With hyperbranched star arms

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). 

The preferred initiator is stannous octoate in catalytic amount [95, 96] and polymerizations occur in bulk at 110°C or in toluene. The synthesis was expanded to stars with 40 and 48 arms by means of a hyperbranched poly(2,2 -bis(hydroxymethyl)propionicacid) or classical dendrimer, respectively [97],... 

Scheme 6.10 Synthesis of the amphiphilic star polymers with a hyperbranched core and amphiphilic diblock graft arms [59].

The branched architecture has great influence on the packing of molecular chains. In general, dendrimers have smaller hydrodynamic radius and the melt and solution viscosity of a hyperbranched polymer is expected to be lower than that of a parent linear polymer. Viscosity measurements performed with a cone viscometer confirmed the decrease of viscosity of star-shape polymers compared to the respective high molecular weight arms (polymers B-R-4 and C-R-4, Tables 1 and 2). This observation is consistent with the decrease of hydrodynamic volume observed for... 

Real-time monitoring of fusion and fission has been reported for polymer vesicles generated from an amphiphilic multiarm copolymer with a hyperbranched poly(3-ethyl-3-oxetanemethanol) core and many oligo(ethylene oxide) arms (HBPO-star-PEO) [152,153], Sonication could partly break the hydrogen bonds and give rise to molecular packing defects on the membrane, which triggers the membrane fusion. 

Hyperbranched polymers have also been used as cores for star polymers. Up to 80 arms of pBA were grown from the core of pBPEA with DP-80. Since number of functional groups is equal to the degree of polymerization, incorporation of just one unit per chain end brings the content of another comonomer to -50% [89]. Hyperbranched polymers prepared by ATRP were modified in just such a way using monomers that were non-polymerizable by ATRP but carried useful functionalities such as epoxies or alkenes, as shown in Scheme 59. 

The group of Yan developed another method for self-assembly using an amphiphilic hyperbranched multiarm copolymer.After addition to a selective solvent like acetone, macroscopic tubes with centimeter-scale length and millimeter-scale diameter were formed. The star polymer was composed of a hyperbranched poly(3-ethyl-3-oxetanemethanol) (HBPO) core and hydrophilic poly(ethylene glycol) (PEO) arms and self-assembled into a multi-walled tube. 

Here the only thing left is to find for each chain topology. Obviously, for a confined linear chain, = D while for a confined star chain with a total of / arms and a number of forwarded arms inside the pore (fin), we showed how c,star depends on / and /m in the previous section. As for hyperbranched chains, Eqs. 5.13-5.16 show that ... 

Various types of block copolymers such as ABA triblock [93,94,105,108], ABC triblock [109-111], PNIPAM-dendrimer [112], and PNIPAM-[star-shaped (or hyperbranched) polymer] [65], and PNIPAM-polyrotaxane-PNIPAM [66] have also been designed/synthesized. For example, Armes et al. have synthesized biocompatible, thermoresponsive ABC and ABA triblock copolymers such as PPO-PMPC-PNIPAM via ATRP using a PPO-based macroinitiator. McCormick et al. have reported the RAFT synthesis of thermally responsive ABC triblock copolymers incorporating M-acryl-oxysuccinimide for facile in situ formation of micelles with cross-linked shells. 

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