the addition of the tert-butyl nitroxide linkage, the purity of the cyclic product was
suspicious and might also have contributed to the higher value.
2.1.3 Ring Closure Through Intramolecular Coupling
of Homodifunctional Linear Polymers (A 2 ).
Linear polymers can form cyclic polymers through the coupling reaction between
their two homofunctional chain ends using external catalysts. For instance, a
dihydroxyl oligo(ethylene glycol) was cyclized to form macrocyclic crown ether
through the traditional Williamson etherification reaction. By treating with
p-toluenesulfonyl chloride (tosyl chloride) in the presence of powdery KOH, one
of hydroxyl groups was converted to tosylate. This monotosylate was further
reacted with another hydroxyl chain end to form the crown ether. Booth and
Scheme 13 Synthesis of cyclic PEO through the formation of an acetal linkage with alkaline as
catalyst
Scheme 14 Synthesis of cyclic poly(ethylene oxide-co-4-glycidyloxy-2,2,6,6-tetramethylpiperydine-1-oxyl) through the formation of an acetal linkage with alkaline as catalyst
Scheme 15 Synthesis of cyclic PSTY through radical trap-assisted atom transfer radical coupling
(RTA-ATRC)
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Z. Jia and M.J. Monteiro