Bio-derived glycidyl ethers: a versatile platform for functional polymers via epoxide polymerization

Abstract

This doctoral thesis covers the synthesis and application of bio-renewable glycidyl ethers (GEs) as novel building blocks for multifunctional polyethers and polycarbonates. The synthetic strategy relies on the transformation of naturally available feedstock, such as terpenoids and lignin-based compounds, into epoxide synthons, rendering them suitable for the ring opening polymerization (ROP). As an emerging trend in polymer science, the substitution of depleting fossil fuel resources with bio-renewable alternatives aims at providing pathways to more sustainable, but highly advanced polymeric materials.

Capitalizing on the facile synthesis and wide application scope of GEs, this underrated monomer class represents the reoccurring theme throughout all thesis chapters. After a brief introduction in Chapter 1, this work focuses on the anionic ring opening (co)polymerization (ARO(c)P) of novel terpenyl glycidyl ethers (TGEs) in Chapter 2, including fundamental studies of the synthetic conditions and resulting properties. Farnesyl glycidyl ether (FarGE), a terpenoid-derived epoxide, represents a central building block, which set the foundation for this chapter. Chapter 3 embodies the potential of combining TGEs with carbon dioxide (CO2) to generate novel thermoplastic elastomer (TPE) soft blocks. A joint venture utilizing lignin-derived glycidyl ethers with stereoselective catalysts for epoxide ROP presents the first stereocomplex formation starting from a racemic monomer mixture, as illustrated in Chapter 4.

Chapter 1 provides a brief introduction into the synthesis and properties of multifunctional polyethylene glycol (mfPEG) structures, available synthons and targeted applications. A detailed illustration on the statistical copolymerization kinetics of epoxides and on the relevance of the reactivity ratios regarding the microstructure is provided, as in situ NMR kinetics represent a key technique of this thesis. The ROP of epoxides is presented with an emphasis on stereoregularity, thus, illuminating the fundamentals of the AROP and catalytic, stereoselective ROP (CSROP). In the context of stereoregularity, the phenomenon of stereocomplexation as a strategy to enhance polymer properties beyond enantiopure or atactic analogues is exemplified with key polymer representatives. As the identification and utilization of biomass-derived monomers for sustainable polymer synthesis poses a key challenge to scientists in both industry and academia, the promising class of terpenes and terpenoids is reviewed as a feedstock for sustainable polymer synthesis.

In Chapter 2, the overlooked class of TGEs as terpene-derived monomer building blocks is explored and fundamentally investigated in the AROP. Despite their easy-to-synthesize nature, TGEs have been scarcely investigated in previous reports. This section consists of three important subchapters.

Chapter 2.1 presents the AROcP of the hydrophobic epoxide FarGE with ethylene oxide (EO). Due to the highly controlled nature of the AROP, series of well-defined, amphiphilic polyethers of block-like and random structure have been synthesized, allowing for facile tunability of the polymer properties by the FarGE monomer feed content. In-depth investigation of the properties in aqueous solution reveals self-assembly of the polymeric amphiphiles into micelles, characterized by very low critical micelle concentrations (CMCs). In summary, these partially biobased, farnesol-derived block copolymers represent an interesting alternative to fossil fuel-based, nonionic surfactants.

In the field of surfactants and surface-active materials, an enhanced solubilization performance offers the potential to reduce both environmental and ecological consequences. In Chapter 2.2 an elaborated synthetic strategy towards partially bio-derived, amphiphilic polyethers as non-conventional surfactants is presented, utilizing FarGE and its hydrogenated analogue hexahydrofarnesyl glycidyl ether (HHFarGE). Banking on previous work by Verkoyen et. al. and the excellent control over the polymerization during crown ether-assisted AROP, two series (using FarGE and HHFarGE) of well-defined, non-ionic block copolymer surfactants with tailor-made amphiphilicity and low CMCs are obtained. In a collaboration with the Sottmann group (University of Stuttgart), the synthesized amphiphiles were applied as co-surfactants to investigate the phase behavior in microemulsions, demonstrating an improved efficiency for all block copolymers. The concept of efficiency boosting relies on the adsorption of amphiphilic copolymers into the surfactant membrane, from where they extend in a mushroom-like conformation into the oil and water interlayer, respectively.

