Synthesis and characterization of novel Methacrylates bearing A 2 (Hydroxymino) Aldehyde group and their copolymers

In the last few decades, “smart”polymers have gained an increasing attention in the scientific community thanks to their large use in several applications, such as drug delivery, sensors, separation techniques and catalysts. The high versatility of smart materials comes from their tunable properties and their adaptability in response to external chemical and physical stimuli, such as temperature, light, pH and electron transfer. Currently, the research on the field of multi-stimuli-responsive polymers has been focused both on design of new polymer bearing two or more functionalities and on the development of novel responsive groups. In particular, hydrophilic multi-stimuli-responsive polymers are of special interest for their potential employments in aqueous solutions. In this context, oximes have been occasionally investigated as stimulus-responsive moieties, although they have a wide use in the field of macromolecules as, for example, photoinitiators for radical polymerization or reversible linker in crosslinked networks and bioconjugate polymers. The introduction of the oxime moiety can be realized applying the α-oximation of aldehydes via organo-somo catalysis developed, recently, in our group. A reaction which affords 2-(hydroxyimino)aldehydes (HIA) compounds from good to excellent yield. 2-(hydroxyimino)aldehyde (HIA) are molecules characterize by an aldehyde group adjacent to an oxime group, which endows them with some interesting properties, such as an unusually low pKa compared to simple oxime molecules and the existence of two configurational isomers E and Z that can be interchanged by light stimulus. Furthermore, the aldehyde group can bind bioactive molecules and the oximate ion can be oxidized reversibly in the range of 0.38 to 0.64 V vs. NHE. The aim of the present work has been the investigation of the possibilities offered by the highly versatile HIA group towards obtaining novel multi-stimuli responsive polymers. Copolymers of OEGMA with methacrylates bearing the 2-(hydroxyimino)aldehyde (HIA) group in the side chain have been prepared in order to combine the properties of the novel moiety HIA with the temperature responsiveness in aqueous solution of OEGMA. In the first part of the work a monomer encompassing the HIA group (HIABMA) was synthetized and characterized. In particular, CNO-H pKa and BDE, oxidation potential of oxime and oximate anion were assessed. Then, random copolymers of OEGMA with different content of HIABMA (from 10% to 50%) and a 1:1 diblock copolymer were prepared through RAFT method. All the obtained copolymers showed low dispersity (Ð less than 1.2), high conversion (up to 94%) and linear dependence of Mn vs. conversion during the reaction. In order to predict the photochemical response of HIA group in a polymer, the second part of the work was dedicated to the study of the photochemical behavior of several small HIAs bearing different substituents adjacent to the oxime group. Depending on HIA substituents, solvent and excitation wavelength (278 nm and 365 nm) two main photochemical phenomena were observed: the classical E/Z isomerization around oxime double bond and Norrish-Yang cyclization to cyclobutanol oxime. The photostimulation of P(HIABMA-co-OEGMA₅₀₀) 3:7 in DMSO-d₆ confirmed the occurrence of Norrish-Yang cyclobutanol formation, along with further photochemical processes that are currently under investigation. In the last part of the thesis, the investigation on thermal behavior of HIA bearing copolymers aqueous solutions was carried out through turbidimetry and DLS measurement. It was found that the presence and the percentage of HIA group in the side chain of the OEGMA copolymer strongly influences the Lower Critical Solution Temperature (LCST) behavior in aqueous solution. Specifically, an increasing amount of HIA moiety in the side chain of copolymers resulted in a progressive lower cloud point that goes from above 80°C for P(HIABMA-co-OEGMA₅₀₀) 1:9 to 47°C for P(HIABMA-co-OEGMA₅₀₀) 1:1. In addition, the photochemical stimulus also play an important role in the LCST behavior of the HIA bearing polymers. Irradiation of OEGMA₅₀₀ copolymer bearing 50% of HIA group leads to structural changes that result in a remarkable increase of Cloud Point in aqueous solution (up to 12-13°C). In conclusion, the research presented in this Thesis covered all aspects of the investigation on a novel stimuli-responsive polymer, from monomer and polymer synthesis and characterization to the effect of sequential application of multiple stimuli (temperature and light).Furthermore, this work constitutes the starting point for a better understanding of the effect of hydrogen bonding patterns provided by aldehydes, oximes and HIAs on polymer solution behavior.