General Aspects

Crosslinkers In order to obtain insoluble resins, given amounts of crosslinking agent have to be used for the synthesis of the beads by suspension polymerization (see also Sect. 3.2.1.2 and Sect. 3.2.1.3). A crosslinking agent is generally a bi- or multifunctional molecule that can be incorporated into two or more growing

Tab. 3.1. Features and applications of frequently used solid-phase resins.

Type

Common features

Representatives

Applications a

Loading [mmolg~']

Gel (microporous)

High capacity

1% or 2%

A, B,

D, E

0.2-5.0

crosslinked PS

(Merrifield resin)

Different solvent

swelling

Reagent access by

PEGA,

A, F

0.4

diffusion

PAP

0.7

Uniform sites

POEPOP

A

0.4-0.7

SPOCC

0.4-1.2

POEPS-3

0.2-0.3

Many unsuitable for

CLEAR

A, F

0.2-0.3

continuous flow

synthesis

Hybrid

High capacity

ROMP

D

3.0

Different solvent

Rasta silanes

D

1.6-3.8

swelling (but less

than microporous

gels)

Encapsulated gel

Suitable for

PolyHIPE

F

1.0-5.0

continuous flow

(acrylamide resin

synthesis

encapsulated in

PS "shell")

Unstable to agitation

Pepsyn-K (similar

A, F

-

resin encapsulated

in inorganic

scaffold)

Graft copolymers

Lower capacity

PEG-PS

D, E

1.2-2.9

More uniform

TentagelĀ®

A, B,

D

0.25-0.6

swelling

Pressure stable

ArgogelĀ®

A, B,

D

0.4-0.5

Suitable for

NovaGelĀ®

A, B,

D

0.5-0.7

continuous flow

synthesis

Rigid

Capacity and

Highly crosslinked

E, F

<1.0

macrorporous

efficiency depend

PS

supports

on specific surface

area, mean pore

volume, pore

distribution, and

mean pore size

Silica

A, E,

F

0.006-0.06

CPG

A, B,

E, F

0.1

a Applications: A, peptide synthesis; B, oligonucleotide synthesis; C, oligosaccharide synthesis; D, synthesis of small organic molecules; E, polymer supported reagents; F, continuous flow synthesis.

Diameter [|im]

Fig. 3.2. Relation between mesh size and bead diameter in micrometers.

Diameter [|im]

Fig. 3.2. Relation between mesh size and bead diameter in micrometers.

chains during the polymerization process, leading to interconnected chains. The most popular crosslinking agent, used in the presence of styrenic monomers, is divinylbenzene (DVB).

Crosslinked polystyrene resins can be further modified in many ways, such as Friedel-Crafts acylations [15, 16], electrophilic substitutions [17, 18], lithiations and so forth. The resulting beads can be used in further reactions to produce resins with various linker functionalities. However, for the synthesis of standard Merrifield resins, the functional group (e.g. chloromethyl styrene) is typically incorporated as a comonomer during the polymerization process.

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