Stereoregular Polystyrene

Figure 1.2 shows sections of polymer chains of these three types the substituent R equals phenyl for polystyrene and methyl for polypropylene. The general term for this stereoregularity is tacticity, a term derived from the Greek word meaning to put in order. ... 

The observation in 1949 (4) that isobutyl vinyl ether (IBVE) can be polymerized with stereoregularity ushered in the stereochemical study of polymers, eventually leading to the development of stereoregular polypropylene. In fact, vinyl ethers were key monomers in the early polymer Hterature. Eor example, ethyl vinyl ether (EVE) was first polymerized in the presence of iodine in 1878 and the overall polymerization was systematically studied during the 1920s (5). There has been much academic interest in living cationic polymerization of vinyl ethers and in the unusual compatibiUty of poly(MVE) with polystyrene. 

After brief discussion of the state-of-the-art of modern Py-GC/MS, some most recent applications for stixictural and compositional chai acterization of polymeric materials are described in detail. These include microstixictural studies on sequence distributions of copolymers, stereoregularity and end group chai acterization for various vinyl-type polymers such as polystyrene and polymethyl methacrylate by use of conventional analytical pyrolysis. 

Because of the chain-stiffening effect of the benzene ring the TgS of commercial materials are in the range 90-100°C and isotactic polymers have similar values (approx. 100°C). A consequence of this Tg value plus the amorphous nature of the polymer is that we have a material that is hard and transparent at room temperature. Isotactic polystyrenes have been known since 1955 but have not been of commercial importance. Syndiotactic polystyrene using metallocene catalysis has recently become of commercial interest. Both stereoregular polymers are crystalline with values of 230°C and 270°C for the isotactic and syndiotactic materials respectively. They are also somewhat brittle (see Section 16.3). 

Polystyrene (PS) is the fourth big-volume thermoplastic. Styrene can be polymerized alone or copolymerized with other monomers. It can be polymerized by free radical initiators or using coordination catalysts. Recent work using group 4 metallocene combined with methylalumi-noxane produce stereoregular polymer. When homogeneous titanium catalyst is used, the polymer was predominantly syndiotactic. The heterogeneous titanium catalyst gave predominantly the isotactic. Copolymers with butadiene in a ratio of approximately 1 3 produces SBR, the most important synthetic rubber. 

As an example we report in this paper the conformational energy maps of two already cited stereoregular polymers, which have been obtained very recently, syndiotactic polystyrene s-PS and syndiotactic polybutene s-PB (Fig. 4 and 5, respectively). In fact, the energy map calculated for s-PS shows... 

We can employ coordination polymerization to produce stereoregular polystyrene. By performing this type of reaction at low temperatures, using Ziegler-Natta or single-site catalysts, we can prepare isotactic and syndiotactic versions of polystyrene. 

Syndiotactic polystyrene is a new polymeric material150 152 of industrial relevance since it shows a high melting point (270°C) and high crystallization rates.153 Syndiotactic polystyrene is a highly stereoregular polymer which can be obtained by using several soluble titanium and, to a less extent, zirconium compounds. 

In the crystal state most stereoregular polymers have helical conformations. Group s(M/N) 1 comprises all the isotactic vinyl polymers [polypropylene, polybutene, polystyrene, etc., M/N = 3/1 poly-o-methylstyrene, etc., 4/1 ... 

Among the uncommon stmctures of stereoregular polymers determined in recent times, is that of isotactic polystyrene first observed by Keller and coworkers in crystalline gels (185) and later studied by Corradini et al. (186). In this case too, a highly stretched helix [ (6/1)] is observed, with unit height h = 5.1 A and imit twist t = 60°. The repeating unit contains two independent monomer units with rotation angles close to 180°. 

Polymers such as polystyrene, poly(vinyl chloride), and poly(methyl methacrylate) show very poor crystallization tendencies. Loss of structural simplicity (compared to polyethylene) results in a marked decrease in the tendency toward crystallization. Fluorocarbon polymers such as poly(vinyl fluoride), poly(vinylidene fluoride), and polytetrafluoroethylene are exceptions. These polymers show considerable crystallinity since the small size of fluorine does not preclude packing into a crystal lattice. Crystallization is also aided by the high secondary attractive forces. High secondary attractive forces coupled with symmetry account for the presence of significant crystallinity in poly(vinylidene chloride). Symmetry alone without significant polarity, as in polyisobutylene, is insufficient for the development of crystallinity. (The effect of stereoregularity of polymer structure on crystallinity is postponed to Sec. 8-2a.)... 

