Cyclic oligomer/chain polymer

Formation of Cyclic Oligomers. Chain transfer reactions occur by intermolecular attack of oxygen from another polyether chain on the a-methylene carbons of the oxonium ion. In an intramolecular attack a distant oxygen of the growing polymer chain itself attacks the a-methylene position of its oxonium center. 

Irrespective of the reaction mechanism, the polymerization of lactams leads to an equilibrium between monomer, cyclic oligomers and polymer. Tobolsky and Eisenberg [9] showed that the thermodynamic parameters are independent of the reaction mechanism, so that the polymerizability may be rationalized in terms of the ease of formation of the cyclic monomer, or, its opening into a linear chain unit. The simple relation between the equilibrium monomer concentration [L]e, temperature, and standard heat and entropy of polymerization. 

Polymer 128 is built upon dppb and forms a zigzag polymer as demonstrated by X-ray studies, and supports the trend where an even number of methylenes in the chain promotes such a structure.115 The use of dppp leads to a cyclic oligomer (129). Dynamic 31P NMR investigations and mass-spectrometry analyses demonstrate the presence of equilibrium between cyclic oligomers and polymers. 

S.H. Chen, H. Shi, J.C. Mastrangelo, J.J. Ou, Thermotropic chiral nematic side-chain polymers and cyclic oligomers. Prog. Polym. Sci. 21, 1211-1233 (1996)... 

In a KCP, cyclics cannot be formed by back-biting , and thus the formation of cyclic oligomers and polymers is indicative for the intermediate formation of linear chains having two reactive end groups. In other words, the formation of cyclics is indicative for simultaneous ROP and (poly)condensation steps. If side reactions are absent, and if the linear chains are mobile enough for cyclization, cyclics will be the only reaction products, and combined ROP and polycondensation is an eflhcient strategy for the preparation of cyclic polymers in high yields. 

An important source of experimental and theoretical studies of equilibria in ring formation is represented by the field of so-called macrocyclisation equilibria (Flory, 1969). Interest in this field appears to have been restricted so far to chemists conventionally labelled as polymer chemists. Experimental evidence of cyclic oligomer populations of ring-chain equilibrates such as those obtained in polysiloxanes (Brown and Slusarczuk, 1965) may be delated to the statistical conformation of the corresponding open-chain molecules (Jacobson and Stockmayer, 1950 Flory, 1969). In these studies experimental results are expressed in terms of molar cyclisation equilibrium constants Kx (14) related to the x-meric cyclic species Mx in equilibrium with the... 

In summary, the production of substituted silylenes and silyl radicals upon exhaustive irradiation at 254 nm of polysilane high polymers suggested that the polymer photochemistry resembled that previously reported for short chain acyclic and cyclic oligomers (39). More recent experiments, however, have suggested that the photochemical mechanism for the degradation of the high polymers is more complex than first envisioned (vide infra) (48). 

Bryant, J. J. L., and Semiyen, J. A., Cyclic polyesters 6. Preparation and characterization of two series of cyclic oligomers from solution ring-chain reactions of polyethylene terephthalate), Polymer, 38, 2475 (1997). 

Nearly all synthetic polymers are synthesized by the polymerization or copolymerization of different "monomers." The chain growth process may involve the addition chain reactions of unsaturated small molecules, condensation reactions, or ringopening chain-coupling processes. In conventional polymer chemistry, the synthesis of a new polymer requires the use of a new monomer. This approach is often unsatisfactory for Inorganic systems, where relatively few monomers or cyclic oligomers can be Induced to polymerize, at least under conditions that have been studied to date. The main exception to this rule is the condensation-type growth that occurs with inorganic dl-hydroxy acids. 

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