Hyperbranched structure branched polymers

The chain architecture and chemical structure could be modified by SCVCP leading to a facile, one-pot synthesis of surface-grafted branched polymers. The copolymerization gave an intermediate surface topography and film thickness between the polymer protrusions obtained from SCVP of an AB inimer and the polymer brushes obtained by ATRP of a conventional monomer. The difference in the Br content at the surface between hyperbranched, branched, and linear polymers was confirmed by XPS, suggesting the feasibility to control the surface chemical functionality. The principal result of the works is a demonstration of utility of the surface-initiated SCVP via ATRP to prepare surface-grafted hyperbranched and branched polymers with characteristic architecture and topography. 

Another definition, taking into account polymerization conversion, has been more recently proposed.192 Perfect dendrimers present only terminal- and dendritic-type units and therefore have DB = 1, while linear polymers have DB = 0. Linear units do not contribute to branching and can be considered as structural defects present in hyperbranched polymers but not in dendrimers. For most hyperbranched polymers, nuclear magnetic resonance (NMR) spectroscopy determinations lead to DB values close to 0.5, that is, close to the theoretical value for randomly branched polymers. Slow monomer addition193 194 or polycondensations with nonequal reactivity of functional groups195 have been reported to yield polymers with higher DBs (0.6-0.66 range). 

Dendrimers have structures similar to that of hyperbranched polymer and can be taken as the perfectly branched polymer with monodispersity. However, they need to be prepared by a multistep procedure. Therefore, very little work has been done on dendritic polyfarylcnc ether)s. Morikawa et al. prepared a series of monomers with a various number of phenylene units.164,165 These monomer were used to prepare poly(ether ketone) dendrons with graded structures (Scheme 6.24). 

Relationships between dilute solution viscosity and MW have been determined for many hyperbranched systems and the Mark-Houwink constant typically varies between 0.5 and 0.2, depending on the DB. In contrast, the exponent is typically in the region of 0.6-0.8 for linear homopolymers in a good solvent with a random coil conformation. The contraction factors [84], g=< g >branched/ <-Rg >iinear. =[ l]branched/[ l]iinear. are another Way of cxprcssing the compact structure of branched polymers. Experimentally, g is computed from the intrinsic viscosity ratio at constant MW. The contraction factor can be expressed as the averaged value over the MWD or as a continuous fraction of MW. 

Ferrocene-based Branched Polymers (Dendrimers). One of the topics in macromolecular chemistry is constituted by dendrimers, or hyperbranched macromolecules of tridimensional globular structure, the surface of which is characterized by a large number of functional groups, Scheme 4. Such functionalities impart to the molecules solubility, viscosity and thermal properties different from those of the common linear polymers.38c,d 44... 

Holter D, Burgath A, Frey H (1997) Acta Polym 48 30 Holter D, Frey H (1997) Acta Polym 48 298. Degree of branching is often used as a descriptor for hyperbranched structures see, e.g., Malmstrom E, Hult A (1997) JMS Rev Macromol Chem Phys 37 555 Dusek K (1997) TRIP 8 268... 

Whereas the well-characterized, perfect (or nearly so) structures of dendritic macromolecules, constructed in discrete stepwise procedures have been described in the preceding chapters, this Chapter reports on the related, less than perfect, hyperbranched polymers, which are synthesized by means of a direct, one-step polycondensation of A B monomers, where x > 2. Flory s prediction and subsequent demonstration 1,2 that A B monomers generate highly branched polymers heralded advances in the creation of idealized dendritic systems thus the desire for simpler, and in most cases more economical, (one-step) procedures to the hyperbranched relatives became more attractive. 

When we do not care much about obtaining a product with precise size and structure, branched monomers can be condensed randomly. The polymeric materials obtained in this way are called hyperbranched polymers (Fig. 3.10). 

The self-condensing copper-catalyzed polymerization of macromonomer of poly(tBA) with a reactive C—Br bond (H-6) affords hyperbranched or highly branched poly(tBA).447 Copolymerization of H-1 and TV-cyclohexylmaleimide induced alternating and self-condensing vinyl polymerization.448 The residual C—Cl bond was further employed for the copper-catalyzed radical homopolymerization of styrene to give star polymers with hyperbranched structures. Hyperbranched polymers of H-1 further serve as a complex multifunctionalized macroinitiator for the copper-catalyzed polymerization of a functional monomer with polar chromophores to yield possible second-order nonlinear optical materials.325... 

Anionic polymerization has proven to be a very powerful tool for the synthesis of well-defined macromolecules with complex architectures. Although, until now, only a relatively limited number of such structures with two or thee different components (star block, miktoarm star, graft, a,to-branched, cyclic, hyperbranched, etc. (co)polymers) have been synthesized, the potential of anionic polymerization is unlimited. Fantasy, nature, and other disciplines (i.e., polymer physics, materials science, molecular biology) will direct polymer chemists to novel structures, which will help polymer science to achieve its ultimate goal to design and synthesize polymeric materials with predetermined properties. 

The structure of the branched polymers produced by any random branching process is the same. Any individual hyperbranched polymer structure made from reacting ABy i monomers can also be made by reacting Ay monomers. The difference between these branching processes is the molar mass distribution—the relative amounts of each structure produced. 

ABy 1 monomers, where A only reacts with B) have both their number-average and weight-average molar masses diverge as their reaction nears completion and they never make network polymers. The hyperbranching reaction makes the same structure of randomly branched polymers as gelation, but with a very different distribution of molar masses. 

Compare dendrimers, hyperbranched polymers, and moderately branched polymers with regard to their molecular architecture. What differences in properties of these polymers would be expected in view of their structural differences Describe several alternative methods of synthesizing hyperbranched polymers and the reactions involved in such syntheses. 

These problems might be overcome by using randomly branched polymer structures as supports [ 13,82,112]. In contrast to dendrimers, hyperbranched polymers are easily available in one reaction step. This allows the production of large quantities of material [82]. They contain dendritic, linear and terminal monomer units in their skeleton and hence can be considered as inter me-... 

These unusual branched structures give rise to unique polymer properties. For example, the topologies of the polyethylenes vary from linear with moderate branching to hyperbranched structures. For the most highly branched systems, the overall branching number and the distribution of short-chain branches can change very little while the architecture... 

Polymerization by the inimer technology has received much attention from Kennedy and Puskas, specifically for the synthesis of hyperbranched polyisobutylenes (PIB)s and copolymers thereof in a one-pot method [67]. While this convergent approach complicates the structural analysis of the branched polymers, fragmentation of the resulting polymer is possible in some cases to allow such analysis [68]. Branching ratios (BR) can be calculated directly from the molecular weight of the branched polymer as per Equation 30.9, to give an indication of the number of branches contained within the molecules, as the ratio of the measured for the polymer obtained to the theoretical... 

Figure 11 shows that within the wide range of pressures the simulations give a constant average number of branches. The microstructure of the polymer, however, is strongly affected by the pressure. Examples of the polymer structures obtained from the simulations are shown in Fig. 11. The polymers obtained at high pressures are mostly linear with a large fraction of atoms located in the main chain, and with relatively short and mostly linear side-chains. With a decrease in pressure the hyperbranched structures are formed. Both, the pressure independence of the branching number and the pressure influence on the polymer topology are in agreement with experimental data for Pd-catalysts [16,18-21].

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