Synthesis, Characterization, and Modification of Hyperbranched Poly(arylene oxindoles) with a Degree of Branching of 100 %

Abstract

Hyperbranched polymers are of high importance as readily available substitutes for dendrimers. Whereas the preparation of the latter requires a multistep synthesis, hyperbranched polymers can be obtained in a single step. [1] However, a degree of branching of 100 %, which is a characteristic property of dendrimers, can be achieved only by postsynthetic modification [2] or when certain requirements for the monomer are met. [3] To date, the only example of a hyperbranched polymer (which necessarily consists exclusively of dendritic and terminal units) was based on an AB 2 monomer possessing an azine and a maleimide functional group, which react in a ™criss-cross∫ cycloaddition. [4] However, to improve features such as accessibility, convenience of functionalization, and attractive physical properties, it is highly desirable to devise new strategies towards hyperbranched polymers with a degree of branching of 100 %.We applied the acid-catalyzed condensation of isatins [5] with aromatic compounds for the synthesis of hyperbranched poly(arylene oxindoles). The linear analogues have been shown to display a high glass-transition temperature (T g ) and excellent thermal stability. [6] The use of para-phenoxybenzophenones as the aromatic reaction partner results exclusively in para-substitution and hence in structural homogeneity of the polymer. [6] Therefore, the AB 2 monomer 1 was designed and synthesized by Stille coupling of the tin derivative 3 and 5-bromo-1-methylisatin (4) [7] (Scheme 1). The methyl group was introduced to enhance the solubility of both the monomer and the polymers and to facilitate the characterization of the latter by NMR spectroscopy. As it is known that this type of condensation of isatin and aromatic compounds exclusively yields 3,3-diaryl oxindoles, [5] we can exclude the possibility of the presence of linear units in the hyperbranched polymers obtained from 1. The tin derivative 3 was obtained by palladium-catalyzed transmetalation of the bromide 2 and hexakis-n-butylditin. The bromide 2 was formed by Friedel± Crafts acylation of diphenyl ether (five-fold excess) with 4-bromobenzoyl chloride.Monomer 1 was polymerized over 24 h using trifluoromethanesulfonic acid, to yield the polymers 5 a±d, which possess 3,3-diaryl oxindole groups as dendritic units and isatin residues at the periphery. These polymers were soluble in chloroform, dichloromethane, and toluene. They were poorly soluble in THF and insoluble in acetone. These solubility properties are in sharp contrast to the linear analogues which were only soluble in highly polar solvents such as N,Ndimethylacetamide and N-methylpyrrolidinone, or in the presence of strong acids. [6] As can be seen from Table 1, the obtained molecular weights are strongly dependent upon the reaction temperature and the concentration. At room temperature, only oligomers were obtained although, according to the literature, [6] the linear polymerization occurred readily at 25 8C. Higher temperatures and higher concentrations favor the formation of higher molecul...

A Convenient A2 + B3 Approach to Hyperbranched Poly(arylene oxindole)s

“…In the 1 H NMR spectrum, the resonance signal for the N-methyl group is shifted from 3.31 ppm in the monomer to 3.29 ppm in polymer 3, indicating the formation of a 3,3-diaryl oxindole group during polymerization. [9] The absence of any other peaks from the methyl groups in the starting isatin indicates that there are only completely reacted A 2 units in the polymer structure. Meanwhile, according to the integration, the ratio of A 2 and B 3 units in the polymer is 1:1, the same as in the feed.…”

“…[9] The B 3 monomer used was 1,3,5-tri-(4-phenoxybenzoyl)-benzene (2) (Figure 1). To the suspension of phenyl ether (25 g, 150 mmol) and AlCl 3 (8 g, 60 mmol) in dichloroethane (70 mL) was added dropwise a dichloroethane (30 mL) solution of benzene tricarbonyl trichloride (2.7 g, 10 mmol).…”

“…[8] We have already shown that this reactivity difference makes isatin a perfect candidate for the preparation of hyperbranched polymers with a degree of branching of 100%. [9] In this light, its use as the A 2 partner in an A 2 þ B 3 approach seems very attractive as well.…”

Synthesis of hyperbranched polymers with controlled degree of branching

“…Thereafter, Smet et al 58 carried out the acid-catalyzed condensation of an isatin-based AB 2 -type monomer with a highly reactive phenoxy group in trifluoromethanesulfonic acid (TFSA), a 'super acid' , for synthesis of hyperbranched poly(arylene oxindole) with a 100% DB (Scheme 11). As it is well known that condensation of isatin and aromatic compounds under acidic conditions exclusively yields 3,3-diaryl oxindoles because of instability of the mono-condensed intermediate, the possibility of generating linear units in the HBP is totally denied, resulting in a 100% DB.…”

“…Moreover, Scheme 11 Synthesis of hyperbranched poly(arylene oxindole) with 100% DB. 58 DB, degree of branching.…”