Hyperbranched polymers properties

Monomers of die type Aa B. are used in step-growth polymerization to produce a variety of polymer architectures, including stars, dendrimers, and hyperbranched polymers.26 28 The unique architecture imparts properties distinctly different from linear polymers of similar compositions. These materials are finding applications in areas such as resin modification, micelles and encapsulation, liquid crystals, pharmaceuticals, catalysis, electroluminescent devices, and analytical chemistry. 

Due to dieir compact, branched structure and to die resulting lack of chain entanglement, dendritic polymers exhibit much lower melt and solution viscosity dian their lineal" counterparts. Low a-values in die Mark-Houwink-Sakurada intrinsic viscosity-molar mass equation have been reported for hyperbranched polyesters.198 199 Dendrimers do not obey diis equation, a maximum being observed in die corresponding log-log viscosity-molar mass curves.200 The lack of chain entanglements, which are responsible for most of the polymer mechanical properties, also explains why hyperbranched polymers cannot be used as diermoplastics for structural applications. Aldiough some crystalline or liquid... 

Hyperbranched polymers generally have very low melt and indinsic viscosities. The large number of chain-end functional groups present in hyperbranched macromolecules have also been shown to dramatically affect physical properties... 

Hyperbranched poly(ethyl methacrylate)s prepared by the photo-initiated radical polymerization of the inimer 13 were characterized by GPC with a lightscattering detector [51]. The hydrodynamic volume and radius of gyration (i g) of the resulting hyperbranched polymers were determined by DLS and SAXS, respectively. The ratios of Rg/R are in the range of 0.75-0.84, which are comparable to the value of hard spheres (0.775) and significantly lower than that of the linear unperturbed polymer coils (1.25-1.37). The compact nature of the hyperbranched poly(ethyl methacrylate)s is demonstrated by solution properties which are different from those of the linear analogs. 

Experimental data on the solution properties and melt rheology of highly branched structures are scarcely found in the literature. This might be because of the structural nonuniformity of hyperbranched polymers, which makes it difficult to obtain reliable data. Because of the purely statistical nature of the poly-... 

In this review, we have described the synthesis of hyperbranched (meth)acry-lates. We have shown that the solution and melt properties are considerably different from their Unear analogs, due to their compact, nonentangled structure. SCV(C)P has become a valuable tool in synthesis of hyperbranched polymers from vinyl monomers. Theoretical investigations help to obtain information on the molecular parameters of the resulting hyperbranched polymers which often could not be obtained experimentally. Studies on the solution and melt properties help one to understand the relationship between the properties and molecular parameters (DB,MW, distribution of branching points), which are extremely valuable from both industrial and scientific viewpoints. 

Owing to multi-functionahty, physical properties such as solubihty and the glass transition temperature and chemical functionahty the hyperbranched (meth) acrylates can be controlled by the chemical modification of the functional groups. The modifications of the chain architecture and chemical structure by SCV(C)P of inimers and functional monomers, which may lead to a facile, one-pot synthesis of novel functionahzed hyperbranched polymers, is another attractive feature of the process. The procedure can be regarded as a convenient approach toward the preparation of the chemically sensitive interfaces. 

Dendrimers with terminal functional groups represent mode compact precursors that are spherical and almost monodisperse, with reactive groups placed on their periphery. Their synthesis, structure and properties have been reviewed in monographs and review articles often together with hyperbranched polymers (cf., e.g. [15-20]), as well as in this book. Application of dendrimers as precursors for conventional materials is limited at this time by their relatively high cost. 

Use of dendritic molecules (mainly hyperbranched polymers) as precursors of a crosslinked (engineering) material in order to modify its processing and materials properties. 

Several applications of hyperbranched polymers as precursors for synthesis of crosslinked materials have been reported [91-97] but systematic studies of crosslinking kinetics, gelation, network formation and network properties are still missing. These studies include application of hyperbranched aliphatic polyesters as hydroxy group containing precursors in alkyd resins by which the hardness of alkyd films was improved [94], Several studies involved the modification of hyperbranched polyesters to introduce polymerizable unsaturated C=C double bonds (maleate or acrylic groups). A crosslinked network was formed by free-radical homopolymerization or copolymerization. 

Frechet, J. M. J., Hawker, C. J., Synthesis and properties of dendrimers and hyperbranched polymers, in Aggarwal, S. L. and Russo, S. (eds), Comprehensive Polymer Science, Second Supplement, Elsevier Science, Oxford 1996, pp. 71-133. 

Dendrimers/dendrons are synthesized almost exclusively via elaborate synthetic procedures that are usually more costly than hyperbranched processes. Even though they display many unique properties, their higher costs do not always justify use in some applications. Consequently, hyperbranched polymers may serve as a more cost-effective alternative when optimum properties are not required. 

Despite the simplicity of chemistries presently employed for the commercially available hyperbranched polymers, these materials are still relatively high priced compared to traditional commodity polymers. This is undoubtedly related to the early stage, low volume demands for these products. Hyperbranched polymers will evolve as substitutes for traditional polymers as their unique properties are used to greatly enhance products on a cost-performance basis. In summary, the enhanced used of hyperbranched polymers in engineered products will depend upon many of the following prerequisites ... 

Unique architecturally driven properties that may be expected from hyperbranched polymers will be largely derived from their (a) amplified number of terminal functional groups, (b) new rheological properties based on less chain entanglement and (c) new architectural arrangements that may modulate crystallinity, flow characteristics and glass transition properties in designed systems. 

The lower cost of synthesizing hyperbranched polymers allows them to be produced on a large scale, giving them an advantage over dendrimers in applications involving large amounts of material, although the properties of hyperbranched polymers are intermediate between those of dendrimers and linear polymers [33]. 

We will focus on the variety of different hyperbranched polymers that have been synthesized, on the specific properties that hyperbranched polymers exhibit, and hopefully stimulate the reader to find new and unique areas where these novel materials can find future applications. 

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