|508. Leibig, D.; Müller, A.H.E.; Frey, H.: Anionic Polymerization of Protected Vinyl Catechols: Reversal of the Monomer Gradient Directed by the Position of the Catechol Moiety in the Copolymerization with Styrene, Macromolecules 49, 4792-4801 (2016) -- DOI: 10.1021/acs.macromol.6b00831|
The catechol containing vinyl monomers 4-vinyl catechol acetonide (4-VCA) and 3-vinyl catechol acetonide (3-VCA) are introduced for carbanionic polymerization in THF, using sec-butyllithium as an initiator. Molecular weights (Mn) ranging from 3.000 to 80.000 g mol-1 were obtained, with polydispersities (Mw/Mn) below 1.10 for 4-VCA and 1.15 for 3-VCA homopolymerization. Furthermore, block copolymers and gradient copolymers with styrene have been prepared via living carbanionic copolymerization. The reactivity of the new monomers 4-VCA and 3-VCA in the copolymerization with styrene and the resulting monomer gradient in the copolymer chains were investigated via in-situ 1H NMR spectroscopic kinetic studies in toluene‑d8. The results show lower reactivity of the 4-VCA monomer than styrene (rS = 4.0, r4‑VCA = 0.24) and a higher reactivity than styrene for 3-VCA (r3-VCA = 2.4, rS = 0.48). Well-defined copolymers of styrene and 4-VCA exhibit a strong gradient structure within the polymer chains with the catechol functionalities preferentially incorporated near the chain terminus. However, in case of 3-VCA, the gradient structure of the copolymers is reversed, and the catechol functionalities are preferentially incorporated in the vicinity of the initiator. The direction of the monomer gradient in the copolymers can be predicted from the difference of the chemical shift of the β-carbon signal of the respective vinyl monomers in 13C NMR spectra, which has general implications for the copolymerization of vinyl monomers. All polymers were characterized by 1H NMR spectroscopy, size exclusion chromatography (SEC), MALDI-ToF mass spectrometry and differential scanning calorimetry (DSC). Quantitative cleavage of the acetonide protecting group under mild acidic conditions rendered well-defined poly(vinyl catechol)s, which were used for pH-sensitive precipitation of iron(III) cations and for surface coating on a variety of materials, showing very stable and permanent catechol-promoted adhesion.