Non-integral linkers

B. Non-integral linkers/handles. In bold, the linker/handle... 

Figure 1.1 Examples of integral (a) and non-integral (b) linkers/handles. Adapted from reference [16, 17].

In the chemical literature, linkers are further divided into integral linkers that are part of the resin and non-integral or grafted linkers, where a part of the linker is attached to the core of the resin. The integral linkers exhibit some disadvantages since the reaction takes place in direct proximity to the resin. Steric and electronic effects will exert a direct influence on the outcome of the reaction. An additional problem to be alleviated is the determination of the exact loading of a resin with an integrated linker [13]. 

The production of polyurethane involves the controlled polymerization of an isocyanate, a long-chain-backbone polyol and a shorter-chain extender or cross-linker. The reaction rates can be controlled through the use of specific catalyst compounds, well known in the industry, to provide sufficient time to pour or otherwise transfer the mix and to cure the polymer sufficiently to allow handling of the freshly demolded part. The use of blowing agents allows the formation of a definite cellular core (thus the term microcellular elastomer ) as well as a non-porous skin, producing an integral sandwich-type cross section. 

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