Macrocyclic oligomer polymerization thermodynamics

Macrocyclic oligomer precursors of polycarbonates, PET, polymers of amides, etherketones, and ethersulfones are candidates for further study of macrocyclic oligomer polymerization thermodynamics [6]. Research extends to cyclic arylates (cyclic aryl-aryl esters) and cyclic alkyl aryl esters going back to isolating a cyclic trimer of poly(ethylene-terephthalate) in 1954 [13]. Much of the work on macrocyclic oligomers as precursors to high-MW macromolecules starts with spiro(bis)indane (SBI) biphenyl monomer to produce macrocyclic carbonates. 

Various authors, who studied the mechanism of cationic ring-opening polymerization, observed the formation of some specific cyclic oligomers there were, however, no attempts to rationalize observed distribution on theoretical grounds, i.e. the preferential formation of certain oligomers or the apparent absence of cyclic oligomers in some systems. Several reviews have been published on macrocyclization but usually they concentrate either on the thermodynamic 5) or synthetic aspects 4). Indeed, both these approaches, thermodynamic and synthetic , were developed simultaneously but have not correlated with each other. In the most papers concerned with synthesis the Jacobson-Stockmayer theory was not even quoted. 

The small contribution of macrocyclization in the polymerization of THF is due to two effects The formation of smaller cyclic oligomers like dimer and trimer is thermodynamically unfavorable (in contrast to the formation of e.g. 1,4-dioxane in the polymerization of ethylene oxide) because 10- and 15-membered cyclic ethers are strained. It is also kinetically hampered since the rate of chain transfer to polymer which leads to ring formation is low due to the lower basicity of the polymer units than that of THF. 

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