Dendrons polymer chains

Scheme 5. Structural and chronological overview of the macromonomers used for synthesis of dendronized polymers chain growth (a) and step growth polymerizations (b)...

Figure 2. Dependence of self-as.sembly of dendronized polymer chains on the degree of polymerization monomers and short polymer chains assemble in spheres longer chains assemble in cylinders. This effect leads to rapid polymerization of such dendron monomers due to self-encapsulation [21],...

Figure 37.11 Cartoon representation of the opening and closing of a valve by contraction and relaxation movement of the dendronized polymer chain (red).

The main chain of dendronized polymers, due to die large size of the mon-odendrons, is usually forced to take a stretched shape thus the whole molecule exists as a rigid rod architecture both in solution and in the solid state.32d Depending on the backbone stiffness, the degree of monodendron coverage, and the size of die monodendron, the architecture of these macromolecules is no longer a sphere but a cylinder this dictates die properties of the dendronized polymers. 

The problems associated with route B also have something to do with steric hindrance. Here the critical point is the steric demand of both monomer and chain end. Incoming monomer will only be connected to the chain end, if steric hindrance is not too high. Otherwise this process will be slowed down or even rendered impossible. Depending on the kind of polyreaction applied, this may lead to termination of the reactive chain end and/or to side reactions of the monomer, like loss of coupling functionality as in some polycondensations or auto-initiation specifically in radical polymerizations. From this discussion it can be extracted that the basic problems for both routes are incomplete coverage (route A) and low molecular weight dendronized polymer (route B). 

In order to estimate whether the true molar masses of the dendronized polymers are larger or smaller than the GPC molar masses reported in Table 1, it is useful to recall that the separation in a GPC experiment is based on the hydro-dynamic volume (Vn) of macromolecules. For flexible coils, Vn depends on the chain length (i. e. contour length) L = MxbxMo1 according to... 

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... 

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. 

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],... 

In situ growth via covalent binding of a hybridizing component to a nanocarbon can be achieved in the case of polymers, dendrons and various other macromolecules which are synthesized in a stepwise manner. The in situ synthesis of such macromolecules potentially increases binding site density while steric effects of the nanocarbon can lead to increased variation in average polymer chain length (polydispersity) [101 103]. 

X-ray reflectivity studies and II-A isotherms revealed a similar model in which the dendrons project the hydrophUic crowns into the water phase and the hydrophobic chains perpendicular to the interface. Amphiphihc dendronized polymers that adopt a cylindrical, rodlike morphology, projecting hydrophihc and hydrophobic dendrons on opposite faces of the stmcture, form stable monolayers exhibiting collapse pressures of approximately 20mN/m (Fig. 11.41). 

The II-A isotherms suggest an orientation that aligns the long axis of the polymer rod in the plane of the interface with the EO chains of the hydrophilic dendrons extended into the water layer and the hydrophobic dendrons pointed upward from the surface. The stability of the dendronized polymers likely arises from the balance between the polar and nonpolar regions exposed at the surface of the polymeric cylinder (Bo et al. 1999 Schluter and Rabe 2000). Ariga and colleagues (2004) constmcted a series of spider web dendritic amphiphiles that project hydrophilic Lys-Lys-Glu tripeptides and hydrophobic chains at each generational level (Fig. 11.42). 

Dendronized polymers are a class of polymers produced by the combination of linear polymers and dendritic molecules as side chain pendant moieties [67-69],... 

Another property of this class of polymers is that the combination of specific dendrons with linear chains provides an opportunity to design a well-defined amphiphilic dendronized polymer system, which can bring about supramolecular aggregates in an aqueous phase [70,71],... 

Several work are concerned with the synthesis, characterization, dielectric behavior and conformational analysis of dendronized Polymers. Poly(methacrylates) containing phtalimidoalkyl moieties in the side chain have been recently studied i.e. poly(3,5-diphtalimido alkylphenyl methacrylate)s with ethyl (P-EthylGi), propyl (P-PropylGi) and butyl (P-ButylGi) chains as spacer groups. Where Gi indicates first generation [112],... 

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. 

Dendrimers usually exhibit spherical (isotropic) shape. However, wedge-like dendrimer fragments ( dendrons ) that have been attached to linear polymers as side groups can be used to create anisotropic nanocylinders , leading to uncoiling and extension of the polymer chains. Synthetic macromolecules of this type can be visualized directly on surfaces and their contour length determined from the images. Unexpected acceleration effects in the self-encapsulated polymerization of dendron monomers used to prepare such polymers as well as the structural consequences of dendritic pieces of cake on linear polymer chains are discussed. 

Preparation of polymers with dendron side-chains... 

The idea to string dendron pieces of cake to a polymer chain as a core, was mentioned by Tomalia and Kirchhoff [16a] in a patent as early as 1987 ( comb-burst polymers). In a recent paper, the results of this work based on strategy (i) have been published in more detail. A poly-... 

In contrast, recent kinetic investigation of the polymerization of spacerless G2 dendron-substi-tuted styrene and methylmethacrylate, respectively, in solution lead to the unexpected conclusion that above a certain critical monomer concentration a strong increase in the rate of the free radical polymerization is observed [21]. The results can be explained by self-organization of the growing polymer chain to a spherical or columnar superstructure in solution, depending on the degree of polymerization (DP, Fig. 2). The rate constants and low initiator efficiency lead one to conclude that the self-assembled... 

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 summary, it is obvious that the attachment of dendron building blocks to common monomers leads to dramatic kinetic and structural consequences. Once more, it should be kept in mind that it is only the shape of the side groups attached that governs the conformation and structure of the resulting polymer chains, not hydrogen-bonding interactions, which are ubiquitous in self-assembly in biological processes. 

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]). 

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