Comb architecture

A few other methods have been used to prepare polypeptide hybrid copolymers. Inoue polymerized Bn-Glu NCA off of amine-functionalized styrene derivatives, and then copolymerized these end-functionalized polypeptides with either styrene or methyl methacrylate using free radical initiators to yield hybrid comb architecture copolymers [38]. Although unreacted polypeptide was identified and removed by fractionation, copolymers were obtained with polypeptide content that increased with feed ratio. There was no mention if the polypeptide interfered with the radical chemistry. In similar work, Imanishi and coworkers converted the amine-ends of polypeptides to haloacetyl groups that were used to initiate the free radical polymerization of either styrene or methylmethacrylate to yield hybrid block copolymers [39]. Studies using CPC showed that the crude product contained mixtures of copolymers and homopolymers, and so removal of the homopolymers by extraction was necessary. 

The transition from hybrid block copolymers with a homopeptide block toward those with sequence-defined peptide segments enriches the self-assembly behavior strongly. Ayers et smdied water-soluble block copolymers composed of a PEO block and a peptide block with comb architecture, which was obtained by the controlled polymerization of oligopeptide... 

In AEM images prepared from more concentrated PS comb solutions, a succession of continuous objects forming a molecular monolayer is observed (Viville et al, 2001 Viville et ah, 2000). Discemable interruptions between macromolecules indicate very little overlapping and interdiffusion of PS branches, in agreement with the highly compact comb architecture. 

ZenteP has drawn attention to the possibility of creating piezoelectric elastomers by the combination of ferroelectric comb architecture (Section 7.6.1.3) within a cross-linked system. The ferroelectric polarization of such a material should be modified by the application of mechanical force to which the macroscopic ordering is vulnerable. That would offer the possibility of transforming a mechanical signal into an electrical response, and the elastomer would behave as a piezoelement. 

As discussed in Section 7.3, conventional free radical polymerization is a widely used technique that is relatively easy to employ. However, it does have its limitations. It is often difficult to obtain predetermined polymer architectures with precise and narrow molecular weight distributions. Transition metal-mediated living radical polymerization is a recently developed method that has been developed to overcome these limitations [53, 54]. It permits the synthesis of polymers with varied architectures (for example, blocks, stars, and combs) and with predetermined end groups (e.g., rotaxanes, biomolecules, and dyes). 

Stars, combs with three-functional branching points along a locally rigid backbone, and planar surface-brushes can also be considered as assemblies of linear chains tethered to df-dimensional objects [9] (df=0, chains tethered to a point, or stars, df=l, chains tethered to a line, combs, and df=2, chains tethered to a surface, brushes). Excellent introductions and reviews on the molecular properties of these different molecular architectures are contained in [2-4,6-9]. 

Freed et al. [42,43], among others [44,45] have performed RG perturbation calculations of conformational properties of star chains. The results are mainly valid for low functionality stars. A general conclusion of these calculations is that the EV dependence of the mean size can be expressed as the contribution of two terms. One of them contains much of the chain length dependence but does not depend on the polymer architecture. The other term changes with different architectures but varies weakly with EV. Kosmas et al. [5] have also performed similar perturbation calculations for combs with branching points of different functionalities (that they denoted as brushes). Ohno and Binder [46] also employed RG calculations to evaluate the form of the bead density and center-to-end distance distribution of stars in the bulk and adsorbed in a surface. These calculations are consistent with their scaling theory [27]. 

The recognition of the two fundamental mechanisms of reptation and arm fluctuation for linear and branched entangled polymers respectively allows theoretical treatment of the hnear rheology and dynamics of more complex polymers. The essential tool is the renormahsation of the dynamics on a hierarchy of timescales, as for the case of star polymers. It is important to stress that experimental checks on well-controlled architectures of higher complexity are still very few due to the difficulty of synthesis, but the case of comb-polymers is an example where good data exists [7]. 

If combs represent one extreme of the topological family of branched polymers, then another extreme is given by the case of dendritic polymers, which retain a branched structure at all timescales. The study of tree-like branched architectures is also motivated by the important commercial low density polyethylene (LDPE), which has remarkable rheological properties making it suitable for many processing operations [3]. 

Fig. 17. Damping functions in shear from the tube model for linear polymers (lowest curve) and various branched architectures. In the cases of comb and tree, the lower curves give the case of the structure with four levels of branching, the upper the limit of large structures hatched area covers published results on LDPE...

Going to more complex systems such as aggregates, micelles, polymer brushes or polymers with architectures hke stars, dendrimers, combs, etc. or gels, the scientific arena is wide open for new investigations reveahng new phenomena and new insights. This is even more true for the dynamics of proteins and biomaterials in general, where at present basically only diffusion processes or very local dynamics have been addressed. 

Lee ML, Schneider GJ. (2001) Scaffold architecture and pharmaco-phoric properties of natural products and trade drugs Application in the design of natural product-based combinatorial libraries. Comb Chem 3 284-289. 

Statistical, gradient, and block copolymers as well as other polymer architectures (graft, star, comb, hyperbranched) can be synthesized by NMP following the approaches described for ATRP (Secs. 3-15b-4, 3-15b-5) [Hawker et al., 2001]. Block copolymers can be synthesized via NMP using the one-pot sequential or isolated macromonomer methods. The order of addition of monomer is often important, such as styrene first for styrene-isoprene, acrylate first for acrylate-styrene and acrylate-isoprene [Benoit et al., 2000a,b Tang et al., 2003]. Different methods are available to produce block copolymers in which the two blocks are formed by different polymerization mechanisms ... 

Although not extensively studied, various architectures such as star and comb polymers have also been synthesized [Bielawski et al., 2000 Goethals et al., 2000]. 

The hydrogen bonding scheme is identical in both crystals as seen in Fig. 11 and Fig. 12. The molecules pack so as to provide what maybe aptly described as a hydrogen-bonded comb, with a GA 36 backbone and caffeine 35 teeth. In fact, the major difference between the two polymorphs on the secondary level of supramolecular architecture resides in the torsion of the GA 36 aliphatic chain. 

At the same time, the macromolecules might be classified according to whether their chains have only one kind of atoms - like carbon - in the backbone (isochains) or different elements (heterochains). Concerning their chain architecture, polymers are subdivided into linear, branched, comb-like, crosslinked, dendritic, or star-like systems. 

Special interest has been devoted in recent years to comb shaped polymers. This architecture consists of a backbone carrying p randomly distributed grafts of known length. 

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