Crosslinking mechanisms polymer formation

Crosslinking of alkyds containing conjugated double bonds results in more carbon-carbon bonds in the crosslinks than in the alkyds containing unconjugated double bonds. The crosslinking mechanism involves the formation and decomposition of cyclic peroxides, to yield radicals that initiate crosslinking by 1,4-polymerization of the polymer molecules. 

Chemical or physical cross-linking of the polymer is applied in order to control membrane swelling, especially for separations of organic mixtures. In addition, this can also enhance mechanical strength as well as chemical stability of a membrane. However, crosslinking decreases polymer solubility, therefore it is often done after membrane formation (cf. Sections 2.4—2.6). 

This technique resembles the formation of antibodies from haptens, and actually a similar mechanism to the imprinting was formerly thought to be the mechanism of formation of antibodies [29], The functional groups in these cavities are located at various points in the polymer chain, and are held in a definite mutual orientation simply by the crosslinking. In this case, the stereochemical information is not carried by a low-molec-... 

For there to be substantial separation in time of the processes of linear polymer formation and of intermolecular crosslink formation, they would have to proceed by different mechanisms. For the relaxation of the internal stresses that occur at the stage of formation of the linear polymer, the polymer must possess sufficient pliability under creep. Because internal stresses may also appear at the stage of formation of the intermolecular crosslinks, the concentration and rate of formation of crosslinks must be minimal. 

Two-phase mixtures of polymers differ from classic ll colloid systems mainly by a transition layer that exists between the system components and is of special significance. The formation of such a layer in mixtures of linear polymers is governed by the kinetic factors of the retarded process of phase sep u ation, by the collid-chemic ll mechanism of formation, or by the adsorption interaction as well as by the segmental solubility [103,104], In mixtures of crosslinked polymers its formation is governed also by the conditions of synthesis. Note also that in the thermodynamic llly nonequilibrium mixtures of polymers in the two-phase systems, the processes of segmental solubility usually have time to reach completion while the macromolecules do not move inside the high-viscosity medium, which ensures the stability of the structure and its mechanical properties [103]. 

The specific behavior of the formation of a crosslinked (co)polymer structure by radical mechanism for a high crosslinker concentration is determined by (1) an increase in the unreacted double bonds concentration from the unsaturated monomer units (at low conversion)and (2) an increase in the number of pendant double bonds in the newly formed structure. 

As a natural consequence of the crosslinking reaction process, the density of the primary polymer differs depending on the time of this primary polymer formation. That is, in the case of the copolymerization of vinyl and divinyl monomers, the generally formed inhomogeneous crosslink formation can be regarded as a natural consequence of the mechanism of crosslink formation. This is true except for die special reaction conditions by favorable timing of the incorporation of divinyl monomer in the polymer chain (formation of pendant double bonds) and consumption of pendant double bonds (formation of crosslinks). These special reaction conditions are used by Flory as simplified conditions when the Flory-Stockmayer theory is applied to the copolymerization of vinyl and divinyl monomers. Flory s simplified conditions include die following three assumptions (1) the reactivities of the monomer and die double bonds in the polymer are all equal (2) any double bond reacts independently and (3) there will be no intramolecular reactions (cyclization) within the finite size molecules (sols). 

Abstract Polymers are macromolecules derived by the combination of one or more chemical units (monomers) that repeat themselves along the molecule. The lUPAC Gold Book defines a polymer as A molecule of high relative molecular mass, the structure of which essentially comprises the multiple repetition of units derived, actually or conceptually, from molecules of low relative molecular mass. Several ways of classification can be adopted depending on their source (natural and synthetic), their structure (linear, branched and crosslinked), the polymerization mechanism (step-growth and chain polymers) and molecular forces (Elastomers, fibres, thermoplastic and thermosetting polymers). In this chapter, the molecular mechanisms and kinetic of polymer formation reactions were explored and particular attention was devoted to the main polymerization techniques. Finally, an overview of the most employed synthetic materials in biomedical field is performed. 

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