Four-arms star

Description Linear Four-armed star Six-armed star... 

Figure 5.15 illustrates the two general types of crosslink. In Fig. 5.15 a), a short crosslink connects adjacent chains to form a four-armed star , the arms of which are normally unequal in length. In Fig. 5.15 b), a longer crosslink connects two adjacent molecules. When... 

The core first method has been applied to prepare four-arm star PMMA. In this case selective degradation of the core allowed unambiguous proof of the star structure. However, the MWD is a little too large to claim that only four-arm star polymers are present [81], Comb PMMAs with randomly placed branches have been prepared by anionic copolymerization of MMA and monodisperse PMMA macromonomers [82], A thorough dilute solution characterization revealed monodisperse samples with 2 to 13 branches. A certain polydispersity of the number of branches has to be expected. This was not detected because the branch length was very short relative to the length of the backbone [83]. Recently, PMMA stars (with 6 and 12 arms) have been prepared from dendritic... 

A four-armed star polymer consisting of silane substituted with different block polymer chains ... 

Although the core-first method is the simplest, success depends on initiator preparation and quantitative initiation under living conditions. This method is of limited use in anionic polymerization because of the generally poor solubility of multifunctional initiators in hydrocarbon solvents [12]. Solubilities of multifunctional initiators are less of an issue in cationic polymerizations, and tri- and tetrafunctional initiators have been used to prepare well-defined three- and four-arm star polymers by this method [7] Except for two reports on the synthesis of hexa-arm polystyrene [27] and hexa-arm polyoxazoHne [26], there is a dearth of information in regard to well-defined multifunctional initiators for the preparation of higher functionality stars. 

Poly[(A)-s-(A )-s-(B)] three-arm star-block copolymer Poly[(A)2-s-(B)] three-arm star-block copolymer Poly[(A)2-s-(B)2] four-arm star-block copolymer Poly[(A)-b-(B)-b-(C)] linear triblock copolymer Poly[(A)-s-(B)-s-(C)] three-arm star-block copolymer N,N- Dimethylacetamide Anionic ring-opening polymerization... 

Fig. 7. Schematic representation of (a) a four-armed star-shaped homopolymer and (b) a star-shaped block copolymer...

Four arm star PIB has been prepared by living polymerization with the 3,3f,5,5f-tetra(2-acetoxy-isopropyl)biphenyl (TCumOAc, 18)/BC13 initiating system in dilute solutions in the -35 to -80 °C range [35]. 

A new class of asymmetric stars, the so-called inverse star block copolymers, were recently reported by Tselikas et al. [62]. These polymers are four-arm stars,... 

Fig. 2. Four-arm star branched copolymer with three outer A blocks and two inner B blocks...

Well-ordered, raspberry-like assemblies of cholesterol-bearing PuL (CHP) particles were prepared by Michael addition of acrylate-group-modified CHP nanogels with four-arm star-shaped PEG bearing thiol groups at the terminus (PEGSH Fig. 3) [79],... 

Exponential dependence of ij0 on Ma, as discussed earlier for star polymers, was also seen in the H-polymers. The crossover where rj0 of polystyrene H-polymers surpasses that of its linear homologues occurs at Mw 600000g/mol. This value is significantly lower than that observed for four-arm star polystyrenes [35]. Furthermore, the average entanglement of each of the five subchains of the H-polymers at this crossover is Ma 7Me, a value far lower than the level of arm entanglement required to reach the crossover in polystyrene star polymers. 

Roovers [34] demonstrated that the added branch point found in an H-polymer significantly alters its relaxation behavior vis-a-vis a star polymer. He showed that the value of v [Equation (5)] was approximately twice that of four-arm star polymers. The time-scales for relaxation of the free-end subchains of H-polymers are very different to the relaxation time-scale associated with the cross-bar subchain that exists between the two branch points of the H-polymer. The free-end subchains of H-polymers relax in a manner nearly identical with that of the arms of a star polymer. However, the cross-bar ... 

Three- and four-arm star-shaped PEA (75 and 77) were synthesized by GTP with tri-and tetrafunctional initiators (74 and 76) according to equations 61 and 62 . ... 

Wnek and coworkers synthesized a four-arm star-shaped PMMA with a cyclic siloxane core, as shown in equation 63. Four ketene silylacetal units were first attached to 1,2,3,4-tetramethylcyclotetrasiloxane (78) to obtain the tetrafunctional core 79, which was used as an initiator of the GTP of MMA with formation of the targeted four-armed star-shaped PMMA (80). 

Chemicals. The four-armed star St-Bd block copolymers with different Bd compositions were synthesized in our laboratories at PSS (Mainz, Germany) by anionic polymerization according to standard procedures (25, 26) modified to give samples with well-known structure and molar mass control. 

Eroducts have the same chemical composition (St-Bd ratio) [A, styrene B, utadiene-1,3 linear (M) + ext. linear (2M) + three-arm star (3M) + four-arm star (4M)] ... 

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