Bifunctional polymers precursor

The third method consists in using a,co-bifunctional polymers of known molecular weight, and in reacting them with adequate plurifunctional reagents17-22. In some cases the a, co-bifunctional precursor polymer is also obtained anionically — this provides for a sharp molecular weight distribution — in other cases it is obtained by other methods. 

In a follow-up study, Dankers et al. extended the UPy-precursor toolkit to linear polymers containing UPy moieties in the main chain [156]. The authors studied the rheological behavior of these materials, reported on their intrarenal behavior and tissue response, and proposed possible therapeutic applications. In another seminal work, Meijer and coworkers developed UPy-based dual-fiber networks as a synthetic analogue to fiber formation in the cytoskeleton [157]. For this purpose, mixtures of mono- and bifunctional polymers were used, which a priori form fibers in water gelation could be achieved by decreasing the pH from 12 to 3. 

The combination of a linear bifunctional telechelic precursor with a tricarboxylate counteranion produced a self-assembly that consisted of three polymer units and two counteranions under dilution. Subsequent heat treatment produced a pair of covalently fixed constitutional isomers, namely manacle-shaped and 0-shaped polymers (Scheme 18.3b) [6, 15]. These relevant pairs of polymeric isomers were formed from a self-assembly of two units of a three-armed star telechelic precursor and three units of a dicarboxylate counteranion [16], as well as from the double-metathesis condensation of an H-shaped telechehc polymer precursor having four alkene end groups (Scheme 18.3c) [17]. 

In this approach proposed by Casassa, the linear polymer precursor to be cydized possesses two identical end groups and the ring dosure requires the use of a bifunctional coupling agent, as illustrated in Scheme 4. 

The azoperester, t-butyl 4-methyl-4-(t-butylazo)peroxypentanoate, cleaves at the azo group on excitation to yield the y-perester radical (231). Cyclisation of (231) by attack on the peroxy linkage to yield the corresponding y-lactone occurs sufficiently slowly (1.5 x 10 s at 22 °C) that the azoperester precursor may be used as a photochemical bifunctional initiator. In contrast thermolysis occurs at the peroxy linkage. Decarboxylation and cyclisation of y-azo radical (230) occur so rapidly that an azo-containing polymer would not be produced.Diarylfur-oxans (232) yield 1,2-diarylacetylenes in low yield on irradiation. ... 

Alternatively, Whittaker et al. utilized the reversible oxidation/reduction of a thiol-terminated linear polymer as a homocoupling reaction to access macrocycles that could be reversibly cyclized and cleaved (Scheme 12.5) [29]. The linear precursors were prepared using reversible addition-fragmentation chain transfer (RAFT) polymerization of styrene from a bifunctional initiator (16). The desired polystyrene with thiol end groups could be isolated in near-quantitative yields by aminolysis of the polymer with terminal dithioester groups (17). The linear dithiols... 

A bifunctional/higher-functional ligand in the presence of a metal or a metal complex precursor is reacting under polymer metal complex formation. 

PPV (62a) has been prepared by a number of different methods which were studied in detail by Horhold and Opfermann [390]. It can be synthesized by bifunctional carbonyl olefination of terephthalaldehyde according to Wittig s reaction and from />-xylylene-bis(diethyl phosphonate), as well as by dehydrochlorination of/p-xylylene dichloride with sodium hydride in A jiV-dimethylformamide and with potassium amide in liquid ammonia. The most popular route to PPV used today is the precursor route, first described by Wessling and Zimmermann [391-394] and Kanabe and Okawara [395], starting from the monomers p-xylylene-bis(di-methylsulphonium tetrafluoroborate) [395] or chloride (Scheme 11.17) [391-394]. The latter is polymerized to yield a water soluble sulphonium salt polyelectrolyte (63d) which is then purified by dialysis [396]. The precursor polymer is converted to PPV (63e) by the thermal elimination of dimethyl sulphide and HCl. The method was later developed by Horhold et al. [397], Lenz and co-workers [398,399], Murase et al. [400] and Bradley [401], One of the major improvements within the last years has been the use of tetrahydrothio-phene instead of dimethyl sulphide in the synthesis of the precursor polymer [402]. The use of the cyclic leaving group facilitates the elimination when the precursor polymers is heated at 230-300°C and leads... 

The end-linking methods provide some control of the structure defects on the molecular scale. Under the usual experimental conditions, the number of dangling chains is expected to be rather low provided the stoichiometry is accurate. Moreover, it is possible to prepare gels containing controlled amounts of dangling chains by using a mixture of monofunctional and bifunctional chains as a precursor polymer. 

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