Example from polystyrene

Example 2.20 A cylindrical vessel with an outside radius of 20 mm and an inside radius of 12 mm has a radial crack 3.5 mm deep on the outside surface. If the vessel is made from polystyrene which has a critical stress intensity factor of 1.0 MN calculate the maximum permissible pressure in this vessel. 

Of the instances of so-called solvent cracking of amorphous polymers known to the author, the liquid involved is not usually a true solvent of the polymer but instead has a solubility parameter on the borderline of the solubility range. Examples are polystyrene and white spirit, polycarbonate and methanol and ethyl acetate with polysulphone. The propensity to solvent stress cracking is however far from predictable and intending users of a polymer would have to check on this before use. 

The principles needed to design a polymer of low flammability are reasonably well understood and have been systematized by Van Krevelen (5). A number of methods have been found for modifying the structure of an inherently flammable polymer to make it respond better to conventional flame retardant systems. For example, extensive work by Pearce et al. at Polytechnic (38, 39) has demonstrated that incorporation of certain ring systems such as phthalide or fluorenone structures into a polymer can greatly increase char and thus flame resistance. Pearce, et al. also showed that increased char formation from polystyrene could be achieved by the introduction of chloromethyl groups on the aromatic rings, along with the addition of antimony oxide or zinc oxide to provide a latent Friedel-Crafts catalyst. 

The isolation of product is usually possible after evaporation of the solvent and extraction with hexane, ether, or toluene. Supported versions, for example on polystyrene grafted with PPh2 groups, have proved unsatisfactory because the rate of deactivation is greatly enhanced under these conditions [37]. Asymmetric versions exist, but the ee-values tend to be lower than in the Rh series [38]. With acid to neutralize the basic N lone pair, imine reduction is fast. Should it be necessary to remove the catalyst from solutions in order to isolate a strictly metal-free product, a resin containing a thiol group should prove satisfactory. A thiol group in the substrate deactivates the catalyst, however. 

Most solid-phase syntheses of pyrazoles are based on the cyclocondensation of hydrazines with suitable 1,3-dielectrophiles. The reported examples include the reaction of hydrazines with support-bound a,(3-unsaturated ketones, 1,3-diketones, 3-keto esters, a-(cyano)carbonyl compounds, and a, 3-unsaturated nitriles (Table 15.19). Pyrazoles have also been prepared from polystyrene-bound 3-(hydrazino)esters, which are generated by the addition of ester enolates to hydrazones (Entry 7, Table 15.19 see also Section 10.3). Benzopyrazoles can be prepared from support-bound hydra-zones using the reaction sequence outlined in Figure 15.11. Oxidation of a polystyrene-bound benzophenone hydrazone yields an a-(acyloxy)azo compound. Upon treatment with a Lewis acid, this intermediate is converted into a 1,2-diazaallyl cation,... 

The ion movement can be controlled by ion exchange or ion transfer membranes, thin sheets of cross-linked organic polymers with ion exchange properties—for example, sulfonated polystyrene-divinylbenzene polymers. Both cation-permeable and anion-permeable membranes are available and have been described (3, 9). To achieve demineralization, these membranes are spaced alternately between a cathode and an anode which introduce direct current. The compartment between each pair of membranes is filled with a saline water. The resulting ion motion is controlled by the membranes, so that one set of compartments—for example, the even-numbered compartments—lose ions and the odd-numbered compartments gain ions. The product from the ion-losing cells is collected and comprises electrically demineralized water. 

Some thermoplastics can also be used in casting processes in these cases polymerisation takes place in the mould which is filled with the monomer. Examples are polystyrene, polymerised from styrene, and polymethylmethacrylate from its monomer. These reactions can take place quite easily in the presence of suitable catalysts. In this way thick sheets can be produced, but also large articles such as PA-6 ship propellers or gear wheels, as polymerisation products from caprolactam. 

Although solution blending has only been used at the lab scale at this time, compared with the in situ process, it may be more industrially friendly, particularly for the primary polymer producers who have operations, which can easily recover and recycle the solvent. High dilution is required and this may have an effect on the production of the PNs and the process is quite dependent on the individual polymer. Some polymers have many solvents from which to choose while others do not. A typical example is polystyrene, which can dissolve in a variety of solvents, so it is easy to find a solvent that is compatible with both the clay and the polymer. Polyolefins, on the other hand, require high boiling solvents and the high temperature may exert an effect of thermal degradation on the modifier. 

Example 14-3 Continuing with the preceding examples for the migration of styrene from polystyrene, this time assume the migration is diffnsion controlled. Assume the product has a 6 month shelf life at room temperature (23 °C). 

Many polymers exhibit neither a measurable stick-slip transition nor flow oscillation. For example, commercial polystyrene (PS), polypropylene (PP), and low density polyethylene (LDPE) usually do not undergo a flow discontinuity transition nor oscillating flow. This does not mean that their extrudate would remain smooth. The often observed spiral-like extrudate distortion of PS, LDPE and PP, among other polymer melts, normally arises from a secondary (vortex) flow in the barrel due to a sharp die entry and is unrelated to interfacial slip. Section 11 discusses this type of extrudate distortion in some detail. Here we focus on the question of why polymers such as PS often do not exhibit interfacial flow instabilities and flow discontinuity. The answer is contained in the celebrated formula Eqs. (3) or (5). For a polymer to show an observable wall slip on a length scale of 1 mm requires a viscosity ratio q/q equal to 105 or larger. In other words, there should be a sufficient level of bulk chain entanglement at the critical stress for an interfacial breakdown (i.e., disentanglement transition between adsorbed and unbound chains). The above-mentioned commercial polymers do not meet this criterion. 

Supported oxo-rhenium catalysts in heterogeneous systems have also been reported, for example, the polystyrene-supported (catecholato)oxo-rhenium(VII) complexes (38), obtained from the reaction of polystyrene-supported catechol with [ReOCl3(PPh3)2], which catalyze alcohol oxidation to ketones or aldehydes with dimethylsulfoxide and epoxide... 

The uniqueness of these polymers is derived from a combination of performance attributes. The SBC family of polymers offers outstanding clarity and excellent impact strength or shatter resistance, and are easy to process. Primarily, these type polymers fill the gap between low-cost commodity materials and high-cost performance polymers. For example, crystal polystyrene offers excellent clarity, but very poor impact resistance, and polycarbonate offers excellent impact resistance, but at a significant cost premium. 

These companies have created core capabilities in areas other than production. They have, for example, used superior deal-making and human resources skills to expand rapidly. Moreover, they have focused their businesses by acquiring and merging business units from less focused competitors. With these, they have built up powerful positions in product innovation or cost leadership - hke that of Nova, for example, in polystyrene. 

In addition polymers, by contrast, the recurring units have the same structures as the monomer(s) from which the polymer was formed. Examples are polystyrene (1-1), polyethylene (1-3), styrene-maleic anhydride copolymers (1-26), and so on. 

A support material bearing a silyl linker (50) can be prepared from polystyrene, according to Farrall and Frechet [59]. This has been successfully applied to the synthesis of diverse prostaglandins [60]. One example of such a synthesis is outlined in Scheme 25. 

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