The general correlations of structure and properties of homopolymers are summarized in Table 2.13. Some experiments which demonstrate the influence of the molecular weight or the structure on selected properties of polymers are described in Examples 3-6 (degree of polymerization of polystyrene and solution viscosity), 3-15, 3-21, 3-31 (stereoregularity of polyisoprene resp. polystyrene), 4-7 and 5-11 (influence of crosslinking) or Sects. 4.1.1 and 4.1.2 (stiffness of the main chain of aliphatic and aromatic polyesters and polyamides). 

These additives usually enhance specific properties of polymers. Thus solid, pure stereoregular polystyrene (PS) is brittle yet, as a result of the addition of the proper impact modifiers and other additives, the modified PS exhibits the properties of a good plastic and rubber. 

The anionic polymerization of styrene to produce stereoregular polymers has been studied by Kern and co-workers (23). They found that the polymerization of styrene by n-amylsodium at low temperatures produced crystalline polystyrene. Braun, HerNER and Kern (24) reported also that the potassium polymerization of styrene produced polymers with increased crystallinity as the temperature was decreased. 

Although many stereoregular polymers have a helical conformation in the solid state (5,96], the conformation is lost in solution in most cases, except in the case of some polyolefins with optically active side groups [12], because the dynamics of the polymer chain are extremely fast in solution. Therefore, isotactic polystyrene [15,16] and polypropylene [17] prepared with an optically active catalyst do not show optical activity due to a helical conformation. However, a helical conformation can be maintained in solution for some polymers having a rigid main chain or bulky side groups that prevent mutation to random conformation, and the conformation may... 

Commercial polystyrene, one of the most representative general-purpose resins, is an amorphous atactic polymer with no melting point (softening temperature ca 100 °C, glass transition temperature ca 70-100 °C). Since the discovery of Ziegler-Natta catalysts, many efforts to produce stereoregular polystyrenes have been made. 

Styrene polymerisation with heterogeneous Ziegler Natta catalysts activated by alkylaluminium compounds generally produces a mixture of isotactic and non-stereoregular polymer. For example, polystyrene produced with the... 

The affect of polymer stereoregularity in the chains on the PAL data has also been studied. Hamielec et al [56] found what appears to be an increased lifetime (hole size) with increased randomness of the chain configuration in a series of polyvinlychloride (PVC) polymers, despite the large degree of scatter in the sample (probably due to the fact that a series of commercially available products were used.). They however found little correlation with tacticity in polypropylene. More recently a PAL study on a series of very well characterized polystyrene and poly(p-methlystyrene) samples of differing tacticity [57] was performed. In addition to finding that the polystyrene samples have smaller free volume holes than the poly(p-methylstyrene) samples, they found that the syndiotactic samples had broader hole distributions than the attactic samples. 

Monocyclopentadienyl complexes of titaninm (Cp TtXs) perform poorly as catalysts for ethylene or propylene polymerization, bnt in the presence of MAO, they polymerize styrene to stereo- and regioregnlar syndiotactic polystyrene, a crystalline material with very high melting point (273 °C) and glass transition temperature (100°C). In this case, the active polymerizing species is a Ti complex (Figure 8). Each styrene monomer inserts in a secondary manner and the stereoregularity is maintained by the conformation of the last inserted unit (chain-end control). 

It was also Staudinger in 1932 who first proposed that the inability of polystyrene to crystallize was due to its lack of stereoregularity which rendered it amorphous. It is its amorphous nature that is responsible for its solubility -though others claimed that polymer solubility was incompatible with very high molecular weight [3]. 

High -cis polybutadiene has relatively high heat resistance, which is advantageous in the processing of HIPS. On the other hand, this type of polybutadiene crystallizes at about 0 °C, owing to its stereoregular structure, with the consequence that the low-temperature toughness of polystyrene, produced in this way, is reduced. 

---