Ring-closure, polymer-analogous

In this paper, we shall discuss, first by a polymerization of unsaturated side-groups (side-chains), second by the polymer-analogous condensation of suitable functional groups, third by ring-closures (cyclization) via electrocyclic reactions and, fourth by cyclization via electrophilic substitution reactions. 

Aromatization experiments show some evidence for relatively small aromatic substructures (naphthalene, anthracene, tetracene) and this independently demonstrates the success of the polymer-analogous cyclization process. The occurence of up to 20% non-cyclized vinylic side-groups is an indication of the statistical nature of the ring-closure reaction [58]. 

The next two approaches for synthesizing structurally defined ladder-type oligomers and polymers, discussed in sects. 4.3. and 4.4., rest on the formation of the single-stranded intermediates via polycondensation of (aromatic) monomers, again followed by a polymer-analogous ring-closure sequence. 

Scherf and Mullen prepared (Scheme 47) the ladder-type polyphenylene (LPPP, 5) with methine bridges [126-129], via a poly(diacylphenylene-co-phenylene) precursor copolymer 103 obtained by an AA-BB type Suzuki polycondensation. The key step is the polymer analogous Friedel-Crafts ringclosing reaction on the polyalcohol 104, obtained by the reduction of 103. This was found to proceed quickly and smoothly upon addition of boron-trifluoride to a solution of 104 in dichloromethane. The reaction appeared to be complete by both NMR and MALDI-TOF analysis, indicating the presence of less than 1% of defects due to incomplete ring closure. LPPPs with num-... 

It can be noted that this reaction Is closely analogous to the free radical inter-intra-molecular polymerization discovered by Butler and Angelo (11) In which a difunctional monomer undergoes such efficient ring closure that a soluble linear polymer is obtained instead of a crosslinked or three-dimensional product. 

An excellent example of the use of Suzuki polycondensation is the synthesis of ladder-type PPPs (67) (see Scheme 6.16) [84]. A precursor polymer 79 is prepared by AA-BB coupling and then converted to the ladder polymers by polymer analogous reactions. Reduction followed by ring closure with boron trifluoride produces a polymer (67a) with bridgehead hydrogens, while addition of methyl lithium instead of reduction leads to Me-LPPP (67b) with methyls at the bridgeheads. 

The success of the polymer-analogous ring closure depends essentially on the substitution pattern of the methylene carbon atom, which, during the course of the conversion, forms the methylene bridge of the ladder... 

A polymer-analogous ring closure similar to that of 2 and 3 afforded the ribbon structure 5, which was composed only of benzenoid rings and, thus, constituted a tme graphene nanoribbon [129, 130] (Scheme 3). As in 2 and 3, a successful two-step protocol was employed. It included the substituted polyphenylene 5a, which was subjected to multiple formation of six-membered rings by reductively coupling the carbonyl-containing substituents. 

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