Step-growth polymerization side reactions

Preparation of Silphenylene-Siloxane Polymers. The basic reaction for the preparation of exactly alternating silphenylene-siloxane polymers from bis(ureido)silanes, Va-Vh, and the disilanol monomer is step-growth polymerization. This reaction was carried out at -20 °C, at which temperature the possible side reactions described previously can be prevented or at least decreased in rate. To obtain a high-molecular-weight polymer, a solution of the bis(ureido)silane monomer was added slowly to a slurry of the disilanol monomer in chlorobenzene (ii). After addition of 95%... 

Step-growth polymerization processes must be carefully designed in order to avoid reaction conditions that promote deleterious side reactions that may result in the loss of monomer functionality or the volatilization of monomers. For example, initial transesterification between DMT and EG is conducted in the presence of Lewis acid catalysts at temperatures (200°C) that do not result in the premature volatilization of EG (neat EG boiling point 197°C). In addition, polyurethane formation requires the absence of protic impurities such as water to avoid the premature formation of carbamic acids followed by decarboxylation and formation of the reactive amine.50 Thus, reaction conditions must be carefully chosen to avoid undesirable consumption of the functional groups, and 1 1 stoichiometry must be maintained throughout the polymerization process. 

Assuming that no intramolecular or side reactions take place and that all groups are equireactive, the polydispersity index, 7P, of hyperbranched polymers obtained by step-growth polymerization of ABX monomers is given by Eq. (2.2), where pA is die conversion in A groups.196 Note that the classical Flory relationship DPn = 1/(1 — pa) holds for ABX monomer polymerizations ... 

Furthermore, Kricheldorf made the following statement All step-growth polymerizations possess the fundamental tendency to yield cycles as stable end products with linear chains being the reactive intermediates or end products of side reaction [183]. 

Equation 3.6, together with Equation 3.4, describes a random distribution of molecular sizes this distribution is also known as the Flory-Schulz distribution or the most probable distribution [5]. Recently, Wutz and Kricheldorf [6] proposed a model describing the frequency distribution (/ ) and formulated the weight distribution (w,) of linear chains in step-growth polymerizations considering the cyclation reaction, which is one of the most important side reactions in step-growth polymerization. 

In step-growth polymerization, the molecular weight continuously increases with time and the formation of polymer with sufficient high molecular weight for practical applications requires very high conversions of the reactive groups (>98-99%). This requirement imposes stringent conditions on the formation of polymers by step polymerization, such as the necessity for a favorable equilibrium and the absence of side reactions. 

In all the preceding polymerization methods we have seen how to utilize the double bond in an unsaturated organic compoimd to link many molecules together into a polymeric chain. Also, in all of these processes the polymer was produced starting from a single monomer. In contrast in this section we will look at polymers that are prepared from the reaction of two difunctional monomers with each other. In all the polymerization reactions that we have seen so far there was no side-product formation. For example, ethylene was converted into polyethylene acrylonitrile was converted into polyacrylonitrile and so on. During this conversion the entire stmc-tural unit of the monomer was incorporated into the polymer without any side-product formation. However, in the preparation of condensation polymers a small molecule (such as water or methanol) is eliminated as the side-product. Another important difference is that condensation polymerization is usually a step-growth polymerization. This means that the polymerization proceeds in a series of steps. To make this point clear let us recall the polymerization of ethylene by the free-radical method. In the free-radical process the polymerization of various chains are initiated by the... 

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