Polyesters, hyperbranched

Additional parameters should be taken into account for polyester networks and hyperbranched polyesters, for example, crosslink density and degree of branching. 

Hyperbranched polyesters are prepared by the step-growth polymerization of A13,-type monomers where A and B are —OH and —COOH groups or derivatives such as CH3COO-, HO-CH2CH2-O-, (CH3)3SiO—, -COOCH3, or -COC1... 

TABLE 2.18 Monomers Used for Synthesis of Hyperbranched Polyesters... 

Due to dieir compact, branched structure and to die resulting lack of chain entanglement, dendritic polymers exhibit much lower melt and solution viscosity dian their lineal" counterparts. Low a-values in die Mark-Houwink-Sakurada intrinsic viscosity-molar mass equation have been reported for hyperbranched polyesters.198 199 Dendrimers do not obey diis equation, a maximum being observed in die corresponding log-log viscosity-molar mass curves.200 The lack of chain entanglements, which are responsible for most of the polymer mechanical properties, also explains why hyperbranched polymers cannot be used as diermoplastics for structural applications. Aldiough some crystalline or liquid... 

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... 

This aliphatic hyperbranched polyester is prepared by the bulk polycondensation of 2,2-bis(hydroxymethyl)propionic acid (bis-MPA) as AB2 monomer and 1,1,1-tris(hydroxymethyl)propane (TMP) as B3 core molecule, according to a procedure... 

Wholly aromatic hyperbranched polyesters based on 3,5-dihydroxyisophthalic acid can be prepared either from bis(acetoxy)isophthalic acid or from its silylated derivative (Scheme 2.67). 

Note This hyperbranched polyester was also prepared by the bulk polymerization of 3,5-bis(trimethylsilyloxy)benzoyl chloride.202 DSC Tg = 190°C. DB = 0.55-0.60 ( H NMR measurements). 

However, dendrimeric and hyperbranched polyesters are more soluble than the linear ones (respectively 1.05, 0.70, and 0.02 g/mL in acetone). The solution behavior has been investigated, and in the case of aromatic hyperbranched polyesters,84 a very low a-value of the Mark-Houvink-Sakurada equation 0/ = KMa) and low intrinsic viscosity were observed. Frechet presented a description of the intrinsic viscosity as a function of the molar mass85 for different architectures The hyperbranched macromolecules show a nonlinear variation for low molecular weight and a bell-shaped curve is observed in the case of dendrimers (Fig. 5.18). 

Aliphatic hyperbranched polyesters, 56 Aliphatic isocyanate adducts, 202 Aliphatic isocyanates, 210, 225 Aliphatic polyamides, 138 Aliphatic polyesteramides, 56 Aliphatic polyesters, 18, 20, 29, 32, 87 degradable, 85 hyperbranched, 114-116 melting points of, 33, 36 structure and properties of, 40-44 syntheses of, 95-101 thermal degradation of, 38 unsubstituted and methyl-substituted, 36-38... 

Hydroxyphenylimide, polybenzoxazole synthesis via, 318 Hydroxy-terminated hyperbranched polyesters, 32... 

Hyperbranched polyesters, 18, 32, 55-58 bulk synthesis of, 64 synthesis of, 114-118 Hyperbranched polyimides, 307-309 Hyperbranched polymers, 8-10, 348-350, 475-476, 481, 519-520 degree of branching in, 57 Hyperbranched polyphenylquinoxalines, 312-314... 

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. 

Similar materials, hyperbranched polyesters based on bis-MPA and a polyol are now commercially available [11] from Perstorp AB under the trade name Boltorn [12], Figure 8.1. The average number of hydroxyl groups per molecule can be tailored between 8 and 64 and molecular weight can be varied between c. 2000 and 11 000. The co-polymerization of bis-MPA and a polyol core keeps the molecular weight distribution fairly low, typically below 2. 

Figure 8.1 A schematic representation of the hydroxy-functional hyperbranched polyester Boltorn H20, based on ethoxylated pentaerythritol and 2,2-bis(methylol)propionic acid [10]...

The degree of branching was initially reported to be close to 0.8 [1], but was recently reevaluated after it was shown that the hydroxy-functional hyperbranched polyesters undergo facile acetal formation. The acetal formation was catalyzed by residual trace amounts of acid remaining in the sample. After reevaluation in DMSO the degree of branching was close to 0.45 which is in accordance with most other hyperbranched polymers. (1. Malmstrom, E., Johansson, M. and Hult, A. Macromolecules, 28, 1698 (1995) 2. Malmstrom, E., Trollsas, M., Hawker, C.J., Johansson, M. and Hult, A. Polym. Mat. Sci. Eng., 77,151 (1997). 

