Step-polymerization

Step polymerization occurs by stepwise reaction between functional groups of reactants. The reaction leads successively from monomer to dimer, trimer, tetramer, pentamer, and so on, until finally a polymer molecule with large DP is formed. Note, however, that reactions occur at random between the intermediates (e.g., dimers, trimers, etc.) and the monomer as well as among the intermediates themselves. In other words, reactions of both types, namely, 

Since most (though not all) of the step polymerization processes involve polycondensation (repeated condensation) reactions, the terms step polymerization and condensation polymerization are often used synonymously. Consider, for example, the synthesis of a polyamide, i.e., a polymer with amide ( ONH-) as the characteristic linkage. If we start with, say, hexamethylenediamine and adipic acid as reactants, the first step in the formation of the polymer (nylon) is the following reaction producing a monoamide  

The reaction continues step-by-step to give the polyamide nylon-6,6. The overall reaction may thus be represented as 

We see that the composition of the repeating unit (enclosed in square brackets) equals that of two monomer molecules minus two molecules of water. Thus a condensation polymer may be defined as one whose synthesis involves elimination of small molecules or whose repeating unit lacks certain atoms present in the monomer(s). 

Problem 1.4 Poly(hexamethylene adipamide) (Nylon-6,6) was synthesized by condensation polymerization of hexamethylenediamine and adipic acid in 1 1 mole ratio. Calculate the acid equivalent of the polymer whose average DP is 440. 

The above classification of polymers according to polymerization mechanism, as shown by the variation of molecular weight with conversion [Figs. 1.2(a) and 1.2(b)], is not without its ambiguities. Certain polymerizations show a linear increase of molecular weight with conversion [Fig. 1.2(c)] when the polymerization mechanism deviates from the normal chain or step pathway. This is observed in certain ionic chain polymerizations, which involve a fast initiation process coupled with the absence of reactions that terminate the propagating reactive centers. Biological syntheses of proteins also show the behavior described by Fig. 1.2(c) because the various 

The first step reaction example involves the preparation of water-based epoxy resins via ring-opening polymerizations of different epoxides with various diamines, dithiols or diols residing in the miniemulsion droplets at 60°C [75]. The basic requirement for successful miniemulsion polymerization is that both reactive components exhibit relatively low solubility in the continuous aqueous phase. The diepoxide Epikote E828, triepoxide Decanol EX-314, and tetraepoxide EX-411 are potential candidates for this purpose. Furthermore, incorporation of conventional costabilizers such as hexadecane into the 

Water-based polyurethane products can also be prepared by homogenizing a mixture of diisocyanates and diols in an aqueous surfactant solution, followed by heating the resultant miniemulsion to the prescribed step polymerization temperature [76]. Satisfactory miniemulsion polyurethanes are obtained only when the following requirements are fulfilled. 

The reaction between the isocyanate groups near the droplet surface layer and water molecules are thought to generate more hydrophobic urea groups that form a passivated surface layer to retard further reaction of the isocyanate groups with water inside the droplets. 

Landfester, N. Bechthold, S. Foster, and M. Antonietti, Macromol. Rapid 

Delgado and M. S. El-Aasser, Macromol. Chem. Macromol. Symp. 31, 63 (1990). 

This proceeds by pairs of molecules, either monomers or oligomers, reacting together (often with the elimination of a small molecule) and the polymer molecules grow throughout the reaction at a steady rate. This may take hours 

Monomer concentration decreases slowly during reaction 

Intermediates not usually isolatable (low concentrations of growing chains) 

MW independent of time MW generally decreases with increasing temperature Rate of polymerization is zero initially, rises to a maximum as active centres are formed from the initiator, remains constant, before falling off when the monomer is consumed 

Polymerization can proceed without catalyst Viscosity increases rapidly at end of reaction Monomer concentration decreases rapidly before any high polymer formed Oligomers can be isolated at any stage 

---

The general definition of a condensation reaction is a one that involves product formation by expulsion of water (or other small molecule) as a by-product. By this definition, activation and methylolation are also condensations. In more precise terms the chain-building process should be described as a condensation polymerization, however, in the jargon of the phenolics industry, the term condensation is usually reserved for the chain-building process. This terminology is not necessarily observed in the literature [88]. Many literature reports correctly refer to methylolation as a condensation reaction. The molecular weight development of the phenol alcohol adducts may also be classified as a step-polymerization. 

