Polymer formation, competition reactions

As seen from the above reaction steps, the DVB procedures for star-branched polymer formation consists of several competitive and consecutive reactions. It was the aim of our work to study various reaction variables which effect this process. As mentioned earlier, these include DVB/RLi molar ratio, reaction... 

Excess alkylaluminum would tend to promote the competitive reaction observed by Milovskaya et al., and, in part, a free polymer would be formed. The fact that alkylaluminum suppresses formation of block polymers tends to support this concept and further suggests that the reaction to form AFR polymer does not reside solely with the aluminum compounds present. In fact, the titanium must be involved to a significant extent. 

These two early findings lead to the concept of competitive ablation and polymerization (CAP) which emphasizes the importance of a balance between polymer formation and ablation [3]. The first finding demonstrated the control of ablation due to extremely reactive fluorine-related species (atomic fluorine, F , etc.) by chemical reactions, and the second demonstrated the role of discharge conditions that control the production of highly ablative species. 

The presence of octamethylcyclotetrasilazane and bicyclic dimer in the pyrolysis products strongly suggests that at least two competitive reactions take place during plasma polymerization of HMCTSN namely, ring enlargement and dimerization. However, dimerization seems to be predominant and it is considered to be the first step of reaction leading to polymer formation. 

When the process involves two competitive reactions, some people prrfer to call those modified polymers interpenetrated polymer networks (IPNs) [5]. The formation of a polyether-urethane network in a loosely crosslinked poly(methyl methacrylate) matrix to increase its toughness can serve as one of the examples. From a general point of view, the analysis of the reaction-induced phase separation is the same (perhaps more complex) for IPNs than for rubber-modified epoxies or for high-impact polystyrene. 

One of the most common outcomes when an RCM reaction fails is the competing CM of the substrate to form linear or cychc dimers, or potentially ohgomers or even polymers. The competition between RCM and CM in the metathesis of dienes will depend on a number of factors, predominantly related to the structure of the target cycloalkene. The strain introduced (AH composed of angle strain, transannular strain, and torsional strain) will depend on the substitution pattern of the diene, while the loss of entropy (AS) will depend on the number of rotors frozen in the product that were otherwise free to rotate in the substrate. The formation of macrocycles will predominantly be influenced by AS, as rings greater than c. 12 members in size typically suffer relatively little strain. Macrocycle formation is not considered in detail here, but has been reviewed recently this section will focus on rings of 5-10 members. 

The claim that site-separation is achieved in the reactions discussed earlier has been questioned (Crowley and Rapoport, 1976 Neckers, 1978). According to these authors, the Claissen condensation of two molecules of an ester resulting in self-condensation cannot be expected to compete with acylation using acid chlorides or acid anhydrides (more powerful acylating agents). Hence, the formation of mixed ketones as the major product cannot be attributed mainly to site-separation. Furthermore, if site-separation did take place by polymer binding, then why were self-condensation products formed in other reactions The evidence for effective site-separation is contradictory because self-condensation has taken place in some cases and not in others. This difference is better explained in terms of a competitive reaction that takes place in one case and not in the other. 

The production of linear polymers by the step polymerization of polyfunctional monomers is sometimes complicated by the competitive occurrence of cyclization reactions. Ring formation is a possibility in the polymerizations of both the A—B and A—A plus B—B types. 

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