Statistically Branched Dendritic Polymers

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. 

Another important issue is the difference between various branching types such as random hyperbranched [31], dendrigrafts and dendrimers. The complexity of synthesis requirements manifested by the statistical dendritic polymers versus the more structurally controlled dendrimers could make the former orders of magnitude more expensive than hyperbranched. Are the structures as significantly unique and of sufficient value effectiveness to justify the higher costs ... 

Poly(ethylene oxide) (PEO) has been employed frequently as a water-soluble catalyst support [9]. Further water-soluble polymers investigated include other linear polymers such as poly(acrylic acid) [10], poly(N-alkylacrylamide)s [11], and copolymers of maleic anhydride and methylvinylether [12], as well as dendritic materials such as poly(ethyleneimin) [10a, c] or PEO derivatives of polyaryl ethers [13]. The term dendritic refers to a highly branched, tree-like structure and includes perfectly branched dendrimers as well as statistically branched, hyperbranched macromolecules. 

Statistical treatments show that batch polymerization of AB2 monomers gives a DB of 0.5. Higher degrees of branching can be obtained using special reaction conditions (slow addition of monomer, addition of a core). Perfect branching (DB = 1) can be found in the dendritic polymers. 

Mathematically, the number of covalent bonds formed per generation (reaction step) in a dendron/dendrimer synthesis varies as a power function of the reaction steps (Fig. 5). This analysis shows that covalent bond amplification occurs in all dendritic strategies. This feature clearly differentiates dendritic processes from covalent bond synthesis found in traditional organic chemistry or polymer chemistry. Polymerization of AB2 or ABj monomers leading to hyper-branched systems also adheres approximately to these mathematics, however, in a more statistical fashion. 

A one-pot solution polymerization was performed at room temperature using partially aromatic monomers, namely 4,4-bis(4 -hydroxyphenyl)valeric acid as AB2 and 3-(4-hydroxyphenyl)propionic acid as AB [118] (see Table 1, entry 5 for monomer combination), in the presence of 4-(A, A dimethylamino) pyridinium 4-tosylate (DPTS) as catalyst and dicyclohexylcarbodiimide (DCC). The dependencies of the DB and the thermal properties of the polymers on the AB AB2 monomer ratio were studied. Polyesters with statistical dendritic topology, controlled DB, and > 35,000 g/mol were obtained. The DB was found to decrease with an increase in the amount of AB monomers and increasing comonomer ratio in the polymer (rp = ratio of AB to AB2) as shown in Fig. 1. Interestingly, the DB for hb homopolyester and branched copolyester at rp = 0.46 was similar (see Fig. la), because of the fact that on adding the small-sized linear AB to the more voluminous AB2 monomers in the reaction mixture, the steric effects decreased, which promoted the formation of dendritic units formed by the AB2 monomer. The thermal... 

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