Materials Based on Polystyrene

Cross-linked polystyrene (copolymer with divinyl benzene) was the original support material used by Merrifield for polypeptide syntheses. The material is actually a terpolymer of styrene, chloromethyl styrene, and divinyl benzene.

Copolymers of styrene and divinyl benzene supports have since received many uses and have undergone many chemical modifications for various reactions as reagents and catalysts. The material can be functionalized in many ways. Thus, it can be nitrated, chloromethylated, sulfonated, lithiated, carboxylated, and acylated. The greatest use has been made of the chloromethylated and lithiated derivatives. These two derivatives can react with nucleophilic and electrophilic reagents, respectively, resulting in a wide range of functionalized polymers. For various modification reactions of polystyrene, see Chap. 9. Two types of cross-linked polystyrene are often favored. One is a polymer that is cross-linked by only 1–2% of divinyl benzene. The material, though fairly strong mechanically, swells and expands significantly in volume when dispersed in proper solvents. It is called micro porous. A copolymer with up to 20% divinyl benzene is the second type. It is prepared in the presence of large quantities of diluents to retain the products in expanded form. As a result, the structures are macro porous or macro reticular. The advantages of micro porous over macro porous structures are faster reactions, less fragility, and easier handling. Although, macro reticular supports are less often used, they have the advantage of being useful in almost any solvent. As a variation of the process, Fre´chet and coworkers introduced reactive filtration [6]. In place of beads, they used discs of cross-linked polystyrene that were cut from a rod of the material. On the surface of these discs, they grafted 2,2-dimethylazlactone. These discs were then used in a filtration to efficiently scavenge excess amines from a reaction mixture. Subsequently, Frechet and coworkers also used such discs in a flow through acylation reaction [7]. To improve the access to the functional groups, Lee and coworkers developed a process of placing most of the functional groups on the outer surface of the polystyrene beads [7]. This was done by surface aminomethylation of preformed cross-linked commercially available polystyrene beads. Such bead usually range in size from 100 to 400 mesh (in.) in diameter and can be functionalized by surface reactions or by surface grafts. This yielded materials with a lower number of functional groups, but with the majority of them being accessible. Lee and coworkers then utilized the beads in a solid peptide synthesis [7]. Lee and coworkers also prepared beads with N-heterocyclic carbene ligands located on the outer shell. In this preparation, they used beads formed in a suspension polymerization [8, 9]. Fre´chet and coworkers [7] reported a variation of the strategy by attaching functional groups to cross-linked polystyrene in the interior of a soluble star polymer matrix. In this process, the catalytic groups are core-confined through the use of low molecular weight macroinitiators that form the surface of the final polymer. Presumably, this allows using simultaneous incompatible reagents, like acids and bases, because they are physically isolated from one another. Lu and Toy [10] illustrated a similar approach: by showing how core-functionalized star polymers are prepared to form sulfonic acid-functionalized core material:

There is a drawback, however, of using the polymer described above, because many functional groups end up imbedded inside the resin and access to them requires the type of solvents that can thoroughly swell the resin. Many polar solvents, however, fail to swell cross-linked polystyrene adequately, yet may be required for specific reactions. That led to modifications, such as, the use of polar cross-linking materials, like oligomers of glycols. One example is a cross-linked polystyrene poly (ethylene glycol) composite, known as Tentagel [10]. It can be illustrated as follows:

Another example is work by Bradley and coworkers who incorporated short oligomer (polyethyl ene glycol) groups into the backbone of the cross-linked polystyrene [11]. The oligomer poly (ethylene glycol) in this preparation also acts as a spacer to separate the polystyrene backbone from locations of the reactions. The material was used efficiently in a solid phase peptide synthesis. Toy et al. [12] demonstrated that by replacing divinyl benzene with more flexible compounds usually increases their mechanical stability and allows them to absorb more solvent. In addition, when the cross-linked compounds contain oligomers of such materials as ethylene glycol, the compatibility with polar solvents increases. Subsequently, Janda and coworkers used polytetrahydrofuran in sus pension polymerization of styrene to prepare general solid support resins for organic syntheses [13].

