Star-P

In another series of experiments [149] the correlation between structure and dynamics was investigated on dilute solutions of 12-arm PS star systems (Mw = 14.9 104 g/mol) in d-tetrahydrofurane, where either only one or all 12 arms were protonated (labelled). 

PS-b-PEO) , n = 3, 4 star-block copolymers were synthesized by ATRP and anionic polymerization techniques [149]. Three- or four-arm PS stars were prepared using tri- or tetrafunctional benzylbromide initiators in the presence of CuBr/bipy. The polymerization was conducted in bulk at 110 °C. The end bromine groups were reacted with ethanolamine in order to generate the PS stars with hydroxyl end groups. These functions were then activated by DPMK to promote the polymerization of ethylene oxide and afford the desired well-defined products (Scheme 73). 

Anionic polymerization and suitable Unking chemistry were employed for the synthesis of 3-arm PCHD-fc-PS star-block copolymers with PCHD either as the inner or the outer block (Scheme 77) [153]. The block copolymers were prepared by sequential addition of monomers. It was shown that the crossover reaction of either PSIi or PCHDLi was efficient and led to well-defined block copolymers. However, in the case of the PCHD-fc-PSLi copolymers, longer polymerization times were needed for long PCHD... 

In the strong segregation phase boundaries were calculated for xN 100 and compared to the experimental results for Pl-arm-PS stars. The results are shown in Fig. 36. 

Fig. 16. Molar mass dependencies of the intrinsic viscosity [rf] for the same samples as shown in Fig. 15 (end-linked PS-stars [94] and randomly crosslinked polyesters [92,93,95]...

Fig. 23. The intrinsic viscosity of several end-linked PS star molecules as a function ofM [95]. In the limit of low and high molar masses asymptotic power law behavior may be derived. That at low molar masses is widely controlled by the presence of non-reacted star molecules, that at high molar masses is expected from theory for randomly branched macromolecules. The exponents of the two asymptotic lines are a =0.49 0.08 for M <0.8x10 g/mol and a =0.18 0.05 for M >2.0xl0 g/mol. Reprinted with permission from [95]. Copyright [1997] American Society...

Fig. 26. Molar mass dependence of the g factor for three pregel and one postgel fraction of end linked PS stars. A good fit was obtained with the Zimm Stockmayer equation (Eq. 69) and an exponent in Eq. (70) of fi 0.63 [95] which agrees well with Kurata s estimation with b-0.6 [129]. Reprinted with permission from [129]. Copyright [1972] American Society...

Asymmetric PS stars of the type (PSA)n(PSB)n were also prepared by the divinyl-benzene (DVB) method [9]. Living PS chains, prepared by sec-BuLi initiation, were reacted with a small amount of DVB producing star homopolymers. The DVB core of the stars contains active anions which, if no accidental deactivation occurs, are equal to the number of the arms that have been linked to this core. These active sites are available for the polymerization of an additional quantity of monomer. Consequently further addition of styrene produced asymmetric star polymers... 

Flolly MK, Dear JW, Flu X, Schechter AN, Gladwin MT, Flewitt SM, Yuen PS, Star RA Biomarker and drug-target discovery using proteomics in a new rat model of sepsis-induced acute renal failure. Kidney International 70 496-506, 2006... 

Four examples of polymer characterization by SEC will be discussed a dextran sample with branching transitions, a pair of brominated polystyrene (PS) samples, aggregation in chitosan, and a PS star polymer. SEC numerical results for dextran, chitosan, and star-branced PS are listed in the corresponding figure captions. 

Fig. 5 The Vise traces (upper chromatogram) and the LS traces (lower chromatogram) are overlayed for a linear and a star-branched polymer with the same M. Star-branching creates a denser polymer with lower [r/] and a smaller Rg. Linear PS = 100,000, [r/] = 0.495 dL/g, Rg = 14 nm. PS-star =...

Several star polymers have been prepared by reacting living polymers with DVB. The method has been applied in the past for the synthesis of PS48-49 and polydiene50 stars. Rather narrow molecular weight distribution PS stars were obtained when the [DVB] [PSLi] ratio was varied from 5.5 to 30 and the corresponding functionality was between 13 and 39. Similar behavior was obtained for polydiene stars when the [DVB] [PDLi] ratio was from 5 to 6.5 and the functionality of the star was varied between 9 and 13. In other cases, broad distributions were observed, caused by the large distribution of the functionalities of the stars prepared by this method. 

DMAPLi was also used by Burchard to synthesize three-arm PS stars (3N-PS) having dimethylamine end groups.72 In this case 1,3,5-triallyloxy-2,4,6-triazine was the linking agent, as shown in Scheme 18. 

The method was first applied to the synthesis of asymmetric PS stars having two arms of equal molecular weights (PSb) and a third one (PSa) having molecular weight either half or twice that of the identical arms.75 The procedure, given in Scheme 22,... 

Taking into account the above observation, a three-arm asymmetric PS star was successfully prepared81... 

Hexaepoxy squalene, HES (Scheme 70) was used as a multifunctional initiator in the presence of TiCU as a coinitiator, di-t-butylpyridine as a proton trap, and N,N-dimethylacetamide as an electron pair donor in methylcy-clohexane/methyl chloride solvent mixtures at - 80 °C for the synthesis of (PIB-fo-PS) star-block copolymers [145]. IB was polymerized first followed by the addition of styrene. The efficiency and the functionality of the initiator were greatly influenced by both the HES/IB ratio and the concentration of TiCU, thus indicating that all epoxy initiation sites were not equivalent for polymerization. Depending on the reaction conditions stars with 3 to 10 arms were synthesized. The molecular weight distribution of the initial PIB stars was fairly narrow (Mw/Mn < 1.2), but it was sufficiently increased after the polymerization of styrene (1.32 < Mw/Mn < 1.88). 

The chain ends of the star polymers can be functionalized. So it is possible to link these sites and form networks or gels of stars. Depending on the chemical nature of the chain end, the crosslinking can be permanent or stimuli-switchable. Patrick-ios et al. [105] have prepared crosslinked double hydrophilic star copolymers made from methacrylic acid (MAA) and poly(ethyleneglycol methacrylate) (PEGMA). Meyer et al. [106] synthesized photocrosslinkable PMAA stars, which could serve as well-defined model polyelectrolyte networks. The polyelectrolyte stars can also be used as template for some nanomaterials. Want et al. [107] prepared strong anionic polyelectrolyte stars by sulfonation of the PS stars. They were used as the template for the preparation of conducting polyaniline. 

Tri- and tetrabromoaryl compounds were lithiated by reaction with sec-butyllithium and then were used as multifunctional initiators to synthesize monodisperse 3- and 4-arm PS stars, respectively (Scheme 3). Tetramethylethylenediamine (TMEDA) was added to improve the solubility of the tetrafunctional initiator in benzene. 

Diphenylethylene derivatives form another dass of linking agents. l,3,5-Tris(l-phenylethenyl)benzene was used for the synthesis of a well-defined 3-arm PS star (Scheme 10),... 

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