Branching hyperbranched

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. 

Condensation polymerizations (polycondensations) are stepwise reactions between bifunctional or polyfunctional components, with elimination of small molecules such as water, alcohol, or hydrogen and the formation of macromo-lecular substances. For the preparation of linear condensation polymers from bifunctional compounds (the same considerations apply to polyfunctional compounds which then lead to branched, hyperbranched, or crosslinked condensation polymers) there are basically two possibilities. One either starts from a monomer which has two unlike groups suitable for polycondensation (AB type), or one starts from two different monomers, each possessing a pair of identical reactive groups that can react with each other (AABB type). An example of the AB type is the polycondensation of hydroxycarboxylic acids ... 

By incorporating such motifs into handcuff -shaped monomers, it should be possible to combine some of the characteristics of supramolecular polymers - self-correcting, thermodynamically-controlled assembly - with the hallmark properties and stability of standard polymers with covalent backbones (Figure 8). Such monomers would, in the presence of a catalyst, spontaneously assemble to form mechanically linked polymers of precise length and architecture defined by the concentration at which they were prepared. Indeed, the various structures (linear, branched, hyperbranched etc.) would be intercon-... 

Poly(ethylene oxide) (PEO) has been employed frequently as a water-soluble catalyst support [9]. Further water-soluble polymers investigated include other linear polymers such as poly(acrylic acid) [10], poly(N-alkylacrylamide)s [11], and copolymers of maleic anhydride and methylvinylether [12], as well as dendritic materials such as poly(ethyleneimin) [10a, c] or PEO derivatives of polyaryl ethers [13]. The term dendritic refers to a highly branched, tree-like structure and includes perfectly branched dendrimers as well as statistically branched, hyperbranched macromolecules. 

Dendrimers are a subset of hyperbranched polymers in that way, that they have no imperfections they branch at each monomer unit. Dendrimers also differ in methods of preparation. They are usually prepared by an iterative synthesis, with purification of intermediate stages or generations . This requires much effort, but allows for perfect branching. Hyperbranched polymers are prepared in one-pot methods that result in imperfect branching. 

As a unique method of controlled/living radical polymerization, ATRP has had a tremendous impact on the synthesis of macromolecules with well-defined compositions, architectures, and functionalities, including star- and comb-like polymers as well as branched, hyperbranched, dendritic, network, cyclic type structures and so forth. 

Cyclic carbonate chemistry has also proven to be useful for the preparation of branched or cross-linked polymers. First branched (hyperbranched) PCs were synthesized by Bolton and Wooley. In one of the published methods of synthesis of hyperbranched PCs they used chloroformate-type l,l,l-tris(4-hydroxyphenyl) ethane (THPE)-based monomers (Scheme 100). The authors synthesized hyperbranched aromatic PCs by the polymerization of A2B and AB2 monomers, which involved the condensation of chloroformate (Scheme 104) functionalities with tert-butyldimethylsilyl-protected phenols (Scheme 104), facilitated by reactions with silver fluoride. 

RAFT polymerisation has also been used in conjunction with CuAAC for the preparation of highly branched, hyperbranched and neo-glycopolymers [134, 135]. For instance, Stenzel and co-workers reported the synthesis of a new class of glycomonomer (4-vinyl-l, 2,3-triazole), followed by its RAFT polymerisation in water/methanol at 60 C (Scheme 1.9) [135]. 

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