Polycondensation of AB Monomers

Fig. 2 Synthetic routes to procure polyesters using lipase catalysis (a) ROP of cyclic esters, (b) polycondensation of AA-BB monomers, and (c) polycondensation of AB monomers...

Although rigid-rod poly(p-phenyleneterephthalamide) analogues having alkyl side chains did not contain cyclic polymers, the polycondensation of silylated m-phenylenediamine and aliphatic dicarboxyhc acid chloride afforded cyclic polyamides predominantly (Scheme 49) [187]. Furthermore, cyclic polymers were also produced in polycondensations for polyesters, poly(ether ketone)s, polyimides, and polyurethanes [183]. These examples are the products in polycondensation of AB monomers or in A2 + B2 polycondensations, but cyclization of oligomer and polymer was also confirmed in polycondensation of AB2 monomers [ 188-195] and in A2 + B3 [ 196-202] and A2 + B4 polycondensations [203-206], which afford hyperbranched polymers. 

Hyperbranched polymers can be prepared by a variety of techniques, including the polycondensation of AB monomers as originally described by Flory [113], the reaction of A2 + B3 monomers, and self-condensing vinyl polymerization [139-141]. The first report [142] of using click chemistry in the synthesis of hyperbranched materials appeared at about the same time as the initial report for dendrimers prepared using CuAAC however, but much fewer examples have been reported that describe hyperbranched materials involving click chemistry. Nevertheless, these polymers represent an important class of materials, and both CuAAC [142-147] and thiol-ene [148] chemistry have found their way into the hyperbranched hterature. 

Case IV. The polycondensation of AB monomers such as co-hydroxy carboxylic acids ... 

Beginn U, Drohman C, Moller M (1997) Conversion dependence of the branching density for the polycondensation of AB monomers. Macromolecules 30 4112-4116... 

As already mentioned by Flory [27], polycondensations of ab monomers do never result in crosslinking, when side reactions are absent, whereas 32 -I- b monomers will yield gels at a2/b ratios > 0.9, and at high monomer concentrations and conversions. 

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... 

As early as 1952, Flory [5, 6] pointed out that the polycondensation of AB -type monomers will result in soluble highly branched polymers and he calculated the molecular weight distribution (MWD) and its averages using a statistical derivation. Ill-defined branched polycondensates were reported even earlier [7,8]. In 1972, Baker et al. reported the polycondensation of polyhydrox-ymonocarboxylic acids, (OH)nR-COOH, where n is an integer from two to six [ 9]. In 1982, Kricheldorf et al. [ 10] pubhshed the cocondensation of AB and AB2 monomers to form branched polyesters. However, only after Kim and Webster published the synthesis of pure hyperbranched polyarylenes from an AB2 monomer in 1988 [11-13], this class of polymers became a topic of intensive research by many groups. A multitude of hyperbranched polymers synthesized via polycondensation of AB2 monomers have been reported, and many reviews have been published [1,2,14-16]. 

A tremendous variety of hyperbranched materials have been synthesized (usually in one-pot reactions) via polycondensation of AB -type monomers, such as polyesters (via condensation... 

Consider the polycondensation of functional monomers of the type R AB/ i. The reaction is assumed to take place between the A group and B group only [1], Nonlinear polymers with a tree structure are formed by reaction. They may have intramolecular cycles, but to find the exact solution we consider only branched tree-type polymers which have no cycles (Figure 3.6). These are sometimes called Cayley trees, named after the mathematician who studied tree-type graphs. The approximation under this assumption of no intramolecular cycles is called the tree approximation. 

Jikei and Kakimoto have prepared the aromatic HBPAs and copolymers by direct polycondensation in the presence of condensation agents by (i) self-polycondensation of ABx-type monomers, (ii) copolymerization of AB and AB monomers and (iii) polymerization of A and B3 monomers [71]. The self-polycondensation of ABx monomers proceeded without gelation to form soluble polymers. Spectroscopic measurements revealed that the copolymers were composed of five kinds of repeating units. The inherent viscosity of the resulting PAs was low, in the range 0.2-0.4 dL/g. All of the resulting PAs were soluble in aprotic polar solvents, such as DMF, NMP and DMSO. The good solubility and adequate solution viscosity of the copol)nners allowed the preparation of transparent... 

