Step polymerization process conditions

An example of a commercial semibatch polymerization process is the early Union Carbide process for Dynel, one of the first flame-retardant modacryhc fibers (23,24). Dynel, a staple fiber that was wet spun from acetone, was introduced in 1951. The polymer is made up of 40% acrylonitrile and 60% vinyl chloride. The reactivity ratios for this monomer pair are 3.7 and 0.074 for acrylonitrile and vinyl chloride in solution at 60°C. Thus acrylonitrile is much more reactive than vinyl chloride in this copolymerization. In addition, vinyl chloride is a strong chain-transfer agent. To make the Dynel composition of 60% vinyl chloride, the monomer composition must be maintained at 82% vinyl chloride. Since acrylonitrile is consumed much more rapidly than vinyl chloride, if no control is exercised over the monomer composition, the acrylonitrile content of the monomer decreases to approximately 1% after only 25% conversion. The low acrylonitrile content of the monomer required for this process introduces yet another problem. That is, with an acrylonitrile weight fraction of only 0.18 in the unreacted monomer mixture, the low concentration of acrylonitrile becomes a rate-limiting reaction step. Therefore, the overall rate of chain growth is low and under normal conditions, with chain transfer and radical recombination, the molecular weight of the polymer is very low. 

In the process of thermal dimerization at elevated temperatures, significant polymer is formed resulting in seriously decreased yields of dimer. Dinitrocresol has been shown to be one of the few effective inhibitors of this thermal polymerization. In the processing of streams, thermal dimerization to convert 1,3-cyclopentadiene to dicyclopentadiene is a common step. Isoprene undergoes significant dimerization and codimerization under the process conditions. 

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. 

The basic sol-gel reaction can be viewed as a two-step network-forming polymerization process. Initially a metal alkoxide (usually TEOS, Si(OCIl2CH )4) is hydrolyzed generating ethanol and several metal hydroxide species depending on the reaction conditions. These metal hydroxides then undergo a step-wise polycondensation forming a three-dimensional network in the process. The implication here is that the two reactions, hydrolysis and condensation, occur in succession this is not necessarily true (8.9). Depending on the type of catalyst and the experimental conditions used, these reactions typically occur simultaneously and in fact may show some reversibility. 

Various combinations of reactant(s) and process conditions are potentially available to synthesize polyesters [Fakirov, 2002 Goodman, 1988], Polyesters can be produced by direct esterification of a diacid with a diol (Eq. 2-120) or self-condensation of a hydroxy carboxylic acid (Eq. 2-119). Since polyesterification, like many step polymerizations, is an equilibrium reaction, water must be continuously removed to achieve high conversions and high molecular weights. Control of the reaction temperature is important to minimize side reactions such as dehydration of the diol to form diethylene glycol... 

Cationic polymerization has been initiated by a variety of protonic and Lewis acids [Kubisa, 1996 Toskas et al., 2001]. The cationic process is more complicated and less understood than the anionic process. Polymerization under most reaction conditions involves the presence of a step polymerization simultaneously with ROP. This appears to be the only way to reconcile the observed (complicated) kinetics for the overall process [Chojnowski and Wilczek, 1979 Chojnowski et al., 2002 Cypryk et al., 1993 Rubinsztain et al., 1993 Sigwalt, 1987 Wilczek et al., 1986],... 

Basilevsky et al. [1982] proposed a mechanism of ionic polymerization in crystalline formaldehyde that was based on Semenov s assumption [Semenov, 1960] that solid-state chain reactions are possible only when the products of each chain step prepare a configuration of reactants that is suitable for the next step. Monomer crystals for which low-temperature polymerization has been observed fulfill this condition. In the initial equilibrium state the monomer molecules are located in lattice sites and the creation of a chemical bond requires surmounting a high barrier. However, upon creation of the primary cation (protonated formaldehyde), the active center shifts toward another monomer, and the barrier for addition of the next link diminishes. Likewise, subsequent polymerization steps involve motion of the cationic end of the polymer toward a neighboring monomer, which results in a low barrier to formation of the next C-0 bond. Since the covalent bond lengths in the polymer are much shorter than the van der Waals distances of the monomer crystal, this polymerization process cannot take place in a strictly linear fashion. It is believed that this difference is made up at least in part by rotation of each CH20 link as it is incorporated into the chain. 

With regard to the chemistry of polymerization processes, we will only introduce the topic superficially. A polymerization reaction is controlled by several conditions such as temperature, pressure, monomer concentration, as well as by structure-controlling additives such as catalysts, activators, accelerators, and inhibitors. There are various ways a polymerization process can take place such as schematically depicted in Fig. 1.1. There are numerous other types of reactions that are not mentioned here. When synthesizing some polymers there may be multiple ways of arriving at the finished product. For example, polyformaldehyde (POM) can be synthesized using all the reaction types presented in Table 1.1. On the other hand, polyamide 6 (PA6) is synthesized through various steps that are present in different types of reactions, such as polymerization and polycondenzation. 

The urea-formaldehyde polymer is formed by a multi-step reaction process between urea and formaldehyde. The initial phase is a methylolation of the urea under slightly alkaline conditions with a formaldehyde-urea (F/U) molar ratio of 2.0 1 to 2.4 1. Condensation of the methylolureas from the methylolat ion reaction is at atmospheric reflux with a pH of 4 to 6. This condensation polymerization continues to a pre-determined viscosity, at which time the pH is adjusted with a suitable base to 7-3 to 8.0. The adhesive is then concentrated to a total solids content of 50 to 60 percent by vacuum distillation. Additional urea is then normally added to produce a final F/U molar ratio of 1.6 1 to 1.8 1. 

In a polymerization process the chain length distribution or molar mass distribution (MMD) is influenced by a large number of factors and conditions the kinetics of the reaction plays a very important role. The calculation of the resulting MMD is thus very complicated. For one of the simplest cases, a step reaction with polycondensation, a first-order approach is given here. As an example we take a hydroxy acid HO-R-COOH, which, upon condensation, forms the chain -[-O-R-CO-]n. 

It must be stated that up to now models that can describe detailed styrene polymerization including all kinds of initiation step are rare. The work of Dhib ei al. [31] is so far the most comprehensive in this respect. It is a common practice tc fit the model to experimental data under different reaction process conditions. 

It is not practical to conduct free-radical polymerizations under conditions where there is an equilibrium between polymerization and depolymerization processes. The polymer synthesis is effectively irreversible in normal radical polymerizations. The course of the reaction is then determined kinetically, and the molecular weight distribution cannot be predicted statistically as was done for equilibrium step-growth polymerizations described in Chapters. 

---