Similarities and differences of epoxide, aldehyde and peroxide initiators for Cp2TiCl-catalyzed styrene living radical polymerizations

Cp2TiCl is the first example of a single electron transfer (SET) agent that both provides initiating radicals from three different types of functionalities (i.e. radical ring opening of epoxides and reduction of aldehydes and peroxides) and doubles as mediator for the living radical polymerization of styrene (St) by reversibly endcapping the growing polymer chains. An initiator (I) comparison was performed using 1,4-butanediol diglycidyl ether (BDE), benzaldehyde (BA) and benzoyl peroxide (BPO) as models. The investigation of the effect of reaction variables was carried out over a wide range of experimental conditions ([Cp2TiCl2]/[I] = 0.5/1–4/1; [Zn]/[Cp2TiCl2] = 0.5/1–3/1, [St]/[I] = 50/1–400/1 and T = 60–130 °C) to reveal living polymerization features such as a linear dependence of molecular weight on conversion and narrow molecular weight distribution (Mw/Mn) for each initiator class. However, progressively lower polydispersities and larger initiator efficiencies are obtained with increasing the [Cp2TiCl2]/[I] and [Zn]/[Cp2TiCl2] ratios and with decreasing temperature. Accordingly, optimum conditions correspond to [St]/[I]/[Cp2TiCl2]/[Zn] = [50–200]/[1]/[2–3]/[4–6] at 70–90 °C. By contrast to peroxides, aldehydes and the more reactive epoxides provide alcohol end groups useful in block or graft copolymers synthesis.