Linear polymerizations, ring

All of these functions are made possible by the characteristic chemical features of carbohydrates (1) the existence of at least one and often two or more asymmetric centers, (2) the ability to exist either in linear or ring structures, (3) the capacity to form polymeric structures via glyeosidie bonds, and (4) the potential to form multiple hydrogen bonds with water or other molecules in their environment. 

Less important methods are the self condensation of w-hydroxy acid and the ring opening of lactones and cyclic esters. In self condensation of w-hydroxy acids, cyclization might compete seriously with linear polymerization, especially when the hydroxyl group is in a position to give five or six membered lactones. 

Furthermore, formation of these products occurs with much greater ease than linear polymerization of units of six or more chain members, and they are more stable to hydrolysis or other ring-opening reactions. 

Figure 1. Number fraction of ring structures per molecule (Nr) as a function of extent of reaction (p) for bulk, linear, and nonlinear polyurethane-forming reactions with approximately equimolar concentrations of reactive groups (r = [NCO]J [OH]0 ss 1) (2,3). Conditions O-linear polymerization, HDI + poly(ethyleneglycol) at 70°, [NCOfo — 5.111 mol/kg, [OH], = 5.188 mol/kg number-average number of bonds in chain forming smallest ring structure (v) = 25.2, and nonlinear polymerization, HDI and POP triol at 70°C, [NCO] — 0.9073 mol/kg, [OH]0 = 0.9173 mol/kg v = 115. Reproduced with permission from Ref. 5.

Neglect of the formation of polymeric rings, however, is sometimes too crude an approximation. It may happen that the cyclisation constant C2 of the linear dimer is larger than the cyclisation constant Cl of the monomer. It may also happen that the concentration of the monomer is comparable to Ct but smaller than C2. When this is the case, the open chain dimer, once formed, will show a higher tendency to cyclise than to react with the monomer to give the linear trimer. Under the above conditions the stepwise polymerisation is truncated after the first step, and the system is described to a useful approximation by scheme (6). 

Recall the discussion in Sec. 2-3 concerning the competition between linear polymerization and cyclization in step polymerizations. Cyclization is not competitive with linear polymerization for ring sizes greater than 7 atoms. Further, even for most of the reactants, which would yield rings of 5, 6, or 7 atoms if they cyclized, linear polymerization can be made to predominate because of the interconvertibility of the cyclic and linear structures. The difference in behavior between chain and step polymerizations arises because the cyclic structures in chain polymerization do not depropagate under the reaction conditions that is, the cyclic structure does not interconvert with the linear structure. 

A are smaller than the sum of the van der Waals radius of carbon and the ionic radius of Tl+ (3.1 A). The structure (164) of Lilia in the solid state contains linear polymeric chains, which are composed of alternating cyclo-pentadienyl rings and thallium atoms, a situation similar to that observed for cyclopentadienylindium(I). The difference between the thallium-ring distances in the gas phase and in the solid state is very great (171). 

Phosphazene sind formal Phosphorsaure-amid-phosphoryliinide. Sie konnen in Form seclis- oder achtgliedriger Ringe sowie linearer Polymere auftreten270 ... 

Gyclosilanes merit special comment because of their tendency to yield polymeric products. Three- and four-membered rings containing silicon polymerize readily under very mild conditions. Silacyclopentanes, on the other hand, give linear polymeric materials [MW (molecular weight) = 1000-2500] when heated with aluminum halides. 

The low affinity of sulfur for alkali metal ions, however, renders template effects of less consequence in the synthesis of polythia macrocycles. Thus, the competition between cycli-zation and linear polymerization is more statistically defined, with cyclization kinetically favored only at high dilution 64,65,66). Consequently, most of the synthetic methods for the synthesis of polythia rings involve high-dilution techniques coupled with relatively long reaction times. Historically, the study of the coordination chemistry of macrocyclic thioethers has been hindered by difficulties in the synthesis of the free ligands. The synthesis of [BJaneSa, first reported by Ochrymowycz and co-workers in 1977 101), illustrates this well. 

The compounds [CymB(Pz)3]Tl (136) and [FcB(Pz)3]Tl show polymeric structures, with bridging B(Pz)3 fragments in the solid state. This is a result of unfavorable steric interaction between the substituent on boron and the hydrogen atoms on the pyrazolyl ring 5-position. The structure of [Cym B(Pz)3]Tl is somewhat related, but it adopts a macrocyclic tetrameric structure rather than a linear polymeric structure. Ligands with secondary donors on the backbone may form additional bonds to the thallium atom. For example, in [HB(3-(2-pyridyl)Pz)3]Tl, weak TF"N interactions between pyridyl nitrogens and the T1 atom have been observed. HB(3-(2-MeOC6Fl4)Pz)3]Tl features close intramolecular TF 0 interactions. ... 

Perhaps the most widely exploited cyclic monomer in reactive processing of composite materials via a stepwise reaction is the oxirane or epoxy group (Hodd, 1989). Epoxy resins are principally used to form three-dimensional networks, but linear polymerization is possible. The main linear polyaddition reactions involve catalysed ring-opening in an ionic chain reaction. However, it is appropriate to consider the chemistry of the oxirane group in its reaction with nucleophilic reagents, principally amines, at this point so that the range of possible reactions may be introduced. 

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