Price et al. [66] developed this method to prepare cyclic PEO as shown in
Scheme 16. However, their attempts to prepare high molecular weight cyclic
PEO by this method were unsuccessful.
Pang et al. [67] modified the reaction conditions by adding a poor solvent,
heptane, for PEG to the reaction solvent THF. They were able to synthesize the
cyclic poly(ethylene oxide-co-ethoxyethyl glycidyl ether) [poly(EO-co-EEGE)]
with molecular weights up to 12,000. The addition of heptane improved the ringclosure reaction efficiency by reducing the end-to-end distance. They produced
pure cyclic by binding linear unreacted or multiblock polymer chains with
α-cyclodextrin.
ATRC usually appears as a side reaction during the ATRP polymerization. For
instance, intermolecular ATRC produces high molecular weight multiblock
copolymers as side products during the polymerization of styrene using a difunctional initiator by ATRP. However, if the reaction is carried out under dilute
conditions, intramolecular radical coupling gives cyclic products. Tillman and
coworkers [68] reported a synthetic procedure for making cyclic PSTY using
ATRC. Linear dibromo-functional PSTY was activated to a diradical intermediate
to form cyclic PSTY through intermolecular radical coupling (ATRC) (Scheme 17).
Cyclization from homodifuctional polymers was also applied for synthesis of
cyclic polymers from RAFT-generated polymer precursor. Monteiro and coworkers
[69] described the process for making cyclic PSTY from linear PSTY diRAFT.
Aminolysis of the two RAFT chain ends produced the active thiol groups that,
through a disulfide linkage, produced cyclic PSTY under dilute conditions
(Scheme 18). However, if the concentration was relatively high, multiblock linear
polymer was obtained as main product. Because the cyclic product was formed
Scheme 16 Synthesis of cyclic PEO through the formation of an ether linkage
Scheme 17 Synthesis of cyclic PSTY through atom transfer radical coupling (ATRC) reaction
Synthesis of Cyclic Polymers via Ring Closure
311
Scheme 16. However, their attempts to prepare high molecular weight cyclic
PEO by this method were unsuccessful.
Pang et al. [67] modified the reaction conditions by adding a poor solvent,
heptane, for PEG to the reaction solvent THF. They were able to synthesize the
cyclic poly(ethylene oxide-co-ethoxyethyl glycidyl ether) [poly(EO-co-EEGE)]
with molecular weights up to 12,000. The addition of heptane improved the ringclosure reaction efficiency by reducing the end-to-end distance. They produced
pure cyclic by binding linear unreacted or multiblock polymer chains with
α-cyclodextrin.
ATRC usually appears as a side reaction during the ATRP polymerization. For
instance, intermolecular ATRC produces high molecular weight multiblock
copolymers as side products during the polymerization of styrene using a difunctional initiator by ATRP. However, if the reaction is carried out under dilute
conditions, intramolecular radical coupling gives cyclic products. Tillman and
coworkers [68] reported a synthetic procedure for making cyclic PSTY using
ATRC. Linear dibromo-functional PSTY was activated to a diradical intermediate
to form cyclic PSTY through intermolecular radical coupling (ATRC) (Scheme 17).
Cyclization from homodifuctional polymers was also applied for synthesis of
cyclic polymers from RAFT-generated polymer precursor. Monteiro and coworkers
[69] described the process for making cyclic PSTY from linear PSTY diRAFT.
Aminolysis of the two RAFT chain ends produced the active thiol groups that,
through a disulfide linkage, produced cyclic PSTY under dilute conditions
(Scheme 18). However, if the concentration was relatively high, multiblock linear
polymer was obtained as main product. Because the cyclic product was formed
Scheme 16 Synthesis of cyclic PEO through the formation of an ether linkage
Scheme 17 Synthesis of cyclic PSTY through atom transfer radical coupling (ATRC) reaction
Synthesis of Cyclic Polymers via Ring Closure
311