Bifunctional carbanionic synthesis of bio-based thermoplastic elastomers in methyl tert-butyl ether
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Abstract
This thesis presents the synthesis of fully bio-based thermoplastic elastomers (TPEs) via anionic polymerization. TPEs are intrinsically more sustainable compared to classical irreversibly vulcanized elastomers. Consequently, bio-based TPEs and novel solvent options are investigated within this work. The first objective is to establish a more efficient synthesis route based on bifunctional initiation, wherein the solvent methyl tert-butyl ether (MTBE) is investigated for its ability to ensure a reliable and controlled anionic polymerization. Additionally, the utilization of renewable monomers as building blocks for TPEs addresses the urgent societal and political demand for increased sustainability. From this perspective, natural building blocks based on terpenes offer great structural diversity and are therefore used within this work to install different material properties. Thus, this dissertation aims to contribute to a more sustainable future of polymer materials.
Aiming at a general overview of the relevance of diene monomers in the anionic polymerization, Chapter ① gives a comprehensive compilation of findings of recent research in the form of a Perspective article. It features how the shift from established fossil fuel-based elastomers, produced from only three primarily used monomers, i.e., styrene, butadiene, and isoprene, towards bio-based feedstocks changes the perspective of research. These recently developed bio-based monomers in terms of polymer properties, mechanistic insights, and behavior in statistical copolymerization are covered.
As some relevant theoretical concepts for this thesis are not included in this first Perspective article, Chapter ② provides further insights. The general concept of TPEs is introduced, covering the origin of the reprocessability and the general concepts of phase behavior in the bulk of block copolymers. Different synthesis pathways towards ABA-type triblock copolymers are discussed. Since peculiar polymer architectures can be achieved by using bifunctional initiators, these are introduced in greater detail. Challenges and former reports of bifunctional initiators are evaluated. Furthermore, as statistical copolymerization is a key method in producing block-like tapered structures in an even more efficient manner, a general introduction is given regarding the concept of reactivity ratios as well as their determination.
The main objective of this work is to develop an efficient synthesis pathway for bio-based TPEs. As the greatest efficiency for the synthesis of ABA-type triblock structures is based on bifunctional initiators, suitable media have to be identified. The poor solubility of bifunctional initiators in hydrocarbons necessitates the use of more polar solvents. Traditionally, tetrahydrofuran (THF) is used on lab-scale. However, translation to industry is prohibited, as the formation of peroxides and proton abstraction at ambient temperatures are major drawbacks of THF. Therefore, Chapter ③ covers the investigation of MTBE as an alternative solvent that comprises solubility of bifunctional initiators, no peroxide formation, and ideally a higher stability towards alkyl lithium compounds. The latter is demonstrated by the 50 times longer half-life of n butyl lithium in MTBE, compared to THF. As little is known about its general performance as reaction medium in the anionic polymerization, key kinetic parameters, i.e., propagation rates of the polymerization of 1,3-dienes are investigated as well. The trend of higher reaction rates compared to the traditionally used hydrocarbon solvent cyclohexane is determined for isoprene and β-farnesene – one “traditional” monomer and a renewable structure. Further, the “living” polydienyl lithium chains are predominantly present as non-aggregated unimers in MTBE. In addition to the homopolymerization, the established copolymerization system of styrene and isoprene is examined. The effect of an increasing MTBE fraction is investigated via in situ near-infrared (NIR) kinetics. The gradient structure, found in pure cyclohexane, successively changes to random monomer incorporation and ultimately yields an inverted, yet still close to random comonomer incorporation in pure MTBE. This is reflected by the reactivity ratios rSMTBE = 1.82 and rIMTBE = 0.55. The great potential identified for MTBE in carbanionic polymerization is used in the ensuing chapters of this work.
In Chapter ④, MTBE is utilized as a reaction medium for the bifunctional initiator 1,3 diisopropylene benzene. The excellent solubility of the bifunctional initiator in MTBE allows easy access to ABA-type triblock copolymers. To alleviate the current environmental challenges and to overcome the shrinking fossil fuel feedstock, a fully bio-based TPE is targeted. Therefore, β-farnesene is used for the middle block, as it can be derived from sugar cane and has a microstructure-independent low glass temperature (Tg). By controlled termination of the telechelic “living” polyfarnesene chains, hydroxyl groups are introduced via adding ethoxy ethyl glycidyl ether (EEGE) to the reaction. This affords low dispersity (Đ = 1.07 to 1.10) and telechelic polyfarnesene macroinitiators, bearing two hydroxyl groups at each chain end. Subsequent organocatalyzed ring-opening polymerization (ROP) of LL-dilactide (LLA) yields H-shaped, fully bio-based triblock copolymers. The material features two distinct Tgs at – 66 °C and 51 °C, as well as gyroid or cylindrical morphologies, determined by transmission electron microscopy (TEM) and small-angle Χ-ray scattering (SAXS). A soft elastic material at room temperature is obtained.
