Hyperbranched polyesters, star-shaped

Polymers are normally classified into four main architectural types linear (which includes rigid rod, flexible coil, cyclic, and polyrotaxane structures) branched (including random, regular comb-like, and star shaped) cross-linked (which includes the interpenetrating networks (IPNs)) and fairly recently the dendritic or hyperbranched polymers. I shall cover in some detail the first three types, but as we went to press very little DM work has been performed yet on the hyperbranched ones, which show some interesting properties. (Compared to linear polymers, solutions show a much lower viscosity and appear to be Newtonian rather than shear thinning [134].) Johansson [135] compares DM properties of some hyperbranched acrylates, alkyds. and unsaturated polyesters and notes that the properties of his cured resins so far are rather similar to conventional polyester systems. 

In a comparison (14) of the polyesterification of silylated 5-acetoxyisophthalic acid and of free 5-acetoxyisophthalic acid, the nonsilylated monomer yielded insoluble products, indicating that a cross-linked material was obtained. The degree of branching for these materials was found to be close to 0.6 and independent of reaction conditions. Star-shaped and hyperbranched polyesters have also been synthesized by polycondensation of trimethylsilyl 3,5-diacetoxybenzoate (15) and a number of hyperbranched polymers based on the trimethylsilylester of )3-(4-hydroxyphenyDpropionic acid have been reported (16). 

It was,and still is, a purpose of our work to illustrate the synthetic potential of "a-b monomers" in the field of aromatic polyethers, polyesters and polyamides (concentrating on polyesters in the present contribution). The preparation of star-shaped and hyperbranched polycondensates is plagued by side-reactions resulting in crosslinks, and thus, clean step-growth processes are a basic requirement for a successfid synthesis. In this connection the potential of silicon mediated polycondensations should be explored, because polycondensations of silylated monomers may be a cleaner process than that of the corresponding nonsilylated (protonated) monomers, for instance, because proton catalyzed side reactions, such as the Fries-rear-rangement, are avoided. 

The syntheses of telechelic, star-shaped or hyperbranched oligo- and polyesters reported in this work are based on two "a-b monomers" 3-hydroxy-benzoic acid (3-Hybe) and 3-(4 -hydroxyphenyl)propionic acid (phloretic acid, Phla). These monomers were selected for the following reasons. Polyesters of 3-Hybe melt below 200°C, and they are soluble in various organic solvents (10). Polycondensations of 2-Hydroxybenzoic acid derivatives show a high tendency to yield cyclic oligomers and substituted and imsub-... 

The synthetic strategy used for Ae preparation of star-shaped polyesters with linear star arms (structures 7 and can also be applied to Ae synthesis of star-shaped polyesters with hyperbranched star arms. For this purpose silylated 3,5-bisacetoxybenzoic acid was polycondensed with the di-, tri- or tetrafunctional phenolacetates 3, 6 or 14. In all cases both viscosity and GPC measurements confirmed that the molecular weights varied with the feed ratio monomer/"star-center". In the case of structure 15 NMR spectroscopy also allowed the determination of the DP which also paralleled Ae feed ratio (Figure 7). The results obtained fi om hyperbranched polyesters of structure 15 are summarized in Table 5 (19). Unfortunately, the "star-center" 6 turned out to be unfavorable for NMR spectroscopic determination, because all its NMR signals were obscured of DP s by the signals of the 3-Hybe units and acetate endgroups. In the case of "star-center" 14 the tert.butyl groups was split of as isobutylene in the course of the polycondensation (20). 

Polycondensations of monomers 19a. 21a and 22a with the tetrafimctional "star-centers" 24, 25 or 26 yielded star-shaped and hyperbranched poly-(ester-amide)s (15). As indicated by viscosity measurements variation of the monomer/star-center ratio allowed a control of the DP. The aliphatic protons of the "star-centers" also allow a determination of the DP by means of 1H-NMR spectroscopy (Figures 14 and 15). In the case of polyesters derived from the piperazine derivative 20 the NMR spectra need to be conducted at temperatures > 80°C, because lower temperatures yield a broad CH2-signal due to cis-/trans-isomerism and slow rotation around the CO-N-bond (27). 

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