Intramolecular crosslinking cyclization

As a summary of the above discussion, four factors including intramolecular cyclization, reduced reactivity of prepolymer, intramolecular crosslinking, and microgelation are not primary factors for the greatly delayed gelation in the radical polymerization of diallyl dicarboxylates. So far, these four factors have been discussed quite often, but we have to look for other reasons for delayed gelation from ideal network formation, especially at an early stage of polymerization, i.e. at least up to the theoretical gel point. 

As described in the Introduction, Dusek [35] emphasized the importance of cyclization and quite recently, Dotson [36] has argued it again. Here it should be noted that they have treated indiscriminately both intramolecular cyclization (2) and intramolecular crosslinking (4) as cyclization and strongly emphasized the marked occurrence of such cyclization at a quite early stage of polymerization where no intermolecular crosslinking occurs. Eventually, the enhanced reactivity of pendant double bonds because of their spacial proximity to the terminal radical site would lead to the formation of microgel at a low conversion. 

For the chain crosslinking (co)polymerization these assumptions are not fulfilled. Here, the strong cyclization or intramolecular crosslinking causes formation of inhomogeneities consisting of densely crosslinked regions surrounded by less... 

For unequal reactivity of the vinyl gronps, the scheme corresponds to that of ternary copolymerization. Therefore, there is exclnsively intramolecular crosslinking at the beginning of process. Cycles of various sizes are formed, with the smallest cycles having the highest probability of formation. A very important cyclization can be observed when the reaction systan contains a large amount of divinyl monomer (Vlad and Vasiliu 2010). However, a sharp decline in the pendant double bonds reactivity appears. This feature of crosslinking (co)polymerization is explainable because many of these links are trapped inside cycles. For a complete conversion of monomers, the presence of donble bonds in the reaction system supports the inaccessibility of these links. 

The polymerization reaction results in (co)polymer molecules rich in bis- or polyunsaturated components that still contain a large number of unreacted double bonds. The propagation, cyclization, and crosslinking reactions lead to weak inter-molecular crosslinked primary nuclei (due to the diluent presence), intramolecular crosslinked microgels, and linear chains soluble in the unreacted monomers. 

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). 

Some Bis reactions may lead to intramolecular cycles that will not affect the elasticity behavior of the gel, and pendant, unreacted, gronps may form where only the crosslinker reacts at one of its double bonds [315]. Tobita and Hamielec [393] found that primary cyclization, i.e., the formation of loop cycles, may consnme as much as 80% of... 

It is difficult to believe that one can chemically advance from generation to generation (i.e., especially G = 2) without substantial intramolecular cycliz-ation and crosslinking,... 

It is difficult to find crosslinking systems that are ideal in that all functional groups are of equal reactivity and intramolecular cyclization is negligible. The crosslinking of vinyl terminated poly(dimethylsiloxane) polymers with tri- and tetrafunctional silanes appears to be an exception. Thus the calculated and experimental pc values were 0.578 and 0.583, respectively, for the tetrafunctional silane and 0.708 and 0.703, respectively, for the trifunctional silane (with r — 0.999) [Valles and Macosko, 1979]. 

The importance of intramolecular cyclization was emphasized when Butler and coworkers found that the radical polymerization of N, N, N, /V-diallyldimethylammonium chloride (DADMAC) gave soluble, uncrosslinked polymers with little or no unsaturation (Eq. 6-101) [Butler and Angelo, 1957 Butler and Ingley, 1951 Wandrey et al., 1999]. There is a very low tendency for radical IV to propagate intermolecularly and undergo crosslinking. The predominant reaction is intramolecular cyclization, and the product is a linear product with cyclic structures in the backbone. The reaction is referred to as alternating intra/intermolecular polymerization or cyclopolymerization. 

---