Given the advantages of the moderately polar solvent MTBE, ensuring reliable solubility of a bifunctional initiator, and the low Tg of polyfarnesene, Chapter ⑤ describes an even more efficient pathway towards ABA-type structures. To allow for a one-step synthesis and to avoid an elaborate multistep pathway, carbanionic polymerization is exclusively used in this chapter. Nopadiene (Nopa) is chosen as a suitable comonomer, as PNopa exhibits a high Tg of ~ 160 °C and can be derived from the renewable feedstock pine tree. Excellent control of carbanionic copolymerization is confirmed by in situ 1H NMR kinetics. By changing the reaction conditions, the reactivity ratios are tailored to yield a steep gradient comonomer incorporation, rFarMTBE = 10.8 and rNopaMTBE = 0.093. Consequently, symmetric, two-sided tapered ABA-type triblock copolymers are accessible in a one-step approach, capitalizing on a difunctional initiator in MTBE. By changing the comonomer ratio, the material properties are tuned in a broad range: highly elastic materials with elongation exceeding 1300% as well as tough materials with Young modulus’ exceeding 500 MPa are obtained. Characterization via SAXS, temperature-modulated differential scanning calorimetry (TM DSC), rheology, and dielectric spectroscopy are employed to relate the material properties to the phase state of the triblock copolymer. This revealed local phase segregation accompanied by respective Tgs of the domains, albeit in the absence of long-range order. Thus, it could be successfully demonstrated that it is possible to produce fully bio-based TPEs that are similar in their properties to fossil fuel-based materials via a one-pot and one-step synthesis.
Bifunctional initiators and functional initiators face similar challenges in their application in the field of carbanionic polymerization. Therefore, Chapter ⑥ involves the translation of previous findings in terms of solubility issues of bifunctional initiators Chapter 4 to functional initiator structures. The project is the result of a collaboration with the synthetic rubber company Arlanxeo. The incorporation of functional groups at the chain end of polymer chains in elastomers is beneficial to improve filler-rubber interaction and helps to improve the performance in, for instance, tires as it lowers the rolling resistance and hence reduces fuel consumption. Further, functional groups at the chain end can be used in a subsequent reaction to access block structures. To this end, a heterotelechelic initiator for the formation of α functionalized polyisoprene is developed. The initiator is accessed via titration using an iodine alkyl precursor and tert-butyl lithium. First, the functional alkyl lithium initiator, bearing two ketal-protected hydroxyl functionalities, is used for the carbanionic polymerization of isoprene. Low dispersity (Đ < 1.1) polymers with a high 1,4-polyisoprene content (60%) are obtained after optimization of the required amount of polar modifiers. Matrix-assisted laser desorption/ionization time-of-flight mass spectroscopy (MALDI-ToF MS) measurements verify an effective end-group functionalization. Second, LLA is grafted from theses (OH)2-PI macroinitiators to access A2B-type block copolymers. Size-exclusion chromatography (SEC), NMR, and diffusion-ordered spectroscopy (DOSY) measurements are used to demonstrate successful chain-extension via lactide grafting. DSC experiments reveal two distinct Tgs, indicating phase-separation within the block copolymers. Hence, a heterotelechelic initiator for both carbanionic polymerization and ROP is introduced.
In the first appendix Chapter A①, an approach for TPEs based on a pure hydrocarbon medium is presented. The bifunctional initiator 1,3-bis(1-phenyl ethenyl) benzene (PEB) is used for its good solubility in cyclohexane. As such, a high 1,4-content can be achieved in the polymerization of 1,3-dienes, which in turn allows for the use of the readily available monomer isoprene. Subsequently, three hydroxyl groups are introduced simultaneously both in α- and ω position by means of end-functionalization of the living anionic dilithiated PI-chains, with 1,2-isopropylidene glyceryl glycidyl ether (IGG) and subsequent acidic deprotection. These multi-hydroxyl PI-macroinitiators are used to initiate LLA in an organocatalyzed ROP. Thereby, super-H-shaped A3BA3-triblock structures are obtained. Thermal characterization reveals two distinct Tgs, suggesting phase-separation within the bulk material. PI-domains feature a low Tg in the range of - 55 °C to - 59 °C and the PLLA-domains show a Tg of 41 °C to 49 °C. Furthermore, the exact phase-separation and morphology is analyzed via TEM and SAXS, confirming cylindrical and lamellar morphologies. The combination of carbanionic synthesis and ROP of lactones is presented as a feasible pathway for complex polymer architectures.
Chapter A② presents a fundamental investigation on binary blends of polyfarnesene and polyisoprene as a model system towards super soft polymer melts. The expertise in the living diene polymerization gained before is used to synthesize a molar mass series of polyfarnesene samples in the range of 3.5 kg∙mol 1 to 720 kg∙mol 1. To prepare polyfarnesene/polyisoprene blends, commercially available polyisoprene samples of the same molar mass are purchased. The addition of linear polyisoprene to the bottlebrush-architecture of polyfarnesene leads to dilation of the reputational “tube” that further reduces the - already low - plateau modulus of polyfarnesene. The work is based on a close collaboration with Prof. G. Floudas, with the syntheses of the polymer samples being contributed in the context of this thesis.
The research article presented in Chapter 4 is additionally published in a German journal. Appendix Chapter A③ is the German translation of Chapter 4.