Other Polymer Architectures

A star polymer contains polymer chains as arms emanating from a branch point. Star polymers can be synthesized via ATRP by using an initiator containing three or more halogens, for example, a 3-arm star polymer is obtained by using a tribromo initiator  

A graft copolymer is a branched polymer containing a polymer chain derived from one monomer to which are attached one or more polymer side chains of another monomer. ATRP produces a graft copolymer when the initiator is a polymer with one or more halogen-containing side groups  

When the polymeric initiator contains many halogens, there will be many grafted side chains, and the product is called a comb or brush polymer. A variety of polymers can be used as the polymeric initiator, including polymers containing vinyl chloride and 4-chloromethylstyrene units, and halogenated natural and butyl rubbers. Graft copolymers are discussed further in Chaps. 5, 6, and 9. 

Hyperbranched polymers (see structure LXII in Sec. 2-16a) are produced from monomers that contain both a polymerizable double bond and an initiating function, such as p-(chloro-methyl)styrene. The product is highly branched with one double bond end group and many 

The synthesis of complex polymer architectures by ATRP (or other living radical polymerizations) is useful but relatively restricted because the occurrence of bimolecular termination increases with the number of initiator sites on a polymeric species. 

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A number of different types of copolymers are possible with ATRP—statistical (random), gradient, block, and graft copolymers [Matyjaszewski, 2001]. Other polymer architectures are also possible—hyperbranched, star, and brush polymers, and functionalized polymers. Statistical and gradient copolymers are discussed in Chap. 6. Functionalized polymers are discussed in Sec. 3-16b. 

Statistical, gradient, and block copolymers as well as other polymer architectures (graft, star, comb, hyperbranched) can be synthesized by NMP following the approaches described for ATRP (Secs. 3-15b-4, 3-15b-5) [Hawker et al., 2001]. Block copolymers can be synthesized via NMP using the one-pot sequential or isolated macromonomer methods. The order of addition of monomer is often important, such as styrene first for styrene-isoprene, acrylate first for acrylate-styrene and acrylate-isoprene [Benoit et al., 2000a,b Tang et al., 2003]. Different methods are available to produce block copolymers in which the two blocks are formed by different polymerization mechanisms ... 

For a spacer length of 11 methylene units, all polymers showed smectic mesophases except for poly-(II-ll) and poly-(VII). Even the flexible poly-(cy-clooctene) main chain prevented a smectic mesophase. Compared to all of the other polymer architectures, poly-(II-ll) and poly-VII present the lowest ratios of mesogens to atoms in the main chain. It can therefore be assumed that smectic phases will only be formed when there is sufficient mesogen density. For the norbornene chain, it is notable that a high Z/E-ratio and a high tactic-ity increased the stability of the smectic A phase. 

With the possibility at hand to directly detect transitions in the conformational space of the hydrated PEG, it is now planned to apply this new tool to various other polymer architectures, particularly those that are known to be less protein resistant... 

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