2.2 Ring Closure Through Hetrodifunctional Linear
Polymers
Linear polymers with two different functional groups on each end of the polymer
chain are usually named α,ω-heterodifunctional polymers. Through the proper
combination of the two functional chain ends, cyclic polymers can be made through
the intramolecular coupling reaction. This methodology has been widely used to
synthesize cyclic polymers with various chemical compositions and chain
structures. The advantage of this method compared to the heterodifunctional bimolecular coupling is that there is no issue caused by inaccurate stoichiometries.
Moreover, with the complimentary of “living” radical polymerization techniques, by
using functional initiators and post-functionalization, these α,ω-heterodifunctional
linear polymer precursors can now be made relatively easily. However, intermolecular
coupling reactions are unavoidable and therefore highly dilute solution conditions are
necessary.
Schappacher and Deffieux [93] first reported the cyclization from heterodifunctional linear polymers. The linear precursor was made from 2-chloroethyl
vinyl ether (CEVE) by living cationic polymerization from a styryl vinyl ether
with hydroiodic acid and ZnCl 2 as catalyst combination. By treating with SnCl 4 , the
iodo endgroup was converted to carbocation and coupled to the styrene chain end to
form the cyclic polymer (Scheme 24). With the same strategy, they successfully
synthesized a series of cyclic polymer derivatives based on the monomer CEVE.
Further extension of this technique used a similar cyclization reaction from
polystyrene by living anionic polymerization [31]. In this case, an acetal functional
anionic initiator was used to polymerize the styrene followed by capping with
1,1-diphenylethene and termination with p-chloromethylstyrene to introduce a
styryl group to another end of the PSTY chain. Then, one of the ethyloxyl groups
of the acetal chain end was converted to an iodo group, making the chain end
similar to that after polymerization of CEVE using the iodo/ZnCl 2 combination.
The cyclization reaction was then catalyzed by SnCl 4 as shown in Scheme 25.
Kubo and coworkers [76] reported synthesis of cyclic PSTY through living anionic
polymerization followed by amidation reaction between the carboxylic acid and
amine chain ends. In their work, ortho-ester initiator was applied to polymerize STY
by living anionic polymerization. The living chain end was then terminated by 2,2,5,5tetramethyl-1-(3-bromopropyl)-1-aza-2,5-disilacyclopentane, a protected amine
bromo compound, to give a linear PSTY precursor with protected carboxylic acid
and protected amine on each end. After deprotection by acid and base, the carboxylic
acid and amine group were then released. This linear precursor was then cyclized via
an amidation reaction catalyzed by 1-methyl-2-chloropyridinium iodide (Scheme 26).
Through modification of initiator structures, the authors then successively reported the
synthesis of cyclic poly(methyl methacrylate) (PMMA) and cyclic poly(tert-butyl
acrylate) (P
t
BA). The latter was further converted to cyclic poly(acrylic acid) and
cyclic poly(potassium acrylate) [77].
Synthesis of Cyclic Polymers via Ring Closure
315
Polymers
Linear polymers with two different functional groups on each end of the polymer
chain are usually named α,ω-heterodifunctional polymers. Through the proper
combination of the two functional chain ends, cyclic polymers can be made through
the intramolecular coupling reaction. This methodology has been widely used to
synthesize cyclic polymers with various chemical compositions and chain
structures. The advantage of this method compared to the heterodifunctional bimolecular coupling is that there is no issue caused by inaccurate stoichiometries.
Moreover, with the complimentary of “living” radical polymerization techniques, by
using functional initiators and post-functionalization, these α,ω-heterodifunctional
linear polymer precursors can now be made relatively easily. However, intermolecular
coupling reactions are unavoidable and therefore highly dilute solution conditions are
necessary.
Schappacher and Deffieux [93] first reported the cyclization from heterodifunctional linear polymers. The linear precursor was made from 2-chloroethyl
vinyl ether (CEVE) by living cationic polymerization from a styryl vinyl ether
with hydroiodic acid and ZnCl 2 as catalyst combination. By treating with SnCl 4 , the
iodo endgroup was converted to carbocation and coupled to the styrene chain end to
form the cyclic polymer (Scheme 24). With the same strategy, they successfully
synthesized a series of cyclic polymer derivatives based on the monomer CEVE.
Further extension of this technique used a similar cyclization reaction from
polystyrene by living anionic polymerization [31]. In this case, an acetal functional
anionic initiator was used to polymerize the styrene followed by capping with
1,1-diphenylethene and termination with p-chloromethylstyrene to introduce a
styryl group to another end of the PSTY chain. Then, one of the ethyloxyl groups
of the acetal chain end was converted to an iodo group, making the chain end
similar to that after polymerization of CEVE using the iodo/ZnCl 2 combination.
The cyclization reaction was then catalyzed by SnCl 4 as shown in Scheme 25.
Kubo and coworkers [76] reported synthesis of cyclic PSTY through living anionic
polymerization followed by amidation reaction between the carboxylic acid and
amine chain ends. In their work, ortho-ester initiator was applied to polymerize STY
by living anionic polymerization. The living chain end was then terminated by 2,2,5,5tetramethyl-1-(3-bromopropyl)-1-aza-2,5-disilacyclopentane, a protected amine
bromo compound, to give a linear PSTY precursor with protected carboxylic acid
and protected amine on each end. After deprotection by acid and base, the carboxylic
acid and amine group were then released. This linear precursor was then cyclized via
an amidation reaction catalyzed by 1-methyl-2-chloropyridinium iodide (Scheme 26).
Through modification of initiator structures, the authors then successively reported the
synthesis of cyclic poly(methyl methacrylate) (PMMA) and cyclic poly(tert-butyl
acrylate) (P
t
BA). The latter was further converted to cyclic poly(acrylic acid) and
cyclic poly(potassium acrylate) [77].
Synthesis of Cyclic Polymers via Ring Closure
315