Step-Growth Polymerizations Polycondensations and Polyadditions

Because it is the extraordinarily large size of the macromolecules which leads to their unusual properties, it would be most sensible to classify polymerization reactions in accordance with the way in which they affect the molecular size and size distribution of the final product, i.e., in terms of the mechanism of polymerization. On this basis, there appear to be only two basic processes whereby macromolecules are synthesized (Zhang et al., 2012 Penczek and Premia, 2012 Moore, 1978 Saunders and Dobinson, 1976 Odian, 2004b Penczek, 2002 Jenkins et al., 1996) (1) step-growth polymerization (polycondensation and polyaddition) and (2) chain-growth (chain) polymerization. 

Zhang, M., June, S.M., Long, T.E., 2012. Principles of step-growth polymerization (polycondensation and polyaddition). In Matyjaszewski, K., Moller, M. (Eds.), Polymer Science A Comprehensive Reference, vol. 5. Elsevier, Amsterdam, pp. 7-47. 

Several polymer types and classes are known to exhibit photoconductivity. Consequently no preferred method of synthesis exists. The known photoconductive polymers are prepared by almost all common methods like free-radical, cationic, anionic, coordination, and ringopening polymerization, step-growth polymerization (polycondensation and polyaddition), and polyanalo-gous reactions. The only common requirement for all photoconductive materials is that they have to be of extreme purity. It is well known [37-39] that even traces of impurities act as traps and have a drastic influence on both quantum yield and carrier mobility. From the structural point of view the photoconductive polymers described in this chapter can be divided into three groups ... 

Step-growth polymerizations can be divided into two main categories polycondensation, in which a small molecule is eliminated at each step, as discussed above and polyaddition, in which, as the name suggests, monomers react without the elimination of a small molecule. These are shown in Equations 2.27 and 2.28, respectively, where R and R are the nonreactive portions of the molecules. 

Polyaddition and polycondensation reactions usually lead to functional polymers, since the polymers produced are terminated with reactive functional groups. A higher degree of functionality is easily affordable if monomers with additional reactive groups are used that do not participate in the step-growth polymerization. In emulsion polymerizations, neither polyaddition nor polycondensation reactions can be carried out consequently, the miniemulsion technique is of special interest as no diffusion of the monomers takes place. The first polyaddition in miniemulsion were performed in 2000, with the reaction of polyepoxides and hydrophobic diamines, bisphenols, and dimercaptanes [105]. Stable latexes of epoxy resins could be obtained, and apparent molecular weights of up to 20 000 g mol were measured. 

Step-growth polymerizations in extruders, both polycondensations and polyadditions, are far less investigated than chain growth reactions. Because the polymer has to remain thermoplastic, only bifunctional monomers should be used, and the molecular weight can be controlled by the addition of a small amount of monofunctional monomers. For both polycondensation and poly-addition reactions the feeding should be very accurate and stochiometrically correct, because otherwise the conversion and therefore pressure built up will be seriously restricted. 

Important polymers that are produced by polyaddition are polyamide 6 (nylon) and all kinds of polyurethanes. In polycondensation one mol of a small molecule (typically H2O) is liberated per step of chain growths, important polymers that are produced by polycondensation are polyamide 6.6, poly(ethylene terephthalate) (PET), polycarbonate, polyarylate, and polysulfide. Step growth polymerization is usually slow, equilibrium limited and isothermal to slightly exothermic. Polyaddition and polycondensation reactions of monomers with three or more reactive end groups lead to three-dimensionally crosslinked resins. 

Step-Growth Polymerization Microwave-assisted step-growth polymerizations -that is, polycondensation and polyaddition reactions - have been studied extensively. According to a review by Wiesbrock et ol. [10], a plethora of reports has been devoted to the microwave-assisted synthesis of polyamides, polyimides, polyethers, and polyesters. For the majority of polymerizations, the reaction rates were significantly increased under microwave irradiation as compared to conventional heating, whilst in many cases the reaction times were shortened from hours, and sometimes days, to about 10 minutes. Moreover, the product purity was improved and the polymers exhibited superior properties, most likely due to a reduction in... 

Flory outlined [3] that the definition of polycondensation is necessarily based on kinetic aspects and not on the structure of polycondensates, because numerous polycondensates can also be prepared by ROP which usually proceeds as chain-growth polymerization. Flory s definition of step-growth polymerization is limited to polycondensations and polyadditions in the melt or in solution, and does not include solid-state polycondensations. Hory s definition of step-growth polymerizations is based on point 1. 

Two different types of step-growth reactions exist polycondensation reactions, where during the polymerization a low molecular by-product is formed (e.g., water or a lower alcohol), and polyaddition reactions, where... 

Hyperbranched polymers are synthesized in a one-step method, often from AB monomers but also by combining A +B (x>3) monomers or variations of those. Polymerization methods have been applied that involve polycondensation, polyaddition, and ring-opening or self-condensing vinyl polymerization. Even though the one-pot synthetic approach leads to imperfectly branched structures because of uncontrolled growth, it is more suitable for the preparation on a larger scale and thus for commercial use. Nowadays, different... 

---