Vinyl and Divinyl Monomers of Different Reactivities

Let us now consider a vinyl (X)-divinyl (YY) system in which the reactivities of the vinyl groups X and Y are not equal, while the two Y groups are equally reactive. If the Y groups are r times as reactive as the X groups, they enter the copolymer r times as rapidly and hence the ratio of Y and X groups in the copolymer, d[Y]/ i[X], is 

An alternative expression to Eq. (7.80) for the gel point conversion can be derived in terms of the reactivity ratios of the two types of vinyl groups in X and YY. Let ri represent the relative reactivity of a X monomer and a Y monomeric 

The composition equation for initial copolymer in terms of monomer group concentration [Y]o becomes [cf. Eq. (7.11)]  

Ignoring the drift of residual monomer composition with conversion and assuming a random distribution of crosslinks, we may predict gelation to occur at a conversion pc given (Odian, 1991) by 

Z)P [Y]o([X]o -h [Y]o)(r2[Y]o + [X]o)2 Thus when the double bonds of the divinyl monomer are more reactive than that of the vinyl monomer (r2 r ), gelation occurs at lower conversions. On the other hand, gelation is delayed until the later stages, if ri r2. 

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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). 

In the FRC of vinyl and multivinyl monomers, a drift in the instantaneous copolymer composition throughout the reaction will be undergone due to the different reactivities of the vinyl groups. This compositional drift is caused by the fact that the more reactive monomer will be consumed faster than the less reactive ones. In the simplest instance, assuming equal reactivity of the vinyl groups in mono-and divinyl monomers present in the reaction system, the reactivity of the crosslinker would be twice that of the monovinyl monomer, and therefore, the polymer chain... 

Atom transfer radical polymerization (ATRP) was selected as an exemplary CRP technique to systematically study the kinetics and gelation behavior during the concurrent copolymerization of monovinyl monomers and divinyl cross-linkers (Scheme 2). The effect of different parameters on the experimental gelation was studied, including the initial molar ratio of cross-linker to initiator, the concentrations of reagents, the reactivity of vinyl groups present in the cross-linker, the efficiency of initiation, and the polydispersity of primary chains. Experimental gel points based on the conversions of monomer and/or cross-linker at the moment of gelation, were determined and compared with each other in order to understand the influence of each parameter on the experimental gel points. 

Assuming that classical chemical kinetics are valid and that the crosslinking reaction rate is proportional to the concentrations of polymer radicals and pendant double bonds, it was shown theoretically that the crosslinked polymer formation in emulsion polymerization differs significantly from that in corresponding bulk systems [270,316]. To simplify the discussion, it is assumed here that the comonomer composition in the polymer particles is the same as the overall composition in the reactor, and that the weight fraction of polymer in the polymer particle is constant as long as the monomer droplets exist. These conditions may be considered a reasonable approximation to many systems, as shown both theoretically [316] and experimentally [271, 317]. First, consider Flory s simplifying assumptions for vinyl/divinyl copolymerization [318] that (1) the reactivities of all types of double bonds are equal, (2) all double bonds... 

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