Hyperbranched polymer step growth

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. 

Dendrimers produced by divergent or convergent methods are nearly perfectly branched with great structural precision. However, the multistep synthesis of dendrimers can be expensive and time consuming. The treelike structure of dendrimers can be approached through a one-step synthetic methodology.31 The step-growth polymerization of ABx-type monomers, particularly AB2, results in a randomly branched macromolecule referred to as hyperbranch polymers. 

Hyperbranched polymers are characterized by their degree of branching (DB). Hie DB of polymers obtained by the step-growth polymerization of AB2-type monomers is defined by Eq. (2.1) in which dendritic units have two reacted B-groups, linear units have one reacted B-group, and terminal units have two unreacted B-groups191 ... 

Assuming that no intramolecular or side reactions take place and that all groups are equireactive, the polydispersity index, 7P, of hyperbranched polymers obtained by step-growth polymerization of ABX monomers is given by Eq. (2.2), where pA is die conversion in A groups.196 Note that the classical Flory relationship DPn = 1/(1 — pa) holds for ABX monomer polymerizations ... 

Hyperbranched step-growth polymers, 14 Hyperbranched structures, polyimide, 284-287... 

The step-growth polymerization of ABx-monomers is by far the most intensively studied synthetic pathway to hyperbranched polymers. A number of AB2-monomers, suitable for step-growth polymerizations, are commercially available. This has, of course, initiated substantial activity in hyperbranched condensation polymers and a wide variety of examples have been reported in the literature [4],... 

The first strategies to random hyperbranched polymers involved exclusively step-growth polymerizations. This limited the potential applications for these architectures to areas where only condensation-type polymers are acceptable. Frechet et al. [21] presented the first example of a hyperbranched vinyl polymerization in 1995, ] initiating the birth of a second generation of hyperbranched... 

The polymerization of AB -functional vinyl monomers is fundamentally different from the step-growth polymerization of AB2-monomers. Condensation of AB2-monomers results immediately in the formation of hyperbranched polymers since the reactivity of the end-groups are the same, regardless of what type of repeat unit (linear or dendritic) that is formed. 

The formation of synthetic polymers is a process which occurs via chemical connection of many hundreds up to many thousands of monomer molecules. As a result, macromolecular chains are formed. They are, in general, linear, but can be branched, hyperbranched, or crosslinked as well. However, depending on the number of different monomers and how they are connected, homo- or one of the various kinds of copolymers can result. The chemical process of chain formation may be subdivided roughly into two classes, depending on whether it proceeds as a chain-growth or as a step-growth reaction. 

Thus a step-polymerization system synthesized from an AB2 monomer should be highly branched but never reach gelation even at full conversion of the available functional groups. This is the basis of the formation of hyperbranched polymers by step-growth polymerization (Jikei and Kakimoto, 2001) and a reaction scheme for AB2 hyperbranching is shown in Scheme 1.11. 

The versatility of polymerization resides not only in the different types of polymerization reactions and types of reactants that can be polymerized, but also in variations allowed by step-growth synthesis, copolymerization, and stereospecific polymerization. Chain polymerization is the most important kind of copolymerization process and is considered separately in Chapter 7, while Chapter 9 describes the stereochemistry of polymerization with emphasis on the synthesis of polymers with stereoregular structures by the appropriate choice of polymerization conditions, including the more recent metallocene-based Ziegler-Natta systems. Synthetic approaches to starburst and hyperbranched polymers which promise to open up new applications in the future are considered in an earlier chapter dealing with step-growth polymerization. 

Many methods have been reported to synthesize hyperbranched polymers. These materials were first reported in the late 1980s and early 1990s by Odian and Tomalia [9], Kim and Webster [10], and Hawker and Frechet [11]. As early as 1952, Hory actually developed a model for the polymerization of AB -type monomers and the branched structures that would result, identified as random AB polycondensates [46], Condensation step-growth polymerization is likely the most commonly used approach however, it is not the only method reported for the synthesis of statistically branched dendritic polymers chain growth and ringopening polymerization methods have also been applied. 

Hyperbranched polymers are synthesized in a one-step method, often from AB monomers but also by combining A +B (x>3) monomers or variations of those. Polymerization methods have been applied that involve polycondensation, polyaddition, and ring-opening or self-condensing vinyl polymerization. Even though the one-pot synthetic approach leads to imperfectly branched structures because of uncontrolled growth, it is more suitable for the preparation on a larger scale and thus for commercial use. Nowadays, different... 

Turner (27) considers hyperbranched polymers to be highly branched, noncross-linked polymers prepared by the step-growth polymerization of (or ABj ) polymers. Thus, A2B means that for every two type A reactive groups there is one type B reactive group, while AB2 means that for every one type A reactive group there are two type B reactive groups. AB2 and AB3 (or A2B and AgB) are the commonest AB (or Aj B) t5q)es where x > 3 tend to be so sterically crowded that polymerization is difficult imless the reactive groups are well separated. 

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