Here, instead of an ordinary diol to react with ACPC, an azo-containing diol such as ACPO or AHPA was used to introduce two kinds of azo groups in the repeating unit. The decomposition temperature of newly incorporated azo groups were higher than that of the ACPC unit, and thus MAI shows dual decomposition temperatures so that a more labile azo group is specifically used for the first step polymerization and another... 

Major fiber-making polymers are those of polyesters, polyamides (nylons), polyacrylics, and polyolefins. Polyesters and polyamides are produced by step polymerization reactions, while polyacrylics and polyolefins are synthesized by chain-addition polymerization. 

L. Vollbracht, in Comprehensive Polymer Science, Vol. 5 Step Polymerization,... 

After the potential step, polymeric oxidation is followed by an oxidation charge to open, swell, and oxidize the compact film. At the start, the charge consumed to relax the compacted polymeric structure is the only component of the oxidation charge. Thus any quantitative information about the... 

One step polymerization using phosgene or diphosgene as coupling agents. Macromolecules. 21, 1925-1929, 1988. 

Keywords. Statistical chemistry of polymers, Chain and step polymerizations, Linear and branched polymers... 

A two step polymerization involving formation of oligomeric phos-... 

Phase mixing can be attributed partly to the irregularity of the copolymer chain. In the case of poly(ester ether) copolymers synthesized by normal step polymerization reactions, the hard segment has a... 

Our future work includes the synthesis of highly regular segmented copolymers via step polymerization of the telechelomer and investigating their structure-property relationships. 

Figure 24.3 Two-step polymerization process for the manufacture of polyethylene terephthalate ...

Alternatively, the one-step polymerization of branched monomers results in what is called a hyperbranched polymer [53] possessing a higher degree of polydispersity and lower degree of branching compared to the analogous dendrimer. 

Table 4 Comparison between addition and step polymerization...

Step polymerizations of linear chains can involve either two different bifunctional monomers in which each monomer possesses only one type of functional group (commonly represented by X-X or Y-Y), or a single monomer containing both types of functional groups (common representation X-Y). However, whatever the monomer type, a linear polymer molecule contains, on average, one functional group of each species per chain (molecule). 

More recent studies, particularly with slower hafnium complexes, have provided more detailed mechanistic insight As a step polymerization, the reaction is "nonideal" in that inequivalent reactivities for different Si-H functional groups in the system are observed. For exaniple, disilanes tend to be more reactive than monosilanes. Beyond disilane formation, the preferred dehydrocoupling reaction appears to involve addition of one silicon at a time to the growing chain, via M-S1H2R intermediates (n = 1 above). The Si-Si bond-forming reactions are also reversible. 

Step polymerization proceeds by the stepwise reaction between the two different functional groups of the monomers. One proceeds slowly from monomer to dimer and larger-sized species ... 

The size of the polymer molecules increases at a relatively slow rate in step polymerization compared to chain polymerization due to the lower rate constants in the former since k is of the order of 10 3-10 liter/mole-sec. One proceeds slowly from monomer to dimer, trimer, tetramer, pentamer, and so on until eventually large polymer molecules have been formed. Any two molecular species containing respectively the two different functional groups can react with each other throughout the polymerization. The average size of the molecules increases slowly with time and high molecular weight polymer is not obtained until near the very end of the reaction (i.e., above 95% conversion). ... 

It should be noted that the product of a step polymerization is a mixture of polymer molecules of different molecular weights. The molecular weight distribution is characterized by the number-average and weight-average degrees of polymerization, X and Xw> respectively, defined by... 

Step polymerization is used to synthesize multiblock copolymeric elastomers (referred to as segmented elastomers). An example is the polyester-polyurethane system produced by the reaction of a diisocyanate with a mixture of macro diol and smallsized diol (Eq. 14). The macro diol (usually referred to as a... 

---