Condensation polymerizations (polycondensations) are stepwise reactions between bifunctional or polyfunctional components, with elimination of small molecules such as water, alcohol, or hydrogen and the formation of macromo-lecular substances. For the preparation of linear condensation polymers from bifunctional compounds (the same considerations apply to polyfunctional compounds which then lead to branched, hyperbranched, or crosslinked condensation polymers) there are basically two possibilities. One either starts from a monomer which has two unlike groups suitable for polycondensation (AB type), or one starts from two different monomers, each possessing a pair of identical reactive groups that can react with each other (AABB type). An example of the AB type is the polycondensation of hydroxycarboxylic acids ... 

The formation of a condensation polymer is a stepwise process. Thus, the first step in the polycondensation of a hydroxycarboxylic acid (AB type) is the formation of a dimer that possesses the same end groups as the initial monomer ... 

Table 4.2 shows how important it is in polycondensation reactions to ensure the exact equivalence of functional groups, since even a 1 mol% excess of one of the two groups limits the maximum attainable degree of polymerization P to less than 200. For polycondensations of the AB type, e.g., hydroxycarboxy-lic acids or amino acids, this equivalence is automatic since the monomer contains both groups. On the other hand, for polycondensations of the AABB type, e.g., between diols and dicarboxylic acids, a small excess of one component causes the reaction to come to a halt when only the end groups of the component present in excess are left because these are unable to react with each other. 

The manufacture of the large variety of polyamides (commonly referred to as nylons) occurs through polycondensation of amino carboxylic acids (or functional derivatives of them, e.g. lactams) and from diamines and dicarboxylic acids. Labeling the amino groups with A and the carboxyl groups with B allows differentiation of the different chemical structures between the two types AB (from amino carboxylic acids) and AA-BB (from diamines and dicarboxylic acids). The number of C atoms in the monomers acts as a code number for the identification of the polyamides. The polycaprolactam manufactured from caprolactam (type AB) is then called polyamide 6 (PA 6). The number of carbon atoms in the diamine is given first for type AA-BB followed by the number of atoms in the dicarboxylic acid, e.g. PA 66 for polyhexamethylenedia-dipic amide from hexamethylenediamine and adipic acid. For copolymers the components are separated by a slash, e.g. PA 66/6 (90 10) is a copolymer composed of 90 parts PA 66 and 10 parts PA 6. 

Many methods have been reported to synthesize hyperbranched polymers. These materials were first reported in the late 1980s and early 1990s by Odian and Tomalia [9], Kim and Webster [10], and Hawker and Frechet [11]. As early as 1952, Hory actually developed a model for the polymerization of AB -type monomers and the branched structures that would result, identified as random AB polycondensates [46], Condensation step-growth polymerization is likely the most commonly used approach however, it is not the only method reported for the synthesis of statistically branched dendritic polymers chain growth and ringopening polymerization methods have also been applied. 

In 1952, Flory desCTibed random AB polycondensation theoretically and showed that highly branched polymers can be synthesized without gelation by poly condensation of an AB monomer (x>2) in which A functional groups can react with B groups [22]. In 1988, the term hyperbranched polymer was... 

Hyperbranched polymers are synthesized in a one-step method, often from AB monomers but also by combining A +B (x>3) monomers or variations of those. Polymerization methods have been applied that involve polycondensation, polyaddition, and ring-opening or self-condensing vinyl polymerization. Even though the one-pot synthetic approach leads to imperfectly branched structures because of uncontrolled growth, it is more suitable for the preparation on a larger scale and thus for commercial use. Nowadays, different... 

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