Polymerization kinetics, polyamides

Saverio Russo is a senior professor of industrial chemistry at Genoa University, Italy. He has been, and still is, project leader of several research programs supported by the European Union, Italian Ministry of University, and Chemical Companies. He has been working for more than 40 yeare in the field of macromolecular science and technology, mainly on advanced polymeric materials synthesis, characterization and applications. Polyamide 6 by the anionic routes has been one of the major topics of his research. He is author of more than 250 scientific publications, mostly in international journals, and six patents. Prof Russo has been member of the Scientific Committee of INSTM (Interuniversity Consortium of Materiab Science and Technology) and director of its S Hon on Functional and Structural Polymeric Materials. He was the co- itor of four volumes of Comprehensive Polymer Science, Peigamon, 1989 and two supplement volumes (1992 and 1996). He was the organizer and co-chairman of two lUPAC Symposia of Free Radical Polymerization Kinetics and Mechanism, in 1987 and 1996. 

The main polymerization method is by hydrolytic polymerization or a combination of ring opening as in (3.11) and hydrolytic polymerization as in (3.12).5,7 9 11 28 The reaction of a carboxylic group with an amino group can be noncatalyzed and acid catalyzed. This is illustrated in the reaction scheme shown in Fig. 3.13. The kinetics of the hydrolytic polyamidation-type reaction has die form shown in (3.13). In aqueous solutions, die polycondensation can be described by second-order kinetics.29 Equation (3.13) can also be expressed as (3.14) in which B is die temperature-independent equilibrium constant and AHa the endialpy change of die reaction5 6 812 28 29 ... 

Others have presented the kinetics of polyamidation differently. At high water concentrations (5-10 mol kg-1), a second-order reaction is given with an activation energy of approximately 86 kJ mol-1.5 612 28 At low water concentrations in die final stages of die polymerization, a mixed uncatalyzed second-order reaction and an acid-catalyzed third-order reaction are observed. The rate constant k in (3.13) can tiien be written as... 

Most addition polymers are formed from polymerizations exhibiting chain-growth kinetics. This includes the typical polymerizations, via free radical or some ionic mode, of the vast majority of vinyl monomers such as vinyl chloride, ethylene, styrene, propylene, methyl methacrylate, and vinyl acetate. By comparison, most condensation polymers are formed from systems exhibiting stepwise kinetics. Industrially this includes the formation of polyesters and polyamides (nylons). Thus, there exists a large overlap between the terms stepwise kinetics and condensation polymers, and chainwise kinetics and addition (or vinyl) polymers. A comparison of the two types of systems is given in Table 4.1. 

Internal esters (lactones) and internal amides (lactams) are readily polymerized through a chainwise kinetic process forming polyesters and polyamides, clearly condensation polymers with respect to having noncarbons in the backbone, but without expulsion of a by-product ... 

Analysis of the non-isothermal polymerization of E-caprolactam is based on the equations for isothermal polymerization discussed above. At the same time, it is also important to estimate the effect of non-isothermal phenomena on polymerization, because in any real situation, it is impossible to avoid exothermal effects. First of all, let us estimate what temperature increase can be expected and how it influences the kinetics of reaction. It is reasonable to assume that the reaction proceeds under adiabatic conditions as is true for many large articles produced by chemical processing. The total energy produced in transforming e-caprolactam into polyamide-6 is well known. According to the experimental data of many authors, it is close to 125 -130 J/cm3. If the reaction takes place under adiabatic conditions, the result is an increase in temperature of up to 50 - 52°C this is the maximum possible temperature increase Tmax- In order to estimate the kinetic effect of this increase... 

Anionic polymerization of 67 by the activated monomer mechanism should occur with the selective cleavage of the CO—NH bond of the monomer to give a polyamide composed of kinetically controlled cis units (68c). However, the cis units isomerize to the thermodynamically more stable trans units (68t) through the proton abstraction from the methine group adjacent to the carbonyl group. This was ascertained by the isomerization experiment in which a polymer consisting of 92% cis unit and 8% trans unit was converted to one containing 40% cis unit and 60% trans unit when heated in dimethyl sulfoxide at 80 °C for 6 hours in the presence of 15 mol% potassium pyrrolidonate. 

