Polystyrene weak links

Richards and Slater (75) used a labelled polystyrene to demonstrate the existence of intermolecular chain transfer in the thermal degradation process. Polystyrene-14C, prepared in the normal way, was mixed with an inactive polystyrene specially prepared with weak links so that it degraded at temperatures where the polystyrene-14C was stable when on its own. Appearance of styrene- C monomer in the volatile degradation products proved the existence of intermolecular chain transfer (Reaction 9). 

In thermal oxidation, initiation (1) results Irom the thermal dissociation of chemical bonds that may arise Irom intrinsically weak links formed as by-products of the polymerization reaction (e.g. head-to-head links) or impurities formed in the polymerization reactor such as hydroperoxides, POOH, or in-chain peroxides as occur in polystyrene from oxygen scavenging. Reaction (1 ) shows that POOH may produce peroxy and alkoxy radicals that may subsequently form alkyl radicals via reaction (3). 

A rapid initial drop in molecular weight followed by a slower decrease is observed when polyvinylpyridine is heated at 250°C [85]. This behaviour is qualitatively similar to that of polystyrene. Scission of weak links may be involved in the fast decay of molecular weight, but random scission may also explain the shape of the curve. As in the case of polystyrene, the mechanistic problem is very complex and many more experiments are needed to solve it. Chelation of 2- and 4-polyvinylpyridine makes those polymers less heat-resistant chain scissions already occur at 100°C while the uncomplexed polymer suffers no damage at this temperature. On heating, a change in the absorption spectrum of 2-polyvinylpyridine copper chelate dissolved in 1M HC1 is observed a new peak is formed at... 

Jellinek [306, 307] suggested that synthetic and natural polymers contain weak links which are more readily broken than the normal backbone bonds. A small fraction of bonds in any polymer sample is more susceptible to scission than the majority. Weak links, due to oxygen becoming incorporated into polystyrene molecules, would certainly give structures that were thermally labile. Oakes and Richards [461] and Davies et al. [181] made a similar suggestion for polyolefins. 

The Tg of the blends are then intermediate between the two polymers. The thermal stability and glass transition temperature of H-H PS are very similar to those of atactic H-T PS despite the structural differences. Although the H-H linkage has been suggested as a possible "weak link" in the commonly manufactured H-T polystyrene, the thermal stability evidence suggests that this is not the case. ... 

Figure 6-22. Influence of the solvent power on the swelling of weakly cross-linked samples of polystyrene (cross-linked with divinylbenzene). From left to right unswollen sample, swelling in the poor solvent cyclohexane (xo high), swelling in the good solvent benzene (xo low).

Diblock copolymers formed firom polystyrene covalently linked to poly( -pentylmethacrylate), P(S-b-nPMA), which have only weak segmental interactions, are shown to exhibit closed-loop phase behavior over a narrow range of molecular weight. 

There are two types of stationary phases commonly used in exclusion chromatography silica gel and micro-reticulated cross-linked polystyrene gels. A third type of exclusion media is comprised of the Dextran gels. Dextran gels are produced by the action of certain bacteria on a sucrose substrate. They consist of framework of glucose units that can form a gel in aqueous solvents that have size exclusion properties. Unfortunately the gels are mechanically weak and thus, cannot tolerate the high pressures necessary for HPLC and, as a consequence, are of very limited use to the analyst. 

Figure 34 A polymer-supported metallocene catalyst (51) with a weakly coordinating anion, [B(C6F5)4] , produced from lightly cross-linked, chloromethylated polystyrene beads for olefin polymerization. (Adapted from ref. 75.)...

Reversed-phase chromatography employs a nonpolar stationary phase and a polar aqueous-organic mobile phase. The stationary phase may be a nonpolar ligand, such as an alkyl hydrocarbon, bonded to a support matrix such as microparticulate silica, or it may be a microparticulate polymeric resin such as cross-linked polystyrene-divinylbenzene. The mobile phase is typically a binary mixture of a weak solvent, such as water or an aqueous buffer, and a strong solvent such as acetonitrile or a short-chain alcohol. Retention is modulated by changing the relative proportion of the weak and strong solvents. Additives may be incorporated into the mobile phase to modulate chromatographic selectivity, to suppress undesirable interactions of the analyte with the matrix, or to promote analyte solubility or stability. 

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