Hyperbranched aliphatic polyesters synthesis

Several applications of hyperbranched polymers as precursors for synthesis of crosslinked materials have been reported [91-97] but systematic studies of crosslinking kinetics, gelation, network formation and network properties are still missing. These studies include application of hyperbranched aliphatic polyesters as hydroxy group containing precursors in alkyd resins by which the hardness of alkyd films was improved [94], Several studies involved the modification of hyperbranched polyesters to introduce polymerizable unsaturated C=C double bonds (maleate or acrylic groups). A crosslinked network was formed by free-radical homopolymerization or copolymerization. 

Frechet and coworkers reported a first example of the synthesis of a hyperbranched aliphatic polyester by polymerization of an AB inimer, i.e., 4-(hydroxyethyl)-s-caprolactone [164]. It is worth noting that the synthesis of this inimer is quite long. Since this pioneering work, other teams have reported the polymerization of a series of AB [165-167] and AB2 [168] inimers, which are shown in Fig. 38. The polymerization of these inimers can be carried out in the absence or presence of a comonomer such as sCL [167]. 

Enzyme-Catalyzed Synthesis of Hyperbranched Aliphatic Polyesters... 

Scheme 33 Enzymatic synthesis of hyperbranched aliphatic polyesters by combining ROP and AB2 polycondensation.

Although low-molar-mass aliphatic polyesters and unsaturated polyesters can be synthesized without added catalyst (see Sections 2.4.1.1.1 and 2.4.2.1), the presence of a catalyst is generally required for the preparation of high-molar-mass polyesters. Strong acids are very efficient polyesterification catalysts but also catalyze a number of side reactions at elevated temperature (>160°C), leading to polymers of inferior quality. Acid catalysts are, therefore, not much used. An exception is the bulk synthesis of hyperbranched polyesters reported in Section 2.4.5.1, which is carried out at moderate temperature (140°C) under vacuum in the presence of p-toluene sulfonic acid catalyst. The use of strongly acidic oil-soluble catalysts has also been reported for the low-temperature synthesis of polyester oligomers in water-in-oil emulsions.216... 

Hyperbranched polymers are formed by polymerization of AB,-monomers as first theoretically discussed by Flory. A wide variety of hyperbranched polymer structures such as aromatic polyethers and polyesters, aliphatic polyesters. polyphenylenes, and aromatic polyamides have been described in the literature. The structure of hyperbranched polymers allows some defects, i.e. the degree of branching (DB) is less than one. The synthesis of hyperbranched polymers can often be simplified compared to the one of dendrimers since it is not necessary to use protection/deprotection steps. The most common synthetic route follows a one-pot procedure " where AB,-monomers are condensated in the presence of a catalyst. Another method using a core molecule and an AB,-monomer has been described. ... 

By virtue of their strong antibacterial properties and low toxicity, Ag-NPs and their compounds have been studied for many years [66]. Meanwhile, studies conducted on Ag-NPs using different polymers have been reported these include the synthesis in poly(vinyl pyrrolidone) [67], poly(vinyl alcohol) [68], hyperbranched polyurethane [69] and aliphatic polyesters [70] to give discrete Ag-NPs. There are significant variations in the average sizes and shapes of the Ag-NPs when the polymers were used due to the different chemical structures and Ag/polymer interactions. 

Schallausky F, Erber M, Komber H, Lederer A (2008) An easy strategy for the synthesis of well-defined aliphatic-aromatic hyperbranched polyesters. Macromol Chem Phys 209 2331-2338... 

Concerted efforts to examine the synthesis and polymerization of such ABx monomers was however not attempted until 1989 when Kim and Webster (17) reported their preparation of hyperbranched polyphenylenes fi-om the AB2 monomers, 3,5-dibromophenyl boronic acid, 7, or 3,5-dibromophenyl magnesium bromide, 8 (Figure 1). In a similar way to dendrimers, the field of hyperbranched macromolecules has been extended considerably since this initial report. Hyperbranched polyesters based on fully aromatic, fiilly aliphatic, or partially aliphatic/aromatic systems have been extensively studied by a number of groups (18-20). Similarly, hyperbranched poly(etherketones) (21), poly(ethers) (22), poly(urethanes) (23), poly(siloxanes) (24), etc. have been prepared using classical step growth chemistry. 

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