Poly(arylenevinylene)s through Ring‐Opening Metathesis Polymerization of an Unsymmetrical Donor‐Acceptor Cyclophane

Abstract

Reported are well-defined donor-acceptor alternating copolymers prepared using ring-opening metathesis polymerization (ROMP). Unsymmetrical cyclophanedienes comprising electron-donating (4-methoxy-1-(2-ethylhexyl)oxy)benzene( MEH) and electron-accepting benzothiadiazole (BT) rings were synthesized from the corresponding [3.3]dithiaparacyclophanes.R OMP of the strained unsymmetrical and "electronically-ambiguous" cyclophanedienes proceeded in ac ontrolled manner in the presence of either Hoveyda-GrubbsI Io rG rubbs II initiator in wake of both steric and electronic encumbrance.T he resulting polymers, comprising alternating BT and MEH-PPV units,are achieved in molecular weights exceeding 20k with values ranging from 1.1-1.4. The living nature of the polymerization is verified through the formation of rod-coil and rod-rod block copolymers.O ur strategy to develop previously unrealized polymers from functional building blocks featuring alocked-in D-A unit is significant in afield striving to achieve well-defined and sequence-specific materials.

Electronically Governed ROMP: Expanding Sequence Control for Donor–Acceptor Conjugated Polymers

“…Upon initiator screening for MEH-BT, we found that polymerizations initiated by Grubbs II and Hoveyda-Grubbs II yielded the highest conversions. 7 Specifically, ROMP of -OEH eclipsed 9b led to 100% conversion, whereas -OMe eclipsed 9a optimized at 75% conversion. We attributed the higher relative reactivity of (9b) to a lessened steric crowding with the -OEH exogenously eclipsed with BT, which exposes the 'backside' of the monomer for CM to occur more efficiently.…”

“…We envisioned that a monomer system incorporating both donor and acceptor moieties could preorganize D-A dyads with electronic bias dictating sequencing (Scheme 6). 7 Whereas KCTP still does not demonstrate a purely electronically biased polymerization, other polymerization methods initiated by metals should be capable of featuring bias. In contrast to KCTP, the ring-opening metathesis polymerization of strained cycloalkenes is likely to exhibit both steric and electronic bias in concert, given metallacyclobutane formation is driven by both factors.…”

“…Notably, electronic and steric factors surrounding both the diene monomer and the Grubbs initiator were expected to have a pronounced effect on ROMP, 7 given the polymerization is contingent on the initial cross-metathesis (CM) event. Typically, CM reactions are electronically governed, with electronically deficient species leading to modest success, 55 while the combination of both electronically deficient and sterically-hindered olefins often results in no metathesis.…”

“…We established the synthesis route outlined in Scheme 7, which demonstrated that MEH-BT pCpd could be obtained in reasonable yields in wake of notable differences in reactivity of the BT core relative to conventional benzenebased precursors. 7 Prior to ROMP, we gained preliminary insights into the steric environment of both conformers using density functional theory with ORCA. 56,57 Each isomer, de facto, presents a suitably strained structure, with the [2.2]cyclophanediene cores are similar to pseudo-gem MEH-pCpd: 53 both systems exhibit boatlike conformations, with - distances encouraged by donor-acceptor interactions (d - = 3.05 Å).…”

“…1 Synthetic methods to engineer small molecules and polymers continue to evolve and have led to the advent of photoredox chemistry, 3 electrochemical syntheses, 4 and other pioneering methods; yet, full emulation of nature's aptitude remains a burgeoning effort. Methods to exert control over monomer sequence [5][6][7][8][9] are commonly being explored in polymeric materials motivated in part by biosystems that display control over architecture and thus, properties. Thus, a holy grail quest in polymer synthesis remains focused on engineering precise monomer sequence.…”