Recent remarkable progress of precision polymerization
techniques
has allowed researchers to synthesize polymers of controlled molecular
weight, tacticity, and monomer sequence with narrow distributions.
However, it is still a challenging subject to synthesize uniform polymers
and oligomers, which possess specific molecular weight, tacticity,
and monomer sequence. Utilizing the advantages of t-butyl 4-azido-5-hexynoate (tBuAH), which we designed recently, for
the synthesis of uniform polymers and oligomers, uniform oligomers
of different stereoregularities possessing a dense 1,2,3-triazole
backbone were synthesized using optically active tBuAH precursors
by iterative procedures containing azidation, deprotection of the
protecting group, and copper(I)-catalyzed azide–alkyne cycloaddition
(CuAAC). The stereoregular uniform oligomers were characterized in
the solid state by powder X-ray diffraction (PXRD) and in the solution
state by pulse-field-gradient spin-echo (PGSE) NMR, ultraviolet (UV)
absorption, and circular dichroism (CD) spectroscopic techniques.
While the solubility of stereoregular uniform oligomers was investigated
using common organic solvents, gelation was observed for solutions
of two octamers of different stereoregularities in some solvents,
e.g., toluene, tetrahydrofuran (THF), acetone, and methanol. It is
noteworthy that the intermolecular attractive interaction depended
on the stereoregularity in aprotic polar solvents, i.e., THF and acetone,
presumably because of the different polarities of octamers.