Polystyrene-dendrimer block copolymers

Yoo, H.-S., Watanabe, T., and Hirao, A. (2009) Precise synthesis of dendrimer-like star-branched polystyrenes and block copolymers composed of polystyrene and poly(methyl methacrylate) segments by an iterative methodology using hving anionic polymerization. Macromolecules, 42,4558 570. 

The structures formed by polystyrene-poly(propylene imine) dendrimers have also been analyzed. Block copolymers with 8, 16, and 32 end-standing amines are soluble in water. They have a critical micelle concentration of the order of 10"7 mol/1. At 3x10 4 mol/l they form different types of micelles. The den-drimer with eight amine groups (80% PS) form bilayers. The dendrimer with 16 amine groups (65% PS) forms cylinders and the dendrimer with 32 amine groups (50% PS) forms spherical micelles [38,130,131]. These are the classical lamellar, cylindrical, and spherical phases of block copolymers. However, the boundary between the phases occurs at very different volume fractions, due to the very different packing requirements of the linear polymer and spherical dendrimer at the interphase. 

For polymer chemists it is interesting to know how well-known linear polymers can be linked with dendritic architectures and what the supramolecular consequences of this approach might be. Combination of dendrimers with linear polymers in hybrid linear-dendritic block copolymers has been employed to achieve particular self-assembly effects. Block copolymers with a linear polyethylene oxide block and dendritic polybenzylether block form large micellar structures in solution that depend on the size (i.e., the generation) of the dendritic block [10]. Amphiphilic block copolymers have been prepared by the combination of a linear, apolar polystyrene chain with a polar, hydrophilic poly(propylene imine) dendrimer [11] as well as PEO with Boc-substituted poly-a, -L-lysine dendrimers, respectively [12]. Such block copolymers form large spherical and cylindrical micelles in solution and have been described as superamphi-philes and hydra-amphiphiles , respectively. 

For a general review on supramolecular chemistry with dendrimers, the reader is referred to an excellent paper of Zimmerman et al. [18]. Because of our acquaintance with the polylpropylene imine) dendrimers, we will restrict ourselves in this Chapter to some examples of supramolecular behaviour of these systems as investigated in our laboratory. Three systems will be discussed (Figure 1) the dendritic box, which can encapsulate guest molecules, the polystyrene-poly(propy-lene imine) block copolymer superamphiphiles, and alkyl-decorated dendrimers, which function as unimolecular micelles, and show surprising aggregation behaviour. However, first the synthesis and properties of the poly(propylene imine) dendrimers will be discussed to demonstrate some typical dendrimer features. 

To obtain polystyrene-poly(propylene imine) block copolymers, the divergent dendrimer synthesis as described in 2.1 was performed onto the PS-CH2-O-CH2-CH2-CH2-NH2 core molecule, with M = 3.2kgmor (based on GPC of PS-CH2-OH). 

The MALDI-TOF spectrum of [G-3] poly(benzyl ether) dendrimer-po-ly(ethylene glycol) triblock copolymer shows a broad band of peaks between 4300 and 6100 D with resolution of the individual ethyleneoxide (44 D) units. The MALDI-TOF spectrum of a [G-3] dendrimer with two polystyrene blocks (molecular peak=8073 D) shows material with 6000-11,000 D and a broad band corresponding to material with 2 M+Ag+. SEC can be used to prove that the latter species is indeed an artifact of the mass spectroscopic method. The authors claim almost exact agreement between the polydispersities derived from MALDI-TOF and SEC [40]. This does, however, not leave any room for the unavoidable column spreading in the latter method. Furthermore, anionically prepared low MW polymers have a minimum polydispersity given by (1 + 1/DP) [41]. 

Gitsov, I. Frechet, J.M.J. Novel nanoscopic architectures. Linear-hlobular ABA copolymers with polyether dendrimers as A blocks and polystyrene as B block. Macromolecules 1994,27 (25), 7309. 

---