Chapter 2.3 expands the general scope of TGE monomers by varying the side chain length and conformation, allowing for fine-tuning of mfPEG properties via copolymerization with EO. To elucidate the microstructure of the copolymers, the copolymerization behavior of each TGE/EO comonomer pair was examined via 1H NMR in situ kinetic measurements. This reveals a change from ideally random to gradient copolyether microstructure (r(EO) > r(TGE)), influenced by the chain length and hydrophobicity of the respective TGE. The resulting statistical copolymers are enriched with unsaturated alkene moieties towards the polymer chain end, paving the way to post-functionalization such as thiol-ene click and hydrogenation. To the best of our knowledge, the diimide reduction using potassium azodicarboxylate is presented for the first time in polymer hydrogenation, which is attractive due to facile purification.

Chapter 3 addresses the sequential synthesis of a novel, fully bio-derived TPE, namely PLLA-b-PCitroGEC b-PLLA. The key aspect of this work is the application of a peculiar, low glass transition temperature polycarbonate, poly(citronellyl glycidyl ether carbonate) (PCitroGEC), and, in turn, phase-separation by combining chemically and structurally dissimilar blocks. Thermal and small-angle X-ray scattering investigations indicate the anticipated phase-separation, allowing for the compilation of an experimental phase-diagram for this series of triblock copolymers. Mechanical measurements demonstrate elasticity of these polycarbonate-polyester TPEs with low E-moduli (1.43 to 2.5 MPa), rendering the PLLA-b-PCitroGEC-b-PLLA triblock copolymers suitable as materials in soft tissue engineering.

Chapter 4 highlights the in situ polyether stereocomplex formation from a racemic phenyl glycidyl ether mixture. Synthetically, the enantioselective and isoselective ROP was explored using both the enantiopure and racemic version of a cobalt salen bimetallic complex, resulting in highly isotactic polyethers. While enantioselective polymerization produces an enantiopure polyether strand, isoselective ROP leads to concurrent formation of both stereoconfigurations (R and S). Isoselective ROP and mixing equimolar amounts of the enantiopure isotactic polyethers (R and S produced separately via enantioselective ROP) both result in stereocomplexation and a considerably higher melting temperature compared to the homochiral parent polymer. Intriguingly, the high polyether tacticity in combination with potential phenyl stacking is sufficient for the formation of a polyether stereocomplex, despite the absence of significant dipole-dipole interactions.

Chapter A1 covers a fundamental investigation of the carbanionic polymerization of 4-trimethylsilylstyrene (4TMSS), featuring ambiguous electronic characteristics. Besides well-defined di- and triblock copolymers, statistical copolymers with isoprene and styrene were prepared. Real-time 1H NMR kinetic measurements unraveled a moderate, gradient-like composition profile due to copolymerization, however, reactivity ratios are inverted for the 4TMSS/S and 4TMSS/I comonomer pairs. Overall, the utilization of 4TMSS allows for the synthesis of tailor-made copolymer structures with respect to thermal properties and overall hydrophobicity.

Chapter A2 investigates the copolymerization behavior of EO and two GEs, respectively, via in situ NMR kinetic measurements with respect to monomer structure and solvent. The experiments reveal preferred incorporation of the respective GE compared to EO, irrespective of the solvent utilized during polymerization. Density functional theory (DFT) calculations emphasize that the microstructure is strongly affected by the chelation capability of GEs with the polymer chain end counterion and, in turn, by the ether-containing monomer side groups. Differences in solvent polarity influence (i) counterion solvation and (ii) chelation capability, leading to a disparity of the reactive ratios with decreasing solvent polarity.

In Chapter A3, a new approach to crosslinkable copolyethers without post-polymerization modifications is presented. To this end, a tailorable amount of cinnamoyl side chains is incorporated into poly(glycerol) by capitalizing on the monomer-activated ROP of a glycidyl ester, namely glycidyl cinnamate (GC), with ethoxyethyl glycidyl ether (EEGE). Subsequent to copolymerization, the primary hydroxyl functionalities are released to increase polymer hydrophilicity, and UV light exposure enables photo-crosslinking via dimerization of the cinnamoyl moiety.