Bio-Based Functional Styrene Monomers Derived from Naturally Occurring Ferulic Acid for Poly(vinylcatechol) and Poly(vinylguaiacol) via Controlled Radical Polymerization

A naturally occurring vinylphenolic compound, 4-vinylguaiacol (4VG: 4-hydroxy-3-methoxystyrene), which is derived from naturally occurring ferulic acid via decarboxylation, was used for the synthesis of well-defined bio-based poly(vinylguaiacol) and poly(vinylcatechol) with phenolic functions. Through one-step chemical conversions of 4VG, a series of 4VG derivatives protected with acetyl (Ac4VG), tert-butyldimethylsilyl (TBDMS4VG), or triethylsilyl (TBDMS4VG) groups as well as bis(triethylsilyl)-protected vinyl catechol (TES2VC) were synthesized with high yields (>90%). The controlled radical polymerization of these protected bio-based phenolic styrene monomers successfully proceeded in the presence of appropriate reversible addition fragmentation chain transfer (RAFT) agents or alkoxyamines with or without radical initiators such as 4,4'azobis(isobutyronitrile) (AIBN) to result in polymers with controlled molecular weights. Bimolecular combination reactions were suppressed even at the high monomer conversions of >95%, especially for TES2VC possessing two bulky substituents, indicating the excellent living character of the polymerization in comparison to other styrene monomers. Deproteciion of the silyl groups was easily attained with hydrochloric acid in THE and water to result in well-defined poly(vinylguaiacol) and poly(vinylcatechol) without loss of the RAFT terminals. These bio-based polystyrenes with phenolic functions were soluble in methanol and alkaline solution. The block copolymerization of TES2VC was accomplished with various common vinyl monomers, such as styrene, methyl methacrylate, methyl acrylate, and n-butyl acrylate, in the presence of their prepolymers as macro-RAFT agents, resulting in well-defined catechol-containing block copolymers after deprotection without any damage to the ester substituents and RAFT terminals.