To attach the molecule of interest to the resin, it is necessary to use a spacer unit and/or linker. This ensures that the molecule can be cleaved from the resin when desired, although the linker should remain inert to the chemistry carried out on the molecule.
A schematic to show the function of linkers in Solid Phase Chemistry:

There are two important properties a linker must possess:
1. It must be stable to all the reactions used in a synthesis. It is particularly important to ensure compatibility with the protecting group strategy used e.g. for peptide synthesis, the linker should be stable to Fmoc and/or Boc chemistries to permit the elongation of the peptide.
2. It must be quantitatively and selectively cleaved under mild conditions to release the target molecule without degradation occurring.
There are several classes of linker currently available for solid phase chemistry and these may be grouped according to their particular method of linkage:
1. Linkers which link through functionality already present in the target molecule e.g. those linkers used in Solid Phase Peptide Synthesis (SPPS) link through amide or ester bonds.
2. Linkers which become incorporated into the target molecule upon cleavage (a means of introducing increased diversity).
3. The introduction of an additional functional group into the target molecule, thus providing a site of attachment for the linker (another means of introducing increased diversity).
4. Traceless linkers, the target compound has no "memory" of the linker or its site of attachment, e.g. silicon and germanium linkers.