Conformationally flexible branching

The conformational flexibility and the lack of difference of the electronic properties of the polyether branches in 32 have been forwarded to explain this zero rotation. Therefore a similar dendrimer 34 has been prepared which carries a more sterically demanding branch, leading to a more rigid structure [66] interestingly, this dendrimer indeed exhibited a very small but measurable optical activity, which underlines the thesis that nanoscopic chirality depends on the rigidity of the investigated structure. 

Along another line of work in our group (S,S)-l,4-bis(dimethylamino)-2,3-dimethoxy butane (DDB), which had been used as cosolvent in asymmetric synthesis [113], was tested as a core moiety for a dendritic amine catalyst. The conformationally flexible DDB-core, which has been synthesized in five steps from diethyl tartrate was coupled with different branches to give dendritically expanded diamines 90-92 (molecular weight 3800 Da) [114] (Fig. 32). 

Examples of rod-like molecules with a branched fluorinated chain at one end (compounds 90-93) are collated in Fig. 26 [166]. It is interesting that smectic phases are retained despite the significant size of these chains. This is mainly a result of the intercalation of the aromatic cores and aliphatic spacers of these molecules, which can compensate this steric distortion. These branched chains remove the B and E phases with crystalline layers and replace them by fluid smectic phases, including SmC phases. The comparison in this figure also shows that the bulky and flexible bis(perfluoropropylene oxide) derived chains (compounds 92, 93) can provide LC materials with especially low melting points and broad mesophase ranges due to the higher conformational flexibility of perfluoroethers compared to linear perfluoroalkyl chains [99, 176]. 

The effect of structure (linear or branched, incorporation of heteroatoms, etc.) and conformational flexibility or rigidity of the fluorous ponytail on the partition coefficients has not yet been studied in detail. 

A quantitative analysis of counterion localization in a salt-free solution of star-like PEs is carried out on the basis of an exact numerical solution of the corresponding Poisson-Boltzmann (PB) problem (Sect. 5). Here, the conformational degrees of freedom of the flexible branches are accounted for within the Scheutjens-Fleer self-consistent field (SF-SCF) framework. The latter is used to prove and to quantify the applicability of the concept of colloidal charge renormalization to PE stars, that exemplify soft charged colloidal objects. The predictions of analytical and numerical SCF-PB theories are complemented by results of Monte Carlo (MC) and molecular dynamics (MD) simulations. The available experimental data on solution properties of PE star polymers are discussed in the light of theoretical predictions (Sect. 6). 

The cytotoxicity of cationic polymers is seen as a major limiting factor in their success as drug or gene delivery vectors. Mechanisms of cytotoxicity caused by polycations is not yet fully understood however, mechanisms are thought to be influenced by different properties of the polymers, such as (i) MW, (ii) charge density and type of the cationic functionalities, (iii) structure and sequence, such as block, random, linear and branched, and finally (iv) conformational flexibility. ... 

The second virial coefficient is not a universal quantity but depends on the primary chemical structure and the resulting topology of their architecture. It also depends on the conformation of the macromolecules in solution. However, once these individual (i.e., non-universal) characteristics are known, the data can be used as scaling parameters for the description of semidilute solutions. Such scaling has been very successful in the past with flexible linear chains [4, 18]. It also leads for branched macromolecules to a number of universality classes which are related to the various topological classes [9-11,19]. These conclusions will be outlined in the section on semidilute solutions. 

It is evident that due to polymeric specificity of LC polymers most of the information on their molecular parameters, i.e. molecular mass, conformational state, polymeric chain flexibility and mobility, optical anisotropy and others, may be obtained from studies of dilute solutions of these compounds. However, taking into account that this branch of polymer science has already been reviewed 134>172-176> we will here confine our treatment only to the initial steps of LC phase formation in polymer solutions. 

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