle phase was greater than that obtained with either oil-continuous or water-continuous microemulsions. This is related to the structure of middle-phase microemulsions, which consist of oily and aqueous bicontinuous interconnected domains. A major difficulty encountered during the thermal polymerization was phase separation. A solid, opaque polymer was obtained in the middle with excess phases at the top (essentially 2-pentanol) and bottom (94% water). The nature of the surfactant had a profound effect on the mechanical properties of polymers. The polymers formed from nonionic microemulsions were ductile and nonconductive and exhibited a glass transition temperature lower than that of normal polystyrene. The polymers formed from anionic microemulsions were brittle and conductive and exhibited a higher Tj,. This was attributed to strong ionic interactions between polystyrene and SDS. 

PCL with the TEMPO 2,2,6,6-tetrameihYlpiperidinoxyl) moiety behaved as a polymeric counter-radical for the polymerization of styrene, resulting in the quantitative formation of PCL-fo-PSt. The radical polymerization was found to proceed in accordance with a living mechanism, without undesirable side reactions. The thermal analysis of the block copolymer indicated that the components of PCL and PSt were completely immiscible and microphase-separated. Incorporation of the TEMPO moiety into PEO chain-ends in the radical form was also achieved [53]. In this case, TEMPO-Na was used as an initiator in a hving anionic polymerization of ethylene oxide (Scheme 11.10), under conditions such that the stable nitroxyl radical at the end of the PEO chain could not be destroyed. 

In many cases of start reactions, a monomer is added on to the initiator to form an active center. The active center may be an anion, cation, or free radical in addition polymerization, or, for example, an electron-deficient compound or an unoccupied ligand position in polyinsertion (see Chapter 19). Spontaneous thermal addition polymerizations of monomers which proceed in the absence of added catalyst or initiator are relatively rare. The free radical thermal polymerization of styrene (Chapter 20) and the charge transfer copolymerization of monomers of opposed polarities (Chapter 22) are examples of genuine spontaneous polymerizations. These genuine spontaneous polymerizations can often only be distinguished with difficulty from nongenuine spontaneous polymerizations which are started by unsuspected impurities remaining in these systems. 

Styrene is one of the few substances that can be polymerized equally well free-radically (thermally and with initiators), cationically, anionically, and with complex catalysts. Free radical, cationic, and most anionic polymerizations yield atactic polymers, whereas certain polymerizations of the polyinsertion type yield isotactic polymers. Only the free radical polymerizations are of commercial interest. 

In order to synthesize an interfacial polyelectrolyte the ionizable groups must be chemically bound to the interface and be an integral part of the polymer molecules comprising the colloidal particles. It is preferable to avoid adsorbed emulsifier, which is usually employed to stabilize these colloids, as it complicates the subsequent purification and quantitative surface characterization of the system. Thus the classical technique of emulsion polymerization, as described by Harkins [2], is not applicable. The systems are actually simpler, employing only monomer(s), water and initiator. It is most important that the initiator be very soluble in the water and that it form ionizable free radicals. The monomer must have a finite solubility in water, although this may be very small (even styrene satisfies this requirement at 0.038 wt.%), but may be present in amounts far in excess of this as an initially separate phase. Initiation of polymerization must then occur in the aqueous solution. For instance, if the initiator is potassium persulfate, K2S2O8, free radical-ions are formed (along with some OH) by the thermal decomposition of the anion ... 

---