Detailed Approach to Investigate Thermodynamically Controlled Supramolecular Copolymerizations

Elucidating
the microstructure of supramolecular copolymers remains
challenging, despite the progress in the field of supramolecular polymers.
In this work, we present a detailed approach to investigate supramolecular
copolymerizations under thermodynamic control. Our approach provides
insight into the interactions of different types of monomers and hereby
allows elucidating the microstructure of copolymers. We select two
monomers that undergo cooperative supramolecular polymerization by
way of threefold intermolecular hydrogen bonding in a helical manner,
namely, benzene-1,3,5-tricarboxamide (BTA) and benzene-1,3,5-tris­(carbothioamide)
(thioBTA). Two enantiomeric forms and an achiral analogue of BTA and
thioBTA are synthesized and their homo- and copolymerizations are
studied using light scattering techniques, infrared, ultraviolet,
and circular dichroism spectroscopy. After quantifying the thermodynamic
parameters describing the homopolymerizations, we outline a method
to follow the self-assembly of thioBTA derivatives in the copolymerization
with BTA, which involves monitoring a characteristic spectroscopic
signature as a function of temperature and relative concentration.
Using modified types of sergeants-and-soldiers and majority-rules
experiments, we obtain insights into the degree of aggregation and
the net helicity. In addition, we apply a theoretical model of supramolecular
copolymerization to substantiate the experimental results. We find
that the model describes the two-component system well and allows
deriving the hetero-interaction energies. The interactions between
the same kinds of monomers (BTA–BTA and thioBTA–thioBTA)
are slightly more favorable than those between different monomers
(BTA–thioBTA), corresponding to a nearly random copolymerization.
Finally, to study the interactions of the monomers at the molecular
level, we perform density functional theory-based computations. The
results corroborate that the two-component system exhibits a random
distribution of the two monomer units along the copolymer chain.