Typical Step-Growth Reactions

Reactions are normally carried out in bulk in the temperature range 420 to 520 K to encourage fast reactions and promote the removal of the low-molar-mass condensation product. Activation energies are about 80 kJ mol.  

When the diamine used is aromatic, only low-molar-mass polymer is formed because of the lower-reaction rates. To produce longer chains, conditions must be readjusted so that both phases are polar and miscible, and vigorous high-speed stirring of the system is necessary. This is used in the reaction between isophthaloyl chloride and /n-phenylene diamine. These aromatic polyamides are particularly versatile materials and of considerable interest. 

The ultimate extension to a homogeneous Systran with inrat polar solvents is used in the synthesis of polyimides. Poly(methylraie 4,4 -diphenylene pyromellita-mide acid) is prepared by mixing equal amounts of pyromellitic dianhydride and foA(4-aminophenyl methane) in N,N -dimethylformamide. The reaction is maintained at 288 K, with stirring, for an hour, and the polymer is isolated by precipitation in vigorously stirred water. 

Polysulfonamides, polyanhydiides, and polyurethanes can also be formed in homogeneous low-temperature reactions. 

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A typical step-growth reaction is one in which a single epoxy ring reacts with the active hydrogen of the curing agent. The general reaction is ... 

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 the growth of polymer chains results from a free-radical reaction, the reaction rate is relatively slow, resembling that of typical step-growth polymerizations. When the number of double bonds equals the number of SH groups, the system described by Eq. (2.100) can be classified as an A2 (ene) + B4 (thiol) reaction. 

The polymerization reaction is typical of step-growth reactions in that it follows second-order kinetics for the reaction of tetracyclone ends with acetylene ends (Figures 1 and 2). 

Flory [6] estimated lO collisions between functional groups before a reaction in a typical step growth polymerization. Even in the case of rodlike molecules, the local dynamics are fast though reorientation is a very slow process. Clearly, a brute force simulation of the polymerization process starting with reactive monomers and allowing them to react to form polymers seems infeasible... 

Epoxy resins are complex network polyethers usually formed in a two-staged process. The first stage involves a base-catalyzed step-growth reaction of an excess epoxide, typically epichlorohydrin with a dihydroxy compound such as bisphenol A This results in the formation of a low-molecular-weight prepolymer terminated on either side by an epoxide group. 

The mechanism of this type of polymerisation reaction follows typical step-growth polymerisation rules, and the diene dithiol (AA BB) ratio is therefore a very important parameter [19]. This observation implies that, in contrast to some of the grafting reactions mentioned above, the dithiol compound cannot be used in excess to compensate for the lower reaction rates with internal alkenes. Hence, these polymerisations are limited to the use of structures bearing terminal double bonds [19]. If this rule is applied to vegetable oils, 10-undecenoic acid and its derivatives are the only species that fulfill this criterion. 

The formation of step-growth polymers, unlike the formation of chain-growth polymers, does not occur through chain reactions. Any two monomers (or short chains) can react. The progress of a typical step-growth polymerization is shown schematically in Figure 27.2. When the reaction is 50% complete (12 bonds have formed between... 

To see why the assumption of equal reactivity is so important to step-growth polymers, recall from Table 1.2 the kind of chemical reactions which produce typical condensation polymers ... 

Another factor in step-growth polymerizations is cyclization versus linear polymerization.1516 Since ADMET is a step-growth polymerization, most reactions are carried out in the bulk using high concentrations of the reactant in order to suppress most cyclic formation. A small percentage of cyclic species is always present but is dependent upon thermodynamic factors, typical of any polycondensation reaction. 

The new polymers 9-28 were synthesized by the Pd-catalyzed reaction of (4,4 - or 5,5 -dibromo-2,2 -bipyridine)-bis(4,4 -terf-butyl-2,2 -bipyridine or 2,2 -bipyridine)ruthenium(II) complexes 5-8 and diethynylarenes (Table 2) in a step-growth polycondensation mechanism (Scheme 4). The typical reaction conditions used for the synthesis of the polymers involved stirring the argon-... 

Cationically catalyzed polymerizations28,72 have not received as much attention as the anionic variety. Typical cationic (acidic) catalysts in this case are Lewis acids. Yields and proportions of the various species are generally very similar to those obtained in anionic polymerizations, although the mechanism is very different. The reaction is thought to proceed through a tertiary oxonium ion formed by addition of a proton to one of the O atoms of the cyclic siloxane. Part of the mechanism may involve step growth, as well as the expected chain growth. 

Step-growth, or condensation, polymers are usually formed in a reaction between two monomers, each of which is at least difunctional. Polyesters, polyamides, polyurethanes, and epoxy resins are typical examples of step-growth polymers. These polymers grow by steps or leaps rather than one monomer unit at a time. 

Sol-gel syntheses are typically carried out in the presence of polar solvents such as alcohol or water media, which facilitate the two primary reactions of hydrolysis and condensation (Eqs. 15 and 16, respectively). During the sol-gel process, the molecular weight of the oxide product continuously increases, eventually forming a highly viscous three-dimensional network (step-growth polymerization - Chapter 5). 

The general types of step-growth polymerizations are shown in Eigure 5.21. Since the resulting polymers contain reactive functional groups on either/both ends, it is referred to as a telechelic macromolecule. Although water is typically released as a byproduct of these polymerizations, other small molecules such as alcohols and alkyl halides may also be generated based on the monomers that co-condense. In its most simplest form, condensation reactions result in linear polymers from the reaction of... 

In step-growth polymerizations, overall costs of monomers, solvent recovery, and preparing the polymer for further processing usually dictate a preference for reactions that are slow at room temperature. (The reasons behind this generalization are summarized in Section 5.3.1.) The ratio of rates of macromolecular growth reactions in typical chain and step-growth polymerizations is often of the order of 10 . ... 

In step-growth polymerizations with unfavorable values of K, it is therefore standard practice to operate at high temperatures and reduced pressures to remove the condensation products. This is typical of the manufacture of linear polyesters where the final stages of the polymerization are at pressures near I mm Hg and temperatures near 280 C. Alkyds (Section 5.4.2) are branched polyesters produced by esterification reactions of mixtures of polyhydric alcohols and acids with varying functionalities. They are used primarily in surface coatings. Alkyd syntheses are completed at temperatures near 240°C. It is not necessary to reduce the pressure to pull residual water out of the reaction mixture, because the final products are relatively low-molecular-weight fluids that are diluted with organic solvents before further use. In one process variation, a small amount of a solvent like xylene is added to the reactants to facilitate water removal by azeotropic... 

FIGURE 20.5 Schematic illustrating the six steps in crystal growth, which can be generally considered as a heterogeneous reaction typical crystal growth proceeds in this fashion. 

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