Star polymers from dendritic cores

Fig. 1 Dendritic core-shell star polymers, (a) Dendritic core-shell star polymers of different architectures with varying densities of the polycationic polymer chains grafted from the dendrimer core as well as varying numbers of positive charges along the individual polymer chains, (b) Cellular uptake of selected macromolecules by ECV-304. The bars represent relative fluorescence units (RFU) measmed in individual cells. Data represent mean values ( SEM after an incubation periods of 6 and 24 h. (c) Cell uptake of polycationic core-shell dendrimers into ECV-304 cells after 15 min. ECV-304 were stained using a green fluorescence cell tracker, whereas the core-shell macromolecules are shown with a red color originating from the PDI core...

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 commercially available hyper-branched polyester derived from bis-MPA was used as the multifunctional initiating core for the ROP of e-CL and this led to the synthesis of hybrid dendritic linear star polymers. The reactivities of the chain-end hydroxymethyl groups in the dendrimer were significantly greater than in the isomeric hyper-branched case. 

Despite the drawbacks of this method, it has been used to prepare a tremendous number of polypeptide hybrid block copolymers (Table 1), and when carefully executed provides reasonably well-defined samples. Synthetic polymer domains have been prepared by addition polymerization of conventional vinyl monomers, such as styrene and butadiene, as well as by ringopening polymerization in the cases of ethylene oxide and e-caprolactone. The generality of this approach allows NCA polymerization off of virtually any primary amine functionality, which was exploited in the preparation of star block copolymers by polymerization of sarcosine NCA from an amine-terminated trimethyleneimine dendritic core [37]. In most examples, the polypeptide domain was based on derivatives of either lysine or glutamate, since these form a-helical polypeptides with good solubility characteristics. These residues are also desirable since, when deprotected, they give polypep-... 

Fig. 5.20. Some simple topologies of polymer liquid crystals (PLCs) derived from calamltic and discotic mesogenic cores main-chain PLCs, copolymers with core and flexible spacer alternating side-chain PLCs with mesogenic cores attached by flexible spacers to the main chain of a conventional polymer and dendritic (or star-shaped) stmctures with mesogens emanating from a central core via flexible spacers.

Figure 35).Another alternative to dendritic monomers involves the preparation of a dendritic core terminated in olefins which are available for metathesis with a catalyst s alkylidene such that the catalyst becomes incorporated at the ends of the dendrimer. From there, ROMP of monomers such as norbomene elongates the dendritic ends into linear arms to yield a star polymer with a dendritic core (Figure 36). Interestingly, prior to ROMP, the catalysts attached to the dendrimer reversibly dimerized at low temperatures and high concentrations. 

A star polymer may also comprise an unknown number of arms connected to the center of the cluster by addition to the reaction mass of a second, multifunctional monomer or SRU that ties all the linear growing polymer chains together into a crosslinked (microgel) core. These are sometimes called star-branch polymers. Cloutet et al., however, describe a star-branched polymer as a dendritic block polymer . A third type of star polymer comprises a dendritic core from which emanate a known number of arms. In any of these types of star polymers, the arms may be all the same, or different arms may have different compositions the latter are known variously as hetero-star, miktoarm star, or variegated star polymers. ... 

One of the primary differences between the star-branched polymers described previously and dendritic or hyperbranched polymers is their chain segment density distribution. For star-branched polymers, the chain segment density is highest at the core and decreases as the distance from the core increases. In contrast, for dendritic and hyperbranched polymers the chain segment density increases as the distance from the core increases (with increasing generations... 

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