Mechanistic Insight into the Photocontrolled Cationic Polymerization of Vinyl Ethers

Abstract

The mechanism of the recently reported photocontrolled cationic polymerization of vinyl ethers was investigated using a variety of catalysts and chain-transfer agents (CTAs) as well as diverse spectroscopic and electrochemical analytical techniques. Our study revealed a complex activation step characterized by one-electron oxidation of the CTA. This oxidation is followed by mesolytic cleavage of the resulting radical cation species, which leads to the generation of a reactive cation–this species initiates the polymerization of the vinyl ether monomer–and a dithiocarbamate radical that is likely in equilibrium with the corresponding thiuram disulfide dimer. Reversible addition–fragmentation type degenerative chain transfer contributes to the narrow dispersities and control over chain growth observed under these conditions. Finally, the deactivation step is contingent upon the oxidation of the reduced photocatalyst by the dithiocarbamate radical concomitant with the production of a dithiocarbamate anion that caps the polymer chain end. The fine-tuning of the electronic properties and redox potentials of the photocatalyst in both the excited and the ground states is necessary to obtain a photocontrolled system rather than simply a photoinitiated system. The elucidation of the elementary steps of this process will aid the design of new catalytic systems and their real-world applications.

Enhancing Temporal Control and Enabling Chain‐End Modification in Photoregulated Cationic Polymerizations by Using Iridium‐Based Catalysts

“…[11,12] Thefoundation for temporal control in these polymerizations is based on two major aspects:1)the stability of the photocatalyst and 2) the delicate interplay between the oxidation of the trithiocarbonate chain end with the excited state photocatalyst to generate carbocations (Figure 1c,step I) and the recapping of the propagating carbocations by the reduced photocatalyst ( Figure 1c,s tep II). [13] Theb alance required for these two steps leads to large differences in the temporal control observed in these polymerizations when small changes are made to the catalyst structure.F or example,t riphenylpyrylium tetrafluoroborate (1a)g ives fast polymerization but poor temporal control owing to the high number of monomer additions per photon absorbed (approx. 35).…”

Enhancing Temporal Control and Enabling Chain‐End Modification in Photoregulated Cationic Polymerizations by Using Iridium‐Based Catalysts

“…Light is arguably one of the most powerful external stimuli used for polymerizations and new developments in this area will enable the synthesis of new functional materials. [13] Theb alance required for these two steps leads to large differences in the temporal control observed in these polymerizations when small changes are made to the catalyst structure.F or example,t riphenylpyrylium tetrafluoroborate (1a)g ives fast polymerization but poor temporal control owing to the high number of monomer additions per photon absorbed (approx. [11,12] Thefoundation for temporal control in these polymerizations is based on two major aspects:1)the stability of the photocatalyst and 2) the delicate interplay between the oxidation of the trithiocarbonate chain end with the excited state photocatalyst to generate carbocations (Figure 1c,step I) and the recapping of the propagating carbocations by the reduced photocatalyst ( Figure 1c,s tep II).…”

“…[10] In 2016, we reported aphotocontrolled cationic polymerization of vinyl ethers using pyrylium-based dyes as the oxidizing photocatalyst (Figure 1a,b). [13] Although the use of 1b gave the first photocontrolled cationic polymerization of vinyl ethers,f urther studies showed monomer conversion occurred after dark periods of several hours or in dark periods at high conversion. [13] Theb alance required for these two steps leads to large differences in the temporal control observed in these polymerizations when small changes are made to the catalyst structure.F or example,t riphenylpyrylium tetrafluoroborate (1a)g ives fast polymerization but poor temporal control owing to the high number of monomer additions per photon absorbed (approx.…”

“…[11,12] Thefoundation for temporal control in these polymerizations is based on two major aspects:1)the stability of the photocatalyst and 2) the delicate interplay between the oxidation of the trithiocarbonate chain end with the excited state photocatalyst to generate carbocations (Figure 1c,step I) and the recapping of the propagating carbocations by the reduced photocatalyst ( Figure 1c,s tep II). [13,14] We therefore sought other photocatalysts for these polymerizations that had increased stability compared to the pyryliums but similar redox potentials to 1b to give enhanced temporal control. 35).…”

“…[13] Potassium ferrioxalate actinometry data of initial reaction rates when targeting a5kg mol À1 polymer revealed that polymerizations with 2a and 2bgave 4and 3monomer additions per photon absorbed, respectively ( Figure 2). In our previously reported photocontrolled polymerizations,p yrylium 1a and 1b yielded 35 and 6m onomer additions per photon absorbed, respectively.…”