Comb-branched structure

This approximation is equivalent to assuming that the differences in internal densities and, consequently, in solvent draining, between a branched chain and the homologous linear chain, when included in their corresponding mean sizes, can describe both the friction coefficient and the viscosity. Besides these theoretical considerations, an empirical correlation in terms of a log-log fit of h vs f was employed by Roovers et al. [51]. Kurata and Fukatsu [48] and Ptitsyn [82] performed a more general Kirwood evaluation of the friction coefficient for different types of ideal branched molecules (uniform and randomly distributed stars, combs and random-branched structures). Their results for different structures are included within the limits l[Pg.60]

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]. 

Calculations of h have since been made for certain combs (20, 35) and randomly branched structures (20). Kurata and Fukatsu (20) found that the ratio h/g% (which would be unity if the hydrodynamic radius had a fixed ratio to the radius of gyration) was not constant, but a function of g0 depending on the type of branching. Also, they found that as the degree of branching (as measured by the number of sub-chains) increased, h/g% approached a limit, also dependent on the type of branching. They suggested however that the inequality ... 

To obtain improved ionic mobility, and thus high conductivity, alternative polymer structures have been developed, for example comb-branched block copolymers such as poly[bis(methoxy ethoxy ethoxide)], usually known as MEEP. Room temperature conductivities of MEEP-based polymer electrolytes of the order of 10-5 S/cm have been achieved, values... 

The extent of LCB and its distribution depends mainly on the catalyst system and the conditions used in the polymerisation. Polymerisation conditions (monomer and comonomer concentration, type of catalyst, temperature and concentration of transfer agents) are important variables to be taken into account when one is looking at the rheological behaviour of the polymers. By decreasing the ethene concentration and increasing the polymerisation time in the reactor the LCB frequency can be enhanced [59, 81]. The polymers made with these catalysts have a complex branching structure composed of comb and tree structures of different lengths. 

The variety of branched architectures that can be constructed by the macromonomer technique is even larger. Copolymerization involving different kinds of macromonomers may afford a branched copolymer with multiple kinds of branches. Macromonomer main chain itself can be a block or a random copolymer. Furthermore, a macromonomer with an already branched or dendritic structure may polymerize or copolymerize to a hyper-branched structure. A block copolymer with a polymerizable function just on the block junction may homopolymerize to a double comb or double-haired star polymer. 

Highly stereoregular PMMA macromonomers, 14, prepared by Hatada and coworkers, have recently been fractionated by supercritical fluid chromatography into completely uniform fractions with no structural distribution [20,21]. They have been oligomerized with a radical (AIBN) or an anionic initiator (3,3-dimethyl-1,1-diphenylbutyllithium). After a new fractionation by SEC comb or star polymers of completely uniform architecture are obtained. No doubt, these samples will be most promising to investigate the branched structure-property relationship. 

Scheme 16 Schematic representation of novel hyperbranched glycostructures illustrating regularly interspaced ligands onto poly(ethyleneimine) backbone (75), comb-branch (GO) structure (76), interspaced spheroidal dendrimer (77), rod-shaped, cylindrical polymer with dendritic branch (78), and dendrigraft (79).

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