Cross-linking during polymerization

We have already discussed cross-linking during polymerization but cross-linking is often carried out after the initial polymer is made. You saw earlier how poly(dimethylsiloxane) can be cross-linked by co-polymerization with MeSiC. An alternative way of cross-linking the linear polymer uses radical reactions to convert silicone oil into silicone putty. Peroxides are used in this process. 

Polymers that cross-link during polymerization are known as thermosetting polymers. They form a three-dimensional structure through the cross-linking reaction, and they can not be reshaped once they are set. 

Fig. 21. Schematic phase diagram expected when there are strong interactions limited to regions along the molecule as in the case of ionomers, segment-segment interactions, or chemical cross-linking during polymerization.

Starting from several monomers, polymer coatings on carrier electrodes are obtained by various techniques. The monomers are activated by a physical or chemical method. The polymers prepared are often insoluble due to cross-linking during polymerization, therefore the determination of real structure by, for example, elemental analysis, IR, Raman or electronic spectra, ESR, ESCA, neutron scattering or SEM is not very easy. Smooth to rough films are obtained. The polymer films often exhibit high electronic conductivity. 

Butyl rubber, polyisobutylene, is an example of cationic polymerization with an acid. Review Chapter 14, Section 2.3. A small amount of isoprene is added to enable cross-linking during vulcanization through the allylic sites. 

The somewhat higher values of N exhibited by the polybuta-dienyl lithium were ascribed to the occurrence of some cross-linking during the polymerization of this monomer, and this was confirmed by use of a special "capping" technique, as shown later. 

Polyether precursors to polymers 57 and 58 were prepared via Negishi crosscoupling between di- and tetra-functionalized macrocyclic monomers, respectively.7,113,116 For 58, one of the two limiting modes of cross-linking during the polymerization via Negishi cross-coupling involved formation of macrocycles... 

Prepolymeric imides with norbornene end groups, I, cross-link during normal polymerization, and those with acetylene end groups, II, presumably cross-link by trimerization to benzene rings ... 

Pre-cross-linked Latex Blends. In these materials the individual latexes are cross-linked during synthesis and then blended, and a film is formed. Because of limited deformation and/or interdiffiision capabilities, such films tend to be weak and only used for special purposes (Zosel and I y 1993 Lesko and Sperry 1997). However, light cross-linking, as occurs in SBR latexes, may be tolerated. Pre-cross-Unked latex blend materials are actually not IPNs, because the definition requires that at least one of the polymers be polymerized and/or cross-linked in the immediate presence of the other. An application of pre-cross-linked suspension-polymerized blends, in animiic and cationic form, is as ion-exchange resins. In suspensions, the particles are larger, usually of the order of 10-200 tm. 

Smooth films of polymers are obtained by exposing monomers to plasma discharge. The rate of surface deposition of the plasma polymerized film depends on the geometry of the chamber, the radio-frequency power and the substrate temperature The details of the structure of the polymers prepared are unknown due to cross-linking during film formation. 

It is also clearly desirable that the double bond should not become involved in polymerization reactions leading to gelation and cross-linking during the polymer manufacture. Additional requirements are that the diene should not seriously influence catalyst reactivity, should react efficiently during vulcanization and should be efficiently converted (or easily recycled, or both) during polymerization. 

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