Multistep polymer, defined

The polymerization of sucrose derivatives (esters, ethers, acetals) bearing a carbon-carbon double bond has been studied (Scheme 45). Polymers can be obtained by polymerization or copolymerization.146,404 414 The monomers are prepared either by multistep synthesis, leading to defined compounds and subsequently rather well-controlled polymerization processes,302,415,416 or by direct functionalization of unprotected sucrose, leading to mixtures of isomers and... 

Redox series of metal-polypyridines still await their practical exploration. The existence of multistep, reversible, sequential reduction processes, each step occurring at a defined potential and being localized at a specific molecular site, is very promising for possible applications in molecular electronics. This would require to organize the active complexes in films, polymers or supermolecules. Up to now, only the electrochromic behavior of some [Ru(N,N)a] + complexes has been explored with potential applications in electrochromic glasses, displays and redox sensors [206, 262, 264]. 

Nevertheless, despite the fact that the optical and electronic properties already converge for relatively short chain lengths of 10-12 phenyls, there are a couple of simple reasons to favor polymers over defined oligomers such as the ladder-type phenylene 11-mer in practical applications (1) in most cases, the synthetic effort toward the defined oligomers is enormous (extensive multistep procedures) and (2) only polymers with a certain molecular weight (favorable >10,000-20,000) allow the processing into high-quality, flexible thin films by solution techniques as favored for the production of polymer electronic devices. 

Conj ugated Ladder Polymers. Since the 1930s double-stranded, ladder-type polymers have been prepared in a multistep process with limited success of cyclization (191,192). Other routes have also been explored such as those for poly(acrylonitrile) (193,194), poly(l,2-butadiene), poly(3,4-isoprene) (195), or poly(butadiyne)s (196). These materials were found to be poorly soluble and unworkable, with a considerable number of defects in the structure (incomplete cyclization, cross-linking, radical sites). The first successful synthesis of a ladder polymer with a completely defined structure was accomplished in 1991 by Sherf and Mullen (197). The first step was the AA/BB-t5q)e polycondensation of an aromatic diboronic acid with a substituted 2,5-dibromo-l,4-dibenzoylbenzene to give a single-stranded precursor PPP-type polymer, followed by cyclization to the ladder structure (Fig. 8). Several other examples exist that have resulted in ladder-type structures. These include angular polyacene (198,199), Diels-Alder polyaddition of AB-type diene-dienophiles (200), AA/BB-type Diels-Alder polyaddition of a bisdiene and a bisdienophile (201), thienylene imits (202),... 

In general, it is far more difficult to synthesize miktoarm star polymers than regular stars having identical arms because there are strict requirements in terms of multistep quantitative reactions that correspond to introducing different arms. In addition, the isolation of intermediate polymers is often required to obtain pure products. Although several successful examples of well-defined p-star polymers have been reported, most of them are composed of less than three different arms. Only a few examples of miktoarm star-branched polymers with four different arms have been synthesized to date. For this difficult synthetic problem, general and versatile methodologies for the synthesis of multiarmed and multicomponent p-star polymers have been vitally desired. 

The reaction sequence described here represents the first synthesis of a structurally defined, soluble band polymer using a multistep process. In addition, it is actually the first known conjugated ladder polymer of defined molecular structure. 

The most striking advantage of ATRP is its applicability [16] to the preparation of polymers with complex topologies and compositions which cannot be prepared by other methods or requires multisteps under stringent conditions or the other methods do not yield well defined materials. 

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