Sequence-regulated copolymerization based on periodic covalent positioning of monomers along one-dimensional nanochannels

Abstract

The design of monomer sequences in polymers has been a challenging research subject, especially in making vinyl copolymers by free-radical polymerization. Here, we report a strategy to obtain sequence-regulated vinyl copolymers, utilizing the periodic structure of a porous coordination polymer (PCP) as a template. Mixing of Cu²⁺ ion and styrene-3,5-dicarboxylic acid (S) produces a PCP, [Cu(styrene-3,5-dicarboxylate)] _n , with the styryl groups periodically immobilized along the one-dimensional channels. After the introduction of acrylonitrile (A) into the host PCP, radical copolymerization between A and the immobilized S is performed inside the channel, followed by decomposing the PCP to isolate the resulting copolymer. The predominant repetitive SAAA sequence in the copolymer is confirmed by monomer composition, NMR spectroscopy and theoretical calculations. Copolymerization using methyl vinyl ketone also provides the same type of sequence-regulated copolymer, showing that this methodology has a versatility to control the copolymer sequence via transcription of PCP periodicity at the molecular level.

Fig. 1

Schematic illustration of sequence-regulated radical copolymerization using PCPs. A vinyl monomer (X) is pre-immobilized at a regular interval along the nanochannels of a PCP (PCP⊃X). Another vinyl monomer (Y) is then incorporated into the PCP, giving a host–guest composite (PCP⊃(X + Y)). Polymerization of the monomers in the composite followed by the removal of host structure produces sequence-regulated copolymers reflecting on the periodicity of the PCP nanochannels

Fig. 2

Crystal structure of host PCP. Views of the nanochannel structure of 1S⊃H₂O along the c-axis (a) and a-axis (b). Atoms: Cu (purple), O (red), C (grey). Styryl groups are periodically aligned along the c-axis at a distance of 6.8 Å on each face of the hexagonal channel. H atoms and one of the disordered vinyl moieties have been omitted for clarity

Fig. 3

XRPD patterns of 1S with guests. XRPD patterns of 1S⊃H₂O, 1S⊃A and 1S⊃A after heating at 70 °C. The same diffraction patterns were observed in all the samples, showing that the crystal structure of the host was maintained during the monomer adsorption and the copolymerization processes

Fig. 4

Structural characterization of polymers using ¹H NMR. The ¹H NMR spectra of (co)polymers synthesized in 1S (P1) and in DMF (R1-6 and polyS). With decreasing S unit in polymers prepared in DMF, peaks for the aromatic protons of the S unit (6.5–8.5 ppm) shifted to lower magnetic field because of the lower shielding effect around S in A-rich copolymers. Although the composition of S in P1 is larger than those of R3 and R4, peaks at 6.5–7.6 ppm were not observed, indicating that S units in P1 are solitary among the repeating A units

Fig. 5

Structural characterization of polymers using ¹³C NMR. ¹³C NMR spectra focusing on carbonyl (a) and methine carbons (b) of polymers, from which S- and A-centred triads can be analyzed, respectively. Plausible monomer sequences of random copolymers (R1–6) are shown together with their NMR spectra. The predominant triads of P1 could be estimated to be ASA, AAS and AAA by comparison of the peak shapes and peak positions for P1, random copolymers (R1–6) and polyS

Fig. 6

Copolymer composition plots for the copolymerization of A with S. A series of A ratio in the generated copolymers were plotted against their initial A ratio for the copolymerization of A with S in 1S (filled red circle) and in DMF (open circle). Note that the maximum initial ratio of A is 0.42 in the channels of 1S due to the limitation of introduction amount of A into the host