Polystyrene, chain-type

The resins can be divided into two groups having major structural differences gel and macroreticular . In the case of gel type resins if the beads are totally dry, then the polymeric matrix collapses and the polystyrene chains will be as close as atomic forces allow. Therefore, swelling ability of the reactants is a prerequisite for catalysis by gel resins. Gel resins are characterized by a divinyl benzene content that is generally below 12%. 

Block copolymers of polystyrene with rubbery polymers are made by polymerizing styrene in the presence of an unsaturated rubber such as 1,4 polybutadiene or polystyrene co-butadiene. Some of the growing polystyrene chains incorporate vinyl groups from the rubbers to create block copolymers of the type shown in Fig. 21.4. The combination of incompatible hard polystyrene blocks and soft rubber blocks creates a material in which the different molecular blocks segregate into discrete phases. The chemical composition and lengths of the block controls the phase morphology. When polystyrene dominates, the rubber particles form... 

In one of several important studies on dendronized polymers [4c, 4d]. Schluter and coworkers explored the stiffening of polystyrene chains through the incorporation of Frechet-type dendrons as side chains [28, 29]. While the G-l and G-2 dendrons were not sufficiently bulky to effectively stiffen the polystyrene chain, the G-3 dendron provides enough steric bulk to force the hybrid polymer into adopting a cylindrical shape in solution [28b], In a complementary study, Neubert and Schluter demonstrated that adding charges to the dendritic wedges leads to an expansion of the chains of the hybrid copolymer in aqueous solution [29],... 

Subsequent polymerization of styrene with these MMA prepolymers as initiators leads to block copolymers of an ABA-type due to the tendency of the growing polystyrene chains to terminate by combination ... 

They possess a gel-type structure due to their styrenic macromolecular chains lightly crosslinked by 1 to 2 % of divinylbenzene. Therefore, the Merrifield resin and its derivatives are able to swell significantly only on good solvents of the polystyrene chain. Their efficiency is therefore strongly lowered in more polar solvents such as alcohol or water. 

Fig. 9.37. Typical model for sorption complexes of proline enantiomers on (.S )-proline- or (S)-hydroxyproline-derived poly.styrene-type sorbents. Retention of (5)-Pro is diminished by the steric interaction with the water molecule crdinatcd in the axial position of the Cu(ll) ion. Retention of (R)-Pro is enhanced by the (favourable in the aqueous mobile phase) hydrophobic interaction with the non polar polystyrene chain (reprinted with permis.sion from Ref. 1403]).

In spite of the different types of crosslinking agents, the degree of bridging, and the concentration of polystyrene chains in the initial solution, hypercrosslinked polymers exhibit narrow pore-size distribution the diameter of the majority of the pores is about 10 A. The exclusion limit of the polymeric gels is rather small polystyrene standards with coils of50-60 A in diameter in chloroform solution (molecular weight of 12,000-16,000 Da) are completely excluded from the polymer phase. It should be emphasized. 

Graft copolymers are a third copolymer type in which the polymerization of a comonomer initiates at reactive sites present on a preformed chain backbone. This technique improves the mixing of two incompatible polymers. As an example, grafting butadiene monomer units onto the polystyrene chain backbone (Figure 1.9) increases the impact resistance of polystyrene. 

Although gel-type resins are the most widely used supports in solid-phase chemistry, it has been found that such supports are not always the most suitable for peptide synthesis. This is due to two main factors polystyrene chains are hydrophobic and are not completely compatible with peptides and, furthermore, bulky peptides can adopt unfavorable conformations within the polystyrene cross-linked matrix. 

Benoit and others (94-98) prepared two types of tagged polystyrene networks (a) type A networks containing labeled (deuterated) cross-link sites. This permitted a characterization of the spatial distribution of the cross-link points, (b) Type B networks containing a few percent of perdeuterated polystyrene chains (see Figure 9.23) (94). Cross-hnking utilized divinyl benzene (DVB). 

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