Step-growth polymerization description

Although these definitions were perfectly adequate at the time, it soon became obvious that notable exceptions existed and that a fundamentally sounder classification should be based on a description of the chain-growth mechanism. It is preferable to replace the term condensation with step-growth or step-reaction. Reclassification as step-growth polymerization now logically includes polymers such as polyurethanes, which grow by a step-reaction mechanism without elimination of a small molecule. 

An analysis of the MWD during step-growth polymerization in a CSTR at steady state can be developed [8] by making the assumption of irreversible polymerization (due, perhaps, to the continuous removal of the condensation product, W). This analysis leads to the following description of the MWD ... 

As for step-growth polymerization, the presentation of the kinetics of radical polymerization must be followed by a description of its equilibrium. The Gibbs free energy, G, for any system at temperature T is defined as H — TS, where H and S are the enthalpy and entropy of the system, respectively. The change in Gibbs free energy, AGp, for the formation of a polymer... 

A detailed description of AA, BB, CC step-growth copolymerization with phase separation is an involved task. Generally, the system we are attempting to model is a polymerization which proceeds homogeneously until some critical point when phase separation occurs into what we will call hard and soft domains. Each chemical species present is assumed to distribute itself between the two phases at the instant of phase separation as dictated by equilibrium thermodynamics. The polymerization proceeds now in the separate domains, perhaps at differen-rates. The monomers continue to distribute themselves between the phases, according to thermodynamic dictates, insofar as the time scales of diffusion and reaction will allow. Newly-formed polymer goes to one or the other phase, also dictated by the thermodynamic preference of its built-in chain micro — architecture. 

The need to drive the polymerizations to completion is common to all step-growth reactions that are carried out under conditions in which polymerization-depolymerization equilibria are significant (Section 5.4.2). This is accomplished in general by removal of a volatile product such as water or an alcohol. The rale of polymerization is often limited by the rate of transfer of such condensation products into the vapor state. A complete kinetic description of the process must then involve both the chemical reaction rate and the rate of mass transfer. The latter depends on the details of reactor design and stirring and therefore so does the rate of polymer production [1]. 

The description of the variety of chemistries that are used to produce thermosetting polymers can be the subject of a whole book and is beyond the scope of this chapter. A description of chemistries involved in the synthesis of several families of thermosets can be found elsewhere [2]. In this section, we focus on some aspects of the chemistry of epoxy polymers because it provides examples of both step-growth and chain-growth polymerizations employed in the synthesis of polymer networks. 

In isodesmic polymerizations, the individual monomers associate with an association constant that is independent of the polymer size. This is comparable to the simplest description of a step-growth polycondensation given by Flory s principle of equal reactivity [10]. The mechanism gives rise to a PDI of 2 in the high-concentration regime. 

The reactions taking place during the synthesis of a polymer are rather complex in nature. The description of the chemistry of a polymerization reaction often involves over 20 different elementary reactions. This means that control of the overall reaction rate that governs the process safety may be rather complicated. Nevertheless the kinetically determining step in polymerization reactions is the chain growth reaction. 

It is to be noted that not all polymers made by the condensation method form a condensate during the reaction. Polyurethanes which are formed by a reaction of isocyanates and alcohols are such an example. Also, ring opening polymerization reactions are considered to be of the addition type even though they form polymers which can also be formed by a condensation reaction, e.g., the polymerization of caprolactam to form nylon 6,6 (see Painter and Coleman, (1994)). As a result, most modem texts do not use the polymerization descriptions, condensation and addition. Rather, the terms step growth and chain are used in place of condensation and addition respectively. 

Regardless of the scenario it must be kept in mind that a ROP plus polycondensation process needs two different definitions of conversion for its proper description. For the ROP part the conversion is defined by the consumption of monomers as usual for a chain-growth polymerization. Yet, for polycondensations the conversion is defined by the consumption of functional (end) groups regardless, if inter- or intramolecular condensation steps take place. 

The description of free-radical chain poljnnerization kinetics must take into account four basic steps initiation, which creates free-radical active centers propagation, which grows the polymer chains termination, which destroys the active centers and ends chain growth and chain transfer, which ends a growing chain and begins another. These classical steps also describe thermal polymerizations however, different descriptions are required for thermal- and photoinitiation. 

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