Living radical polymerization star polymers

Comb or densely grafted polymers are defined as polymers that have at least one polymeric chains per monomer unit of the main chain, and Figure 29 shows examples obtained by metal-catalyzed living radical polymerization. Comb polymers possess physical properties similar to those of star polymers in solution. 

As discussed in Section 7.3, conventional free radical polymerization is a widely used technique that is relatively easy to employ. However, it does have its limitations. It is often difficult to obtain predetermined polymer architectures with precise and narrow molecular weight distributions. Transition metal-mediated living radical polymerization is a recently developed method that has been developed to overcome these limitations [53, 54]. It permits the synthesis of polymers with varied architectures (for example, blocks, stars, and combs) and with predetermined end groups (e.g., rotaxanes, biomolecules, and dyes). 

The core first method starts from multifunctional initiators and simultaneously grows all the polymer arms from the central core. The method is not useful in the preparation of model star polymers by anionic polymerization. This is due to the difficulties in preparing pure multifunctional organometallic compounds and because of their limited solubility. Nevertheless, considerable effort has been expended in the preparation of controlled divinyl- and diisopropenylbenzene living cores for anionic initiation. The core first method has recently been used successfully in both cationic and living radical polymerization reactions. Also, multiple initiation sites can be easily created along linear and branched polymers, where site isolation avoids many problems. 

One of the most distinguishable characteristics of the metal-catalyzed living radical polymerization is that it affords polymers with controlled molecular weights and narrow MWDs from a wide variety of monomers under mild conditions even in the presence of a protic compound such as water. This permits the synthesis of a vast number of polymers with controlled structures such as end-functionalized polymers, block copolymers, star polymers, etc., where they are widely varied in comparison with those obtained by other living polymerizations. This is primarily due to the tolerance to various functional groups and the polymerizability/controllability of various vinyl monomers as mentioned above. 

Block copolymers between alkyl or related methacrylates (B-1,132 198,357 B-2,198 and B-3115,146,148) were prepared via the ruthenium-, copper-, and nickel-catalyzed living radical polymerizations. These block copolymers can be synthesized both via sequential living radical polymerizations and via the living radical polymerization initiated from isolated polymers. For example, the ruthenium-catalyzed sequential living radical polymerization of MMA followed by nBMA affords AB block copolymers B-1 with narrow MWDs (Mw/Mn = 1.2), which can be extended further into ABA block copolymers B-2 with similarly narrow MWDs (Mw/Mn = 1.2).198 Star block copolymers with B-1 as arm chains were similarly synthesized but with multifunctional initiators.357... 

Telechelic and star polymers can be obtained by a so-called multifunctional initiator method where metal-catalyzed living radical polymerizations are initiated from halogen compounds with plural reactive carbon—halogen bonds. This method can give multiarmed or star polymers with a predetermined number of arms that corresponds to the number of the carbon—halogen bonds in the initiator. Numerous polyhalogen compounds are accessible and synthesized from various polyfunctional compounds as summarized in Figures 27 and 28, where the arm numbers may be varied between 2 and 12. 

The third method is based on a polymer-linking reaction where the liner polymers are obtained by the living radical polymerization with divinyl compounds. This can afford star polymers with a relatively large number of arms, up to several hundred per molecule, while the number of arms by definition involves a statistical distribution in a single sample. 

Functional groups can also be introduced in the spacer units. Bifunctional initiators with bipyridine units such as MI-17 and MI-18 induced the living radical polymerizations of styrene and MMA, respectively, with copper catalysts to give polymers that carry a coordination site at the middle of the chain.87,333 These polymers can be connected together into star polymers with a ruthenium cation at the core, where the arm numbers are varied among three, four, five, and six in combination with the polymers obtained from the monofunctional initiator with a bipyridine unit (FI-21 and FI-22 Figure 13).416 A... 

Another tetrafunctional ester (MI-36) is the smallest number of a series of dendrimer-type initiators such as MI-46 and MI-53 for 6- and 12-arm star polymers, respectively.414 419 420 These initiators induce the living radical polymerizations of MMA with Ni-2 to give the corresponding multiarmed polymers with controlled molecular weights although the arm number with MI-53 is slightly lower than 12 due to incomplete initiation from all the carbon—bromine bonds. 

Other multifunctional initiators include star polymers prepared from initiators via living radical or other living polymerizations. In particular, all of the star polymers via metal-catalyzed living polymerization, by definition, carry a halogen initiating site at the end of each arm, and thus they are potentially all initiators. Thus, star-block copolymers with three polyisobutylene-Mock-PMMA arms and four poly-(THF) -A/oc/F polystyrene or poly(THF)-Woc/c-polysty-rene-Wock-PMMA were synthesized via combination of living cationic and copper-catalyzed living radical polymerizations.381,388 Anionically synthesized star polymers of e-caprolactone and ethylene oxide have... 

Five synthetic strategies for the construction of star-like polymers have been identified, mainly through variations in the core construction. In particular, con-trolled/living radical polymerization (C/LRP) techniques, that originally were developed during the mid-1990s, have provided quite simple routes to polymers with a star-like architecture [12-14, 21]. 

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. 

Back, K.-Y., Kamigaito, M., and Sawamoto, M. (2001). Core-functionalized star polymers by transition metal-catalyzed living radical polymerization. 1. Synthesis and characterization of star polymers with PMMA arms and amide cores. Macromolecules, 34(22) 7629 7635. 

Bernard, J., Favier, A., Zhang, L. et al. (2005) poly(vinyl ester) star polymers via xanthate-mediated living radical polymerization from poly(vinyl alcohol) to glycopolymer stars. Macromolecules, 38,5475-5484. 

Terashima, T., Ouchi, M., Ando, T. et al. (2006) Metal-complex-bearing star polymers by metal-catalyzed living radical polymerization Synthesis and characterization of poly(methyl methacrylate) star polymers with Ru(ll)-embedded microgel cores. Journal of Polymer Science Part A-Polymer Chemistry, 44,4966. 

The development of PPE synthetic chemistry makes the synthesis of PPEs with various structures possible. Recently, PPE-based polymers with different topological structures including linear random copolymers, block copolymers, star polymers, miktoarm polymers, and brush and hyperbranched polymers have been synthesized. Among them, linear homopolymers or random copolymers of PPEs are perhaps the most studied. Different block copolymers with AB, ABA, and ABC architectures have been synthesized by controlled ROP. By the combination of ROP of PPE with other controlled polymerization methods, such as living radical polymerization, or click chemistry, more complex architectures including miktoarm, comb, or graft copolymers can be synthesized. The richness of structures has allowed the convenient adjustment of material properties of PPE for biomedical applications. 

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