Star-shaped polyesters

This is potentially a method to make complicated polyester structures such as star-shaped polyesters. 

The core-first approach is based on the initiation of polymerization by a multifunctional initiator. The number of arms is then defined by the number of functional units present on the core. In order to have a good control of the molecular structure of star-shaped polyesters, the initiation must be quantitative and fast. It is also mandatory to avoid possible side-reactions between the initiating species on the core. 

Fig. 34 Usual initiators used for the s) thesis of star-shaped polyesters...

Fig. 35 Synthesis of star-shaped polyesters from a spirocyclic initiator...

Multifunctional initiators made up of metal alkoxides rather than alcohols have been less used for the synthesis of star-shaped polyesters than have the tin (II) bis-(2-ethylhexanoate)/alcohol system. Nevertheless, Kricheldorf initiated the polymerization of sCL using a spiro-cyclic tin(IV) aUcoxide to obtain a tin-containing height-shaped polyester whose final hydrolysis resulted in the formation of a star-shaped polyester (Fig. 35) [25, 159-161]. 

Aliphatic star-shaped polyesters of l-LA have been synthesized [114, 115] with multifunctional hydroxy compounds as initiators. The crystallinity of the star-shaped poly(L-LA) was found to be higher than that of the corresponding linear counterpart. Star-shaped poly(L-LA) has also been block copolymerized with trimethylene carbonate/e-CL [116] This resulted in a less brittle and considerably toughened material. 

Davaran et al. have synthesized cyclodextrin-based star-shaped polyesters bearing PLGA side chains, which are covalently attached via the hydroxyl groups in cyclodextrin. They have demonstrated that such copolymers can be used for sustained delivery of peptides such as insulin and anti-cancer drugs. 

All star-shaped polyesters described in this work were prepared in such a way that a suitable "star-center" was directly polycondensed with the di-fimctional monomers. The star-centers were selected, so that they contained isolated methyl groups which yield sharp singlet signals in the NMR spectra to allow an easy identification and quantification (analogous to bisphenol-P). 

A second series of star-shaped polyesters (10) was prepared by polyconden-... 

Table 3 Yields and properties of the star-shaped polyesters 7 prepared from silylated 3-acetoxybenzoic acid and "star-center" 6 inbulkat270°C/5h...

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). 

A change of architecture is another route that enables diversification of the properties of aliphatic polyesters. This review will focus on star-shaped, graft, macrocyclic, and crosslinked aliphatic polyesters. It must be noted that the ROP of lactones has been combined with several other polymerization mechanisms such as ROP of other heterocyclic monomers, ionic polymerization, ROMP, and radical polymerization. Nevertheless, this review will not cover these examples and will focus on polymers exclusively made up of poly(lactone)s. 

Finally, Lecomte and coworkers reported the synthesis of mikto-arm star-shaped aliphatic polyesters by implementing a strategy based on click chemistry (Fig. 36) [162]. Firstly, the polymerization of sCL was initiated by a diol bearing an alkyne function. The chain-ends were protected from any further undesired reaction by the esterification reaction with acetyl chloride. The alkyne was then reacted with 3-azidopropan-l-ol. The hydroxyl function located at the middle of the chain was then used to initiate the ROP of sCL and y-bromo-s-caprolactone. Finally, pendant bromides were reacted successfully with sodium azide and then with N, N-dimethylprop-2-yn-l-amine to obtain pendant amines. Under acidic conditions, pendant amines were protonated and the polymer turned out to exhibit amphiphilic properties. 

Several review articles on biodegradable polymers and polyesters have appeared in the literature [12-22]. Extensive studies have been carried out by Al-bertsson and coworkers developing biodegradable polymers such as polyesters, polyanhydrides, polycarbonates, etc., and relating the structure and properties of aliphatic polyesters prepared by ROP and polycondensation techniques. In the present paper, the current status of aliphatic polyesters and copolyesters (block, random, and star-shaped), their synthesis and characterization, properties, degradation, and applications are described. Emphasis is placed primarily on aliphatic polyesters derived by condensation of diols with dicarboxylic acids (or their derivatives) or by the ROP of cyclic monoesters. Polyesters derived from cyclic diesters or microbial polyesters are beyond the scope of this review. 

Biela T, Duda A, Pash H, Rode K (2005) Star-shaped poly(L-lactide)s with variable numbers of hydroxyl groups at polyester arms chain-ends and directly attached to the star-shaped core -controlled synthesis and characterization. J Polym Sci Part A Polym Chem 43 6116-6133... 

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. 

MALDI can be used to follow dendrimer growth. Some papers report the characterization of dendrimers grown on a star-shaped nucleus made of and of some aromatic polyester dendrimers. Figure 10.37 shows the MALDI-TOF mass spectra of a dendrimer which grows on a star-shaped nucleus made of PEG, at various stages of its growth. Initially, the molar mass is around 25000, then it changes to 28000 and to 33000, and eventually it reaches 39000. 

ROP can also be used to generate star-shaped block copolymers in a combinatorial manner.Meier et al7 employed a five-arm star-shaped PEG compound as a macroinitiator for the ROP of 8-caprolactone monomer. Using this strategy, the PEG compound formed the core of the star while the outer shell was comprised of polyesters. Polymerizations were performed on a Ghemspeed ASW2000 by mixing monomer and initiator at different ratios and heating to 130 °C. Polymerizations were demonstrated to be reproducible by the low variation in measured MW values in iterative experiments. Moreover, a linear relationship was observed between MWs (calculated by both GPC and NMR) and different [M]/[I]... 

In 1997, Jeong and Kim et al. reported biodegradable IP systems using a triblock copolymer of PEG and PLLA, PEG-h-PLLA-h-PEG [32]. After this achievement, many block copolymers with combinations of PEGs and aliphatic polyesters were reported [45-47] with various molecular architectures. Linear block, star-shaped block, and graft topologies... 

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). 

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