Лако-красочные материалы — производство

In order to terminate a growing polymer chain, it is necessary to effect the removal of the free radical from the polymer chain. This may be accomplished in one of several ways, but all involve either a “combination” reaction, a “disproportion” reaction or a “transfer” reaction. The exact mechanism will be dependent on the chemical structure of the monomer, the polymer chain and the nature of any other species present.

(i) Termination by Combination

This is the simplest form of chain termination. It involves two growing polymer chains which combine with the mutual extinction of the radicals.

This type of termination reaction will result in a “head to head” linkage of the two polymer chains involved.

(ii) Termination by Disproportion

This occurs when the radical of a propagating chain, abstracts an H atom from another propagating polymer chain, to achieve the mutual extinction of the radicals.

In this case the two polymer chains do not combine, but remain as separate entities, one chain containing an unsaturated end unit, and the other a saturated end unit.

R — CH2— CHX 4- CH2— CHX4- CH2— CHX + XHC — CH2-f XHC — CH2i XHC ^ CH — R L Jn L Jm

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R — CH2— CHX 4- CH2— CHX4- CH2— CH2X + XHC = CH — f XHC — CH24- XHC — CH2— R

L — — J n L_ J m

Figure 1-3

A special case of intra-molecular abstraction can occur when the H atom is removed from the same chain as the abstracting radical. This process (called "backbiting") may occur where long chain polymers take up configurations which bring labile H atoms on the polymer chain, into close proximity with the radical at the chain end. This process will result in the termination of the growing chain and give rise to a C=C bond at the position on the polymer chain where the abstraction occurred.

Whether a polymer terminates by a disproportionation or by a combination reaction depends upon the configuration of the monomers involved. If there are no labile H atoms available, then termination will be by a combination reaction, e. g. isobutyl styrene homopolymers. If labile H atoms are available, then termination may occur by both combination and disproportionation reactions, e. g. acrylic monomers.

When disproportionation occurs, the product will contain polymer chains with unsaturated chain ends or with unsaturated sites along the polymer backbone, as a result of the abstraction of H atoms.

These C=C bonds are capable of reactivation by a free radical.

Where the unsaturation is at the chain ends, propagation will continue, in the presence of sufficient monomer, as before.

Where the unsaturated sites are along the polymer backbone, the restart of chain propagation will result in branching of the polymer chain.

The above illustrates a branched chain polymer formed by activation of a double bond situated on a previously formed polymer chain.

R — CH2— CHX — CH2= CX— CH2— CHX— CH2— CHX + R

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R — CH2— CHX — CH2— CXR — CH2— CHX- CH2- CHX

I nCH2=CHX

R — CH2— CHX — CH2— CXR — CH2— CHX- CH2- CHX

CHX •CHX

Figure 1-4