Step-growth polymerization definition

Step-growth polymerizations can be schematically represented by one of the individual reaction steps VA + B V —> Vab V with the realization that the species so connected can be any molecules containing A and B groups. Chain-growth polymerization, by contrast, requires at least three distinctly different kinds of reactions to describe the mechanism. These three types of reactions will be discussed in the following sections in considerable detail. For now our purpose is to introduce some vocabulary rather than develop any of these beyond mere definitions. The principal steps in the chain growth mechanism are the following ... 

Based on the classical definitions given in organic chemistry, the step-growth polymerization process can involve either condensation steps or addition steps. The former proceeds with elimination of by-products while the latter takes place without elimination of by-products. This is illustrated by Eqs (2.1) and (2.2), for the particular case of difunctional molecules ... 

In step-growth polymerization of bifunctional monomers, each molecule, whether monomer or polymer, carries two functional groups There are always half as many molecules as functional groups. Thus, when the number of unreacted groups has decreased to a fraction 1 —/F of the initial, the number of molecules has decreased to that same fraction of its initial. By virtue of its definition 10.15, the number-average degree of polymerization (monomer included in averaging) is the reciprocal of this fraction ... 

It is important to define the terms used in describing functionality and to clearly distinguish between the actual and potential functionality and to show the relationship between stoichiometry and functionality. Functionality can be defined as the number of other molecules that a compound can react with. This definition of functionality also means that within step-growth polymerizations the actual functionality is dependent on stoichiometry. The phenol-formaldehyde reaction is a typical step-growth reaction in... 

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. 

Condensation polymers by the above definition are usually produced by step-growth polymerizations but not all step-growth syntheses are condensation reactions. Thus there is no elimination product in polyurethane synthesis from a diol and a diisocyanate (cf. reaction (1-12)) ... 

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. 

Theoretical and experimental research of the past 40 years allow for a more precise definition of step-growth polymerization by addition of to more points ... 

In summary, two conclusions may be drawn. First, non-stoichiometric polycondensations of class ni may give much higher molar masses than polycondensations of a-b monomers. Second, the kinetic course of the polymerizations deviate largely from that of a normal step-growth polymerization, and the term non-stoichiometric polycondensation is rather a formal label than a correct terminology. In the extreme case, when cyclization is totally suppressed, class III polymerization exactly obeys the definition of an aa + condensative chain polymerizaion . This term was proposed by lUPAC, and numerous condensative chain polymerizations are known from a-b monomers as discussed in Chap. 16. 

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. 

While these extremely simplified cases are met in practice, some very important exceptions occur that show the need for caution in making generalizations. For example, nylon 6 would normally be considered as a step-growth polymer because it is a polyamide. However, it can also be produced by a ring-opening polymerization that is a kind of chain-growth polymerization. Thus, these definitions are useful but can sometimes be misleading. 

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