The material became known as Janda Gel [13]. The preparation of this gel with n equal to one is illustrated above [14]. An interesting application of cross-linked styrene resins that are similar to the Janda gel was developed by Kobayashi and coworkers, who used such materials to encapsulate metal catalysts [15]. The techniqueusedwastofirstentrapthemetalcatalystsbycoacervationwithlinearpolystyrenethatwasfunctionalized both by epoxide groups and oligo poly (ethylene glycol). The polymer becomes cross-linked upon heating through reaction of the epoxide groups with polyethylene glycol entrapping the metal:

 The above solid catalysts were used successfully to catalyze various reactions, like the Suzuki–Miyura reaction [15], such as amino carbonylation, amidation, and the Heck reactions [16]. Polymers containing scandium triflate, ruthenium, platinum, or gold were prepared to perform Mukayama aldol, alcohol, and sulfide oxidation, hydrogenation, and indole syntheses [17]. Incorporation of some of the metals, like palladium, is improved by incorporating phosphine ligands into the polymer. One such ligand can be illustrated as follows [19]:

Hoveyda, Schrock, and coworkers [19] reported using chiral cross-linking compounds immobilized on heterogeneous polystyrene resins. The chiral moiety was then used as a ligand in asymmetric catalyses. In one application, they used the material to prepare a recyclable chiral molybdenum olefin metathesis catalyst that was used in enantioselective ring opening and ring closing metathesis reactions. The material can be illustrated as follows:

The products that were isolated possessed only slightly lower enantiomeric excess than those obtained with the corresponding small molecule catalyst. In a similar manner, Sellner et al. [20] prepared a variety of polystyrene beads with embedded a,a,a,a-tertiaryl-l,3-dioxolane-4,5-dlmethanol groups. They were subsequently used to form an immobilized catalyst with recyclable chiral titanium Lewis acid for addition of Bu2Zn to benzaldehyde.

They also reported preparations of various cross-linkers containing tertiary aryl groups with polymerizable carbon-carbon double bonds [20]. Itsuno et al. [21] synthesized a cross-linked polymer support with a chiral 1,2-diamine for enantioselective ruthenium transfer hydrogenation catalysis of aromatic ketones. Avariation on the process was to functionalize polystyrene by incorporating groups like hydroxy, acetoxy, or nitrile onto the backbone of polystyrene, through copolymerization of monomers [22]. Following are two examples:

Similar to the work by Frechet who grafted functional groups to heterogeneous polystyrene, Hodges used living free-radical polymerization to prepare what he referred to as Rasta resin [23]. The Merrifield resin was first functionalized with reduced TEMPO to form a resin core. With the help of Rhodamine dye, it was confirmed that the grafts and associated functional groups were located Similar to the work by Frechet who grafted functional groups to heterogeneous polystyrene (see above), Hodges used living free-radical polymerization to prepare what he referred to as Rasta resin [23]. The Merrifield resin was first functionalized with reduced TEMPO to form a resin core. With the help of Rhodamine dye, it was confirmed that the grafts and associated functional groups were located

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مواضيع ذات صلة

Immobilized Nonenzymatic Catalysts

Immobilized Catalysts

Special Gels for Drug Release

Support Materials Other Than Polystyrene

Polymer Supports for Reagents, Catalysts, and Drug Release

Degradation of Polymeric Materials by Ionizing Radiation

Oxidation of Chain-Growth Polymers

Oxidative Degradation of Polymers

Thermal Degradation of Cellulosic Materials

Thermal Degradation of Polyimides, Polyoxidiazoles, and Polyquinoxalines

Thermal Degradation of Aromatic Polysulfones

Thermal Degradation of Epoxy Resins