Polycondensation one-step

A majority of the hyperbranched polymers reported in the literature are synthesized via the one-pot condensation reactions of A B monomers. Such one-step polycondensations result in highly branched polymers even though they are not as idealized as the generation-wise constructed dendrimers. The often very tedious synthetic procedures for dendrimers not only result in expensive polymers but also limit their availability. Hyperbranched polymers, on the other hand, are often easy to synthesize on a large scale and often at a reasonable cost, which makes them very interesting for large-scale industrial applications. 

The rate is slower in basic aprotic amide solvents, and faster in acidic solvents such as / -cresol. In general, the imidization reaction has been shown to be catalyzed by acid (14,32,33). Thermal imidization of poly(amic acid)s is catalyzed by tertiary amines (34). High temperature solution polymerization in -cresol is often performed in the presence of high boiling tertiary amines such as quinoline as catalyst. Dialkylaminopyridines and other tertiary amines are effective catalysts in neutral solvents such as dichlorobenzene (35). Alkali metal and zinc salts of carboxylic acids (36) and salts of certain organophosphorus compounds (37) are also very efficient catalysts in one-step polycondensation of polyimides. 

Whereas the well-characterized, perfect (or nearly so) structures of dendritic macromolecules, constructed in discrete stepwise procedures have been described in the preceding chapters, this Chapter reports on the related, less than perfect, hyperbranched polymers, which are synthesized by means of a direct, one-step polycondensation of A B monomers, where x > 2. Flory s prediction and subsequent demonstration 1,2 that A B monomers generate highly branched polymers heralded advances in the creation of idealized dendritic systems thus the desire for simpler, and in most cases more economical, (one-step) procedures to the hyperbranched relatives became more attractive. 

The pol3mrerizations were performed by two different routes two-step and one-step polycondensations. The imidization in two-step synthesis was carried out thermally and chemically (Figure 1)... 

The one-step polycondensation in nitrobenzene was less sensitive to the stoichiometry of reagents than the two-step synthesis.PAtss] PT-IR spectra of the obtained Pis showed intensive characteristic imide bands at 1773 cm (imide C=0 asymmetrical stretching), 1714 cm (imide C=0 S3nnmetrical stretching), and 1380 cm (imide CNC axial), confirming the complete imidization. 

None of these conventional methods was effective enough for cellulose synthesis. In addition to the present-day knowledge of chemistry at the time, enzymes were introduced as catalysts for cellulose synthesis [14]. Then, taking the hypothesis of an artificial substrate as shown in Fig. 2B into consideration, y0-cellobiosyl fluoride (y0-CF) was designed as substrate monomer for catalysis by cellulase, an extracellular hydrolysis enzyme. It was found that the reaction proceeded smoothly to produce synthetic cellulose via a one-step polycondensation that liberated the HF molecule (Scheme 6). In the reaction y0-CF acted as both donor and acceptor [31]. 

To appreciate the growth characteristics of highly branched rigid aromatic FPs obtained by one-step polycondensation, we have to understand the nature of the structural elements involved. When linear polymers are prepared by polycondensation from difunctional monomers AA and BB, aromatic diadd and aromatic diamine for instance, the number average degree of polymerization, DP, is related to the fraction, p, of consumed monomer AA (or BB) by [39]... 

An important outcome of the above was the realization [664] that, because there is almost no energetic difference between syn- and anti-placements [412, 665] in stiff aromatic polyamides, there is practically no energetic advantage to either placement Therefore, during the one-step polycondensation, monomers... 

Schulz et al. [497] described a series of poly(oxadiazoles) for potential LED applications. Some of their structures have been depicted earlier in Fig. 12-21. These were prepared by a one-step polycondensation of the aromatic dicarboxylic acids with hydrazine hydrate in polyphosphoric acid, which acted as a solvent as well as dehydration agent. 

Hyperbranched pol)miers are prepared by the direct, one-step polycondensation of AB monomers with two different types of ftmctional groups (A and B) that can react with each other to form a covalent bond, and the total number of reactive sites is x + 1 (x > 2) [48]. Hyperbranched polymers are highly branched like dendrimers, but their structure is not regular or highly symmetrical because linear segments can be formed, as shown in Scheme 10. It is important to note that in 1952 Flory [49] recognized the unique polymer structural type that... 

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