Andrea Hufendiek

Temperature responsive cellulose-graft-copolymers via cellulose functionalization in an ionic liquid and RAFT polymerization

Well-defined cellulose-graft-polyacrylamide copolymers were synthesized in a grafting-from approach by reversible addition-fragmentation chain transfer polymerization (RAFT). A chlorine moiety (degree of substitution DS(Cl) ≈ 1.0) was introduced into the cellulose using 1-butyl-3-methylimidazolium chloride (BMIMCl) as solvent before being substituted by a trithiocarbonate moiety resulting in cellulose macro-chain transfer agents (cellulose-CTA) with DS(RAFT) of 0.26 and 0.41. Poly(N,N-diethylacrylamide) (PDEAAm) and poly(N-isopropylacrylamide) (PNIPAM) were subsequently grafted from these cellulose-CTAs and the polymerization kinetics, the molecular weight characteristics and the product composition were studied by nuclear magnetic resonance spectroscopy, X-ray photoelectron spectroscopy, and size exclusion chromatography of the polyacrylamides after cleavage from the cellulose chains. The number-average molecular weights, Mn, of the cleaved polymers ranged from 1100 to 1600 g mol(-1) for PDEAAm (dispersity Đ = 1.4-1.8) and from 1200 to 2600 g mol (-1) for PNIPAM (Đ = 1.7-2.1). The LCST behavior of the cellulose-graft-copolymers was studied via the determination of cloud point temperatures, evidencing that the thermoresponsive properties of the hybrid materials could be finely tuned between 18 and 26 °C for PDEAAm and between 22 and 26 °C for PNIPAM side chains.

Catalytic transesterification of cellulose in ionic liquids: sustainable access to cellulose esters

Catalytic transesterifications of cellulose were studied under homogeneous conditions using the ionic liquid 1-butyl-3-methylimidazolium chloride (BMIMCl) as a solvent. Cellulose was thus efficiently converted into cellulose esters employing various methyl esters and 10 mol% of 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) as catalyst. 1H NMR analysis of the products revealed up to 2.3 turnovers of the methyl esters per catalyst molecule, leading to degrees of substitution (DS) of up to 0.69. Although a comparatively low turnover number (TON) is observed, the developed methodology represents the first successful homogeneous catalytic reaction on cellulose. Furthermore, the new method is an important step forward in terms of sustainability, since the BMIMCl–DMSO mixture can be recycled and reused for the reaction, and toxic and corrosive chemicals commonly employed for cellulose esterification (such as anhydrides, acid chlorides and bromides, organic bases, all in overstoichiometric amounts) are avoided. To demonstrate the versatility of this transesterification, an aromatic (cellulose benzoate), an aliphatic (cellulose butyrate), and a fatty acid containing cellulose ester (cellulose 10-undecenoate) were prepared. Additionally, cellulose 10-undecenoate was successfully used for thiol–ene grafting onto reactions employing two thiols for efficient thiol–ene addition reactions.

Quantifying photoinitiation efficiencies in a multiphotoinitiated free-radical polymerization.

Online size exclusion chromatography-electrospray ionization-mass spectrometry (SEC/ESI-MS) is employed for quantifying the overall initiation efficiencies of photolytically generated radical fragments. In a unique experiment, we present the first quantitative and systematic study of methyl-substituted acetophenone-type photoinitiators being employed in a single cocktail to initiate the free-radical polymerization of methyl methacrylate (MMA) in bulk. The photoinitiators are constituted of a set of two known and four new molecules, which represent an increasing number of methyl substituents on their benzoyl fragment, that is, benzoin, 4-methylbenzoin, 2,4-dimethylbenzoin, 2,4,6-trimethylbenzoin, 2,3,5,6-tetramethylbenzoin, and 2,3,4,5,6-pentamethylbenzoin. The absolute quantitative evaluation of the mass spectra shows a clear difference in the initiation ability of the differently substituted benzoyl-type radical fragments: Increasing the number of methyl substituents leads to a decrease in incorporation of the radical fragments.

Renewable, fluorescent, and thermoresponsive: cellulose copolymers via light-induced ligation in solution

We introduce a mild photochemically driven strategy for the synthesis of fluorescent cellulose copolymers using filter paper as the starting material. Thermoresponsive behaviour in water is imparted to the brush-like structures by grafting of poly(N-isopropylacrylamide) side chains. All reactions take place in homogenous solutions, allowing the design of novel advanced materials from renewable resources.

Polycycloacetals via polytransacetalization of diglycerol bisacetonide

Diglycerol bisacetonide was synthesized and isolated from acetone and diglycerol, which is renewable and available in large scale. DGA was directly used in polytransacetalization with 1,4-cyclohexanedione or 4,4′-bicyclohexanone as diketone monomers. Polycycloacetals were obtained with molecular weights (Mn) of up to 28 kg mol−1, broad dispersities (Đ = 1.5–4.0) and as semi-crystalline polymers with high melting points (Tm = 210–241 °C) and glass transition temperatures (Tg) of 48 °C or 65 °C. Introducing di(trimethylolpropane) (di-TMP) in the polymerization of DGA and 1,4-cyclohexanedione resulted in copolymers as confirmed by 1H-NMR and 13C-NMR spectroscopy. Increasing the di-TMP content from 10 to 50 mol% reduces the crystallinity of the polycycloacetals and increases the Tg, eventually yielding amorphous polymers (Tg = 60–71 °C). For the amorphous polycycloacetals, Youngs moduli could be determined by tensile strength testing (E = 1.1–1.4 GPa). The polycycloacetals with renewable carbon contents in the range of 33–100% cover a wide range in material properties and are stable against hydrolysis at pH > 1–3, depending on the polycycloacetal composition.