Preparation of poly(vinyl alcohol) with enhanced stereoselectivity via hydrolysis of poly(vinyl acetate) produced by radical polymerization using 7-membered ring controlling agents

Controlling tacticity is crucial in polymer chemistry since it impacts the characteristics of synthetic polymers. Poly(vinyl alcohol) (PVA), generally obtained by hydrolysis of poly(vinyl acetate) (PVAc), is a prevalent polymer whose properties are susceptible to even a small change in the main chain stereostructure. Thus, new polymerization methods that improve the tacticity of PVA or its precursor polymer are highly desirable. Herein, 7membered ring compounds (tropolone (Tro-H), 2-tosyloxytropone (Tro-OTs), potassium tropolonate (Tro-K), methanesulfonic acid, 1,1,1-trifluoro-,7-oxo-1,3,5-cycloheptatrien-1-yl ester (Tro-OTf), potassium 4-((7-oxocyclohepta-1,3,5-trien-1-yl)oxy)butane-1-sulfonate (Tro-SO3K) are explored for the first time as controlling agents for the radical polymerization of VAc with azobisisobutyronitrile (AIBN) as initiator. The obtained PVAc was hydrolyzed to obtain PVA. Process parameters such as monomer concentration, amount of controlling agent and initiators were optimized. The results revealed that the hydrolysis of PVAc produced in the presence of controlling agents gave PVA with improved stereoselectivity. Higher stereoselectivity, that is, isotactic triad (mm) = 24.24% of PVA, was obtained by the hydrolysis of PVAc produced with Tro-SO3K compared to VAc polymerization with other controlling agents. Moreover, hydrolysis of PVAc produced with Tro-SO3K under a wavelength radiation source (lambda max, 365 nm) gave PVA with increased mm to 24.59% under similar conditions. In addition, flow chemistry, an emerging technology, was also used for VAc polymerization under optimized reaction conditions to obtain PVA with a mm of 24.62%. This work therefore provides an effective strategy to produce PVA with improved stereoselectivity via both batch and continuous flow techniques, enabling technical advantages for potential large-scale uses.