Dendrons side chains

S. Setayesh, A.C. Grimsdale, T. Weil, V. Enkelmann, K. Mullen, F. Meghdadi, E.J.W. List, and G. Leising, Polyfluorenes with polyphenylene dendron side chains toward non-aggregating, light-emitting polymers, J. Am. Chem. Soc., 123 946-953, 2001. 

A. Pogantsch, F.P. Wenzl, E.J.W. List, G. Leising, A.C. Grimsdale, and K. Mullen, Polyfluorenes with dendron side chains as the active materials for polymer light-emitting devices, Adv. Mater., 14 1061-1064, 2002. 

Dendrons attached as side chains on linear polymer chains behave different from free dendrimers and dendrons. Block copolymers, poly(3,5-bis(3,5-bis (benzyloxy)benzyloxy)-benzyl methacrylate-random-methacrylic acid)-block-poly(2-perfluorooctylethyl acrylate), possess poly(benzylether) dendrons and perfluorinated alkyl chains in their side chains (Fig. 4) [85], While an LB film of a copolymer with a medium substitution fraction of poly(benzylether) dendron side chain in poly(methacrylic acid) displays flat surface, a copolymer with high fraction of poly(benzylether) dendron side chains produces the zone texture. Dendron rich blocks are hydrophobic and oleophilic but perfluorinated blocks are solvophobic. Therefore, in this case, the solvophobicity-to-solvophilicity balance must be considered. As a result, copolymers with medium fraction of dendron are laid on solid substrate, but dendron blocks of copolymers with high fraction prefer to arrange at air side of air/ water interface and the fluorocarbon blocks are enforced to exist close to water subphase, resulting in the zone texture [86]. These situations of molecular arrangements at air/water interface are kept even after transfer on solid substrate. By contrast, when perfluorooctadecanoic acids are mixed with block copolymers with high dendron fraction, the flat monolayers are visualized as terrace [87], The monolayers are hierarchized into carboxyl, per-fluoroalkyl, and dendron layers, that is, hydrophilic, solvophobic, and oleophilic layers. In this case, perfluorooctadecanoic acids play a role for ordering of block copolymers. 

Preparation of polymers with dendron side-chains... 

Consequently, cylindrical shape is also expected for polymers with large dendron side chains. Of course, biomolecules such as DNA or RNA with rigid cylindrical or wormlike shapes are well known, although in these cases the super-molecular structure (i.e., the secondary and tertiary structure) is the result of well-controlled secondary bonds. 

In contrast the materials 47 with di-ferf-butylphenyl and 48 with first generation dendron side-chains were processable from toluene [73]. The degrees of polymerisation for these polymers were rather low, with the den-dronized polymer 48 having a Mn of only lOOOOg/mol (n = 10) probably due to the bulkiness of the substituents. Polymer 48 was also found to be exceptionally thermally stable, with no significant mass loss being observed in thermal gravimetric analysis (TGA) until 570 °C (cf. 463 °C for 89 [73]). 

Setayesh, S., et al. 2001. Polyfluorenes with polyphenylene dendron side chains Toward nonaggregating, light-emitting polymers. J Am Chem Soc 123 946. 

Figure 24 Schematic representation of the noncovalent assembly of dendronized side-chain functional polymers. Reproduced from ten Cate, A. T. Kooijman, H. Spek, A. L. etal. J. Am. Chem. Soc. 2004, 126,3801-3808. ...

The adsorption of surfactant polymers onto hydrophobic substrates are found to be influenced by the hexyl side chains, while a protective antifouling layer is formed by the maltose dendron side chains inhibiting platelet adhesion. 

Jen a al. have developed dendronized polymeric NEO materials that have shown significantly improved poling efficiency by encapsulating chromophore with dendritic substituents that can electronically shield the core, ii-electrons, and form spherical molecular shapes." "" " Figure 6 illustrates different molecular architectures of dendronized side-chain NLO polymers with crosslinkers. The diverse selections of molecular architectures provide additional flexibility in the molecular engineering of high-performance polymeric NLO materials. Moreover, the unique nanoscale environment created by the shape and size, dielectric properties, and distribution of chromophores in crosslinkable polymers with dendrons and dendrimers can all play critical roles in maximizing the macroscopic EO properties of polymeric NEO materials. 