This characteristic feature of cationic polymerization of THF allows the important synthetic application of this process for preparation of oli-godiols used in polyurethane technology and in manufacturing of block copolymers with polyesters and polyamides (cf., Section IV.A). On the other hand, the cationic polymerization of THF not affected by contribution of chain transfer to polymer is a suitable model system for studying the mechanism and kinetics of cationic ring-opening polymerization. 

Polymerization in the melt is widely used commercially for the production of polyesters, polyamides, polycarbonates and other products. The reactions are controlled by the chemical kinetics, rather than by diffusion. Molecular weights and molecular weight distributions follow closely the statistical calculations indicated in the preceding section, at least for the three types of polymers mentioned above. There has been much speculation as to the effect of increasing viscosity on the rates of the reactions, without completely satisfactory explanations or experimental demonstrations yet available. Flory [7] showed that the rate of reaction between certain dicarboxylic acids and glycols was independent of viscosity for those materials, in the range studied. The viscosity range had a maximum of 0.3 poise, however, far below the hundreds of thousands of poises encountered in some polycondensations. 

Polyamides clearly dominate the field of thin-film composites by interfacial polymerization. The composition and morphology of the membranes depend on different parameters, including the concentration of the reactants, their partition coefficients and reactivities, the kinetics and diffusion rates of the reactants, the presence of by-products, competitive side-reactions, cross-linking reactions and postreaction treatment... 

The polycondensation processes generally produce polyamides that are mixtures of polymer molecules of different molecular weights, the distribution of which usually follows a definite continuous function according to the most probable distribution model by Schulz-Flory [3]. This distribution function may, in principle, be derived from the kinetics of polymerization process, but is more readily derived from statistical considerations. In this case, the extent... 

Recently, several reviews have been written detaihng miniemulsion systems, with attention focused on the kinetics of miniemulsion polymerization [1, 2], the structure of the obtained nanoparticles [3], and their applications in medicine [4] and for catalysis [5]. As a consequence of the mechanism of miniemulsion s formation, and on the basis of their colloidal properties and stability, a wide range of different, industrially relevant polymers colloids can be generated using different types of polymerizations. Examples that have been reported in miniemulsions include polystyrene (first reported in 1973 [6]) or poly(vinyl chloride) (in 1984) [7] by radical polymerization sihcone (in 1994) [8] or polyamide (in 2005) [9] by anionic polymerization polyethylene (in 2000) [10] by catalytic polymerization epoxies (in... 

Although many step polymerizations do involve condensation reactions, e.g. in the formation of polyesters and polyamides, there are examples where this is not the case. The formation of polyurethanes from diols and diisocyanates, and the formation of polyphenylene oxides (PPO) shows step growth kinetics. 

The strong interactions between lithium halide and polyamides which cause the relevant melting point depression of the polymers (Figure 2), can be better understood if we analyze the behavior of the model system lactam-halide. In this way, we can also gain add1 tional information on mechanisms and kinetics of the anionic polymerization. 

Recent progress in kinetic and reactor modeling make it possible to assist the reactor design and process operations with unprecedented exactness. A recent analysis of the nylon-6,6 process [138] looked for improvements based on expansion of the solid-state polymerization, but the gain was minor compared to what happened with PET. The reason is the much reduced sensitivity of polyamides to by-product removal as compared to polyesters, since the equilibrium constant of the former is much greater. 

Thomas et al. [16] used CSLM to observe in situ the formation of polyamide membranes and the measurements were used to study polymer precipitation kinetics. Turner and Cheng [17] applied CSLM and hydrophilic fluorescent probes of varying molecular weights to image the size distribution of poly(methacrylic acid) (PMAA) hydrogel domains in polydimethylsiloxane (PDMS)-PMAA interpenetrating polymer networks. The combination of CSLM with AFM, SEM and X-ray spectroscopy allowed characterization of the structure of stimuli-responsive polymeric composite membranes [18]. 

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