Photoinitiated Metal Free Living Radical and Cationic Polymerizations

Abstract

Although conventional photoinitiated radical and cationic polymerizations are widely used in many industrial applications, they usually proceed in an uncontrolled manner. Recent developments in photomediated living/controlled radical and cationic polymerization made it possible to prepare various well-defined polymers with complex architecture under mild conditions. Various methods described for cationic and radical polymerization utilizes metal or Lewis acid catalysts. Several strategies eliminating the use of such additives in photoinduced controlled/living radical and cationic polymerizations have been developed. This article describes various photoinitiation systems acting in UV-vis range. Their mechanistic details are also evaluated and several specific examples involving combination both free radical and cationic routes are presented.

Mechanistic Insight into the Photocontrolled Cationic Polymerization of Vinyl Ethers

“…Although the plot of conversion versus time clearly illustrates that polymerization proceeds only in the presence of light with 1a (Figure 14A), the corresponding plot for 1b shows that polymerization is not halted in the dark (Figure 14B). Consequently, pyrylium 1b should be considered a fine catalyst for photo initiated , rather than photo controlled , cationic polymerization, 1f as it allows for predictable M n and a Đ of ∼1.2 but offers no temporal control of chain growth. This outcome also suggests that the recapping step (step IV) is much slower for 1b than for 1a , which can be explained by the difference in ground state redox potentials ( ${E°}_{\mathrm{bold1}{\mathbf{a}}^{+}/\mathrm{bold1}{\mathbf{a}}^{•}}=-0.50\text{ V versus SCE},\text{ and }E{°}_{\mathrm{bold1}{\mathbf{b}}^{+}/\mathrm{bold1}{\mathbf{b}}^{•}}=-0.32\text{ V versus SCE}$).…”

“…1 The intrinsic resolution of light enables unparalleled spatial control over these polymerizations, a factor that may prove desirable in a wide array of settings. Photocontrolled variants of living radical polymerizations, including atom-transfer radical-polymerization (ATRP), 2 organotellurium-mediated radical polymerizations (TERP), 3 and reversible addition–fragmentation chain transfer (RAFT) polymerizations, 4 have already demonstrated usefulness with a variety of monomers.…”

Simple Photochemical Route to Block Copolymers via Two‐Step Sequential Type II Photoinitiation

“…Upon irradiation, the singlet excited state of Type II photoinitiators undergoes intersystem crossing to form triplet states which essentially abstracts hydrogen from suitable hydrogen donors, such as amines, ethers, and alcohols forming free radicals capable of initiating free radical polymerization ( Scheme ). Besides conventional free radical polymerization, Type II photoinitiatiors can be used in photoinduced metal free atom transfer radical polymerization, the preparation of hyperbranced polymers with different branching densities through self‐condensing vinyl polymerization and nanocomposites …”

“…Photoinitiated free radical polymerization by Type II photoinitiators. free atom transfer radical polymerization, [39] the prepara tion of hyperbranced polymers with different branching densities through selfcondensing vinyl polymerization [40] and nanocomposites. [41]…”

Synthesis of Polysulfone Based Amphiphilic Graft Copolymers by a ‘Grafting to’ Approach

“…21,22 Such processes provide numerous benefits including practicality at low temperatures, reduced energy consumptions, rapid reaction rates, and minimized side reactions. 21,22 Such processes provide numerous benefits including practicality at low temperatures, reduced energy consumptions, rapid reaction rates, and minimized side reactions.…”