NLO chromophore 1=1 Crosslinkable moiety Figure 6 Molecular architectures of dendronized side-chain NLO polymers with crosslinkers. 

Figure 10 Molecular structures of guest/host polymer 27, conventional side-chain NLO polymer 28, and dendronized side-chain NLO polymer 29.

The PPhE bearing the PBE dendron as the repeating side chains is also soluble in THF, whereas the rigid main chain itself does not dissolve in any solvent. The blue-luminescence dendron-grafted rigid polymer forms thin films by spin coating [18]. 

Dendronized polymers are a subclass of comb polymers [7]. The synthetic access to comb polymers with dendrons in the side chain is somewhat more difficult and time consuming compared to most other polymers of this class. It is therefore not too far fetched to ask whether the exciting properties described above... 

The authors refer to compound 9 as a monomer with tapered side chains and have only recently begun to use dendrimer terminology. Since the side chain meets the criteria for a G-l dendron, compound 9 is considered here. 

SIDE-CHAIN FUNCTIONALIZED OR DENDRONIZED COPOLYMER HYBRIDS... 

Another type of architecture featuring a linear main chain surrounded by dendritic side-chains has emerged over the last decade [4], The highly descriptive term dendronized , coined by Schliiter [4] aptly describes this novel type of macromolecular architecture. Though three separate routes can be used to prepare such dendronized hybrids (Figure 7.8), the most successful approach to date has generally involved the polymerization of dendronized monomers. 

The concept of making brush-type polymers in which a linear polymer is funtionalized with dendritic side-chains was first suggested by Tomalia in a 1987 patent, though actual experimental work on his approach was only reported recently recently [15]. Hawker and Frechet were first to document the preparation of a vinyl copolymer containing a few pendant Frechet-type dendrons (Figure 7.9). 

Other efforts based on the macromonomer approach to homopolymers having dendritic side chains, include the work of Draheim and Ritter on acrylate and methacrylate derived structures having dendritic chiral side chains based on L-aspartic esters [17a], and of Xi and coworkers with poly(methacrylate) structures containing very small benzyl ether dendritic side-chains [17b]. Unfortunately, both of these approaches met with limited success due to a significant drop in degree of polymerization (DP) when the size of the dendron used as pendant group in the macromonomers increased from G-l to G-2. 

In one of several important studies on dendronized polymers [4c, 4d]. Schluter and coworkers explored the stiffening of polystyrene chains through the incorporation of Frechet-type dendrons as side chains [28, 29]. While the G-l and G-2 dendrons were not sufficiently bulky to effectively stiffen the polystyrene chain, the G-3 dendron provides enough steric bulk to force the hybrid polymer into adopting a cylindrical shape in solution [28b], In a complementary study, Neubert and Schluter demonstrated that adding charges to the dendritic wedges leads to an expansion of the chains of the hybrid copolymer in aqueous solution [29],... 

If the backbone as well as the side chains consist of flexible units, the molecular conformation arises out of the competition of the entropic elasticity of the confined side chains and the backbone [ 153 -155]. In this case, coiling of the side chains can occur only at the expense of the stretching of the backbone. In addition to the excluded volume effects, short range enthalpic interactions may become important. This is particularly the case for densely substituted monoden-dron jacketed polymers, where the molecular conformation can be controlled by the optimum assembly of the dendrons [22-26,156]. If the brush contains io-nizable groups, the conformation and flexibility may be additionally affected by Coulomb forces depending on the ionic strength of the solvent [79,80]. 

These observations indicated that 1) the dendrons adopt a stable folded state in aqueous media, 2) the terminal chains experience a shift in equUibrium toward the lower energy gauche conformational state (as compared to the pentaethylene glycol side chain), and 3) the dendron secondary structure and the terminal chain conformations are strongly correlated. 

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