Discovery of an Anionic Polymerization Mechanism for High Molecular Weight PPV Derivatives via the Sulfinyl Precursor Route

Abstract

The polymerization of PPV via the sulfinyl precursor route has been investigated with respect to its mechanism. When polymerized in sec-butanol, a purely radical polymerization mechanism is observed as in most precursor polymerization routes. Accordingly, an increase in the reaction temperature induced an increase in the overall yield alongside with a reduction of the average molecular weight of the polymer. Upon changing the monomer concentration in solution before addition of the base NatBuO, an increase in molecular weight is observed, signifying that the polymerization is faster than the mixing of the two reaction components. When changing the solvent to NMP, a competition of anionic and radical polymerization has been established while in THF an anionic polymerization mechanism occurs exclusively. To prevent termination reactions, LDA and LHMDS were introduced as base whereby LHMDS shows less propensity to initiate anionic chain growth due to higher steric hindrance. With polymerizations in presence of the radical quencher TEMPO, the anionic polymerization mechanism could unambiguously be proven.

Anionic PPV polymerization from the sulfinyl precursor route: Block copolymer formation from sequential addition of monomers

“…In our previous studies we demonstrated that a pure anionic PPV polymerization can be achieved [20,21]. As mentioned above, however, deviations from ideal polymerization were still observed before and doubts remained about the initiation efficiency of the employed initiator.…”

“…All earlier published [20,21] and above described polymers were synthesized via the anionic route using regular Schlenk lines (vacuum/N 2 ). For comparison, experiments using different equivalents of anionic initiator 1 were repeated on a high vacuum line anionic polymerization setup with a high vacuum pump which is available at the University of Groningen.…”

“…Polydispersities of the PPV blocks remained, however on a level of roughly 2 or larger as must be expected for a chain transfer polymerization. Also, in previous studies, we recently demonstrated that the sulfinyl synthesis route allows for switching between the radical polymerization route and an anionic polymerization mode depending on the choice of solvent and base used to start the reaction (Scheme 1) [20]. In the sulfinyl route, a strong base is generally required to eliminate a leaving group from a precursor monomer to form a polymerizable quinodimethane system (see Scheme 1 for structure).…”

Living Polymerization via Anionic Initiation for the Synthesis of Well‐Defined PPV Materials

“…To reach this aim, the free energy of the initiation step must defi ned. [ 14 ] Therefore, it is a matter of priority to identify more effi cient control methodologies for PPV polymerization. [ 9 ] Already, a few years ago, it was shown that the anionic polymerization could be suppressed by using a protic environment (thus quenching active chain propagating sites) and the radical polymerization could be stopped by addition of a radical scavenger such as 2,2,6,6-tetramethylpiperidine-N-oxyl.…”

“…[ 8 , 15 ] Only recently, however, we reported on conditions under which a selective anionic polymerization for reasonable yields and molecular weights could be achieved. [ 14 ] To reach this aim, polymerizations via the sulfi nyl precursor route are carried out in tetrahydrofuran (THF; rather than an alcohol, which is otherwise often used as solvent) and the bulky lithium hexamethyldisilazide (LHMDS) as the base. Still, although polymerization solely proceeded via an anionic polymerization mechanism, broad-molecular-weight distributions and only little control over the molecular weight could be obtained because the base acted both as the coinitiator of the reaction (by formation of the quinodimethane) and as the anionic initiator of the polymerization.…”