When polymerizing A2B monomers there is a possibihty of losing the unique focal point due to intramolecular cyclization. The loss of the focal point in a hyperbranched polyester based on 4,4-(4 -hydroxyphenyl)pentanoic (Fig. 7) acid was closely examined by Hawker et al. [45]. The study showed no significant occurrence of intramolecular cyclization. One disadvantage of polycondensation polymers is that they are sensitive to hydrolysis, that is depolymerization, which might restrict their use. Some hyperbranched polymers are synthesized via substitution reactions which provide less hydrolytically unstable polymers. 

Considerable attention has been paid to aromatic hyperbranched polyesters synthesized from monomers derived from 3,5-dihydroxybenzoic acid (DBA). The thermal stability of DBA is not good enough to allow direct esterification of DBA, and therefore chemical modifications are necessary. Some aromatic monomers used for the synthesis of hyperbranched aromatic polyesters are presented in Fig. 6. 

Turner et al. [71, 72] also report on hyperbranched polyesters derived from 3,5-bis(trimethylsiloxy)benzoyl chloride and from 3,5-diacetoxybenzoic acid, which both yield phenolic polyesters after hydrolysis of the end groups. The same group investigated the hyperbranched polyesters obtained in the melt condensation of 5-acetoxyisophthalic acid and 5-(2-hydroxy)-ethoxyisophthalic acid respectively. The latter yields a soluble product while the former results in an insoluble polymer due to formation of anhydride bridges. 

Feast and Stainton [63] reported on the synthesis of aromatic hyperbranched polyesters from 5-(2-hydroxyethoxy)isophthalate copolymerized with 1,3,5-benzenetricarboxylate (core molecule) as a moderator of the molecular weight. The degree of branching was found to be 0.60-0.67 as determined by C-NMR. Apparent molecular weights (M ) were found to be 5-36 kDa according to SEC characterization using linear polystyrene standards. 

Structural variations of hyperbranched polyesters have also been achieved by copolymerizing an A2B-monomer with an AB-functional monomer, although no properties were reported for these copolymers [71]. 

The use of aliphatic monomers for hyperbranched polyesters has been debated because aliphatic monomers are said to be prone to thermal degradation reactions such as decarboxylation, cyclization, or dehydration [77]. The only commercial hyperbranched polymer is a hydroxy-functional aliphatic polyester, Boltorn, available from Perstorp AB, Sweden. 

Hawker et al. report on the synthesis of a similar hyperbranched polyester based on the corresponding AB4-monomer that is, the preformed dendron of the second generation was used in the condensation reaction [79]. 

A somewhat different approach was presented by Rannard and Davis where they first reacted bis-MPA with carbonyl diimidazole, allowing a highly selective base-catalyzed reaction to form a hyperbranched polyester. The resulting polymers were hydroxy-functional and reported to be water-soluble [84]. 

Hult et al. [36] have described semi-crystalline hyperbranched aliphatic polyesters where the crystallinity was induced by attachment of long alkyl chains as end groups. The crystallization was affected by several factors such as length of the end groups and the molecular weight of the hyperbranched polyester. The crystallization was proposed as being either intra- or intermolecular depending on the size of the hyperbranched polyester onto which the alkyl chains were attached. 

Fig. 10. Complex dynamic viscosity as function of temperature for three different aliphatic hyperbranched polyesters based on bismethylol propionic acid and having different end-group structure - (O) propionate end-groups, ( ) benzoate end-groups, ( ) hydroxyl end-groups [118]...

Another demonstrated application is the use of epoxidized hyperbranched polyesters as toughening additives in composites [120-122]. 

A combination of enhanced reactivity and reduced viscosity for alkyd resins has been achieved by using hyperbranched polyester structures as discussed in Sect. 4.2.3 [ 123]. This study clearly showed the benefits of using highly branched structures in coating applications to obtain improved properties. 

An important application of polymers in medicine is in advanced drug-delivery systems. These materials control the drug concentration and delivery rate in the body. Hyperbranched polyesters have been suggested for such systems [111]. However, most applications within this field, described in the literature, deal with dendrimers and not with hyperbranched polymers. 

In this chapter we describe the synthesis of new hyperbranched polyester-amides carried out with standard melt condensation technology as well as the properties of these new structures. 

There are two ways to make hyperbranched polyester amides water soluble. One is to functionahze the periphery of the molecule with hydrophilic groups, the other the use of suitable anhydrides to obtain water solubihty via the core. These approaches are shown in Fig. 24. 

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