During the preparation of higher molecular weight polymers by the step growth "click" coupling of homo- and hetero-telechelic polystyrenes in DMF solution it was noted that a lower molecular weight polymer was formed. This was assumed to be the result of an intramolecular "click"cyclization.(1,2)
α,ω-Diazido-terminated polystyrene oligomers were prepared by ATRP of styrene with dimethyl 2,6-dibromoheptadioate as an initiator followed by nucleophilic displacement of the bromine end groups with NaN3. The resulting difunctional homotelechelic "monomer" was chain extended with propargyl ether at room temperature to afford higher molecular weight polystyrene. Because the click reaction was conducted in N,N-dimethylformamide (DMF), no additional ligand was necessary to solubilize the CuBr catalyst. α-Alkyne-ω-azido-terminated polystyrene was prepared by ATRP of styrene with propargyl 2-bromoisobutyrate as an initiator and subsequent post-polymerization nucleophilic substitution of the bromine end groups by reaction with NaN3. The resulting heterotelechelic polystyrene was click coupled in DMF with CuBr as the catalyst. A one-pot, two-step ATRP-nucleophilic substitution-click coupling process was also successful. For all three approaches, click coupling of the telechelic polystyrene "monomers" (Mn = 960 - 2590 g/mol) yielded moderate to high molecular weight polystyrene (Mn up to 21500 g/mol) with molecular weight distributions characteristic of step growth polymers (Mw/Mn = 2 - 5). However in all cases some intramolecular click coupling also occurred in DMF producing cyclic structures, as evidenced by the formation of product with lower hydrodynamic volume than the starting material. The cyclization reaction was found to be concentration dependant and more dilute solutions provided a higher yield of cyclized product.
A modification of this approach was taken by Grayson(3) who effectively maintained a low concentration of the α-alkyne-ω-azido-terminated polymer the Cu(I)Br and 2,2'-bipyridine catalyst in a warm DMF solution by using a syringe pump to add a 2 mM solution of the l-PS-N3 in DMF over 25 hours. The product, c-PS, was isolated by extraction and precipitation. This approach has also been applied to polymers prepared by nitroxide mediated polymerization.(4) A polystyrene bearing a benzyl chloride on the α-terminus was converted to an azide by SN2 displacement, followed by oxidative replacement of the alkoxyamine at the ω-terminus with an acetylene grouup using ceric ammonium nitrate and propargyl alcohol. Macrocyclization using copper(I) catalyzed Huisgen 1,3-dipolar cycloaddition resulted predominately in macrocyclic polymers.
A different shaped "tadpole" molecule, one with a cyclic polystyrene as the head and a linear poly(N-isopropylacrylamide) as the tail was successfully synthesized by combination of reversible addition-fragmentation chain transfer (RAFT) polymerization and a "click" reaction.(5) The synthesis involves two main steps, initial preparation of a linear acetylene-terminated PNIPAAM-b-PS with a side azido group anchored at the junction between two blocks followed by an intramolecular cyclization reaction to produce the cyclic PS block using "click" chemistry under high dilution. A difference in the surface properties between the tadpole-shaped polymer and its linear precursor was observed with the water contact angles on the former surface being greater than that on the latter surface. A figure eight shaped molecule was formed when the first functional polymer contained two azide groups at the junction of the two polymer segments.(6) Another amphiphilic tadpole molecule consisting of a polystyrene ring and a poly(ethylene oxide) tail was synthesized via ATRP and click chem.(7) Self-assembly of a c-PS-b-PEO sample and its linear precursor in water was preliminarily investigated by TEM and the vesicles formed from the c-PS-b-PEO were much larger than those formed from the linear analog.
(1) Tsarevsky, N. V.; Sumerlin, B. S.; Golas, P. L.; Matyjaszewski, K. Polymer Preprints 2005, 46, 179-180.
(2) Tsarevsky, N. V.; Sumerlin, B. S.; Matyjaszewski, K. Macromolecules 2005, 38, 3558-3561.
(3) Laurent, B. A.; Grayson, S. M. Journal of the American Chemical Society 2006, 128, 4238-4239.
(4) O'Bryan, G.; Ningnuek, N.; Braslau, R. Polymer 2008, 49, 5241-5248.
(5) Shi, G.-Y.; Tang, X.-Z.; Pan, C.-Y. J. Polym. Sci., Part A: Polym. Chem. 2008, 46, 2390-2401.
(6) Shi, G.-Y.; Pan, C.-Y. Macromol. Rapid Commun. 2008, 29, 1672-1678.
(7) Dong, Y.-Q.; Tong, Y.-Y.; Dong, B.-T.; Du, F.-S.; Li, Z.-C. Macromolecules 2009, 42, 2940-2948.