Biobased thermoplastic polyurethanes synthesized by isocyanate-based and isocyanate-free routes

Thermoplastic polyurethane elastomers (TPUs) are widely applied and consumed in large quantities because of their excellent properties, such as good transparency, tunable stiffness, good wear resistance, excellent biocompatibility, etc. Meanwhile, considering that crude oil is depleting day after day, renewable resources are potential substitutes for crude oil as feedstock for polymer materials. Besides, biobased compounds have chemical structures different from the conventional petrochemical matters, offering polymers with special properties. Therefore, it is interesting to study the synthesis and the properties of TPUs from renewable resources. This thesis describes the synthesis and characterization of biobased TPUs. Prior to the synthesis of TPUs, the diolpolyester precursors were prepared from biobased monomers. Not only semi-crystalline polyester diols, such as poly(1,3-propylene adipate) (PPA), poly(1,4-butylene adipate) (PBA) and poly(1,12-dodecylene sebacate) (PDS), but also two novel amorphous polyester diols poly(1,2-dimethylethylene adipate) (PDMEA) and poly(1,2-dimethylethylene succinate) (PDMES) were prepared via transesterification polymerizations catalyzed by an organic superbase 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD). TBD was previously found to be an efficient catalyst for the ring-opening polymerizations of cyclic esters or cyclic carbonates. It was discovered in our current study that TBD is also a promising catalyst for the polycondensation of diesters with diols, at mild reaction conditions. The polymerizations to prepare PPA catalyzed by TBD were carried out at 120 °C while the temperature required for the same polymerizations catalyzed by the commonly-used titanium tetrabutoxide (TBO) is at least 150 °C. Fully dihydroxyl-terminated polyesters were obtained by adjusting the feed ratio of the diol and diester starting materials. In chapter 3, we describe the synthesis and the characterization of TPUs based on hexamethylenediisocyanate (HDI), different polydiols, and different chain-extenders (1,6-diaminohexane DAH and water). The flow temperature (Tfl) of the TPUs can be adjusted by changing the hard segment length (LHS) or the soft segment length (LSS), from which the materials with proper flow transition can be selected. For example, when water was applied as the chain-extender instead of DAH, by which the LHS of the TPUs decreases from 4U (which means the hard segment consists of 4 urethane/urea groups) to 3U. As a result, the Tfl of the TPUs decreases from approx. 250 °C (far above the degradation temperature of PUs) to about 150 °C (safe for PU processing). By increasing the LHS, the hard segment content can also be decreased, which results in a reduction in the Tfl values, from 250 °C to 175 °C. The TPUs based on the novel amorphous polyester diol, PDMEA, exhibit similar thermal and mechanical properties to those polymers based on the amorphous polyether poly(1,2-propylene glycol) (PPG), e.g. the E-moduli of these materials are in a range of 5 ~ 15 MPa and the strains at break are from 500% to 1000%. The phase separation of the PDMEA-based TPUs is better than the TPUs based on PPA, PBA or PPG, evidenced by the morphology of the materials as determined by atomic force microscopy. The biodegradability in a solution of pseudomonas cepacia lipase (Lipase PS) at 37 °C for all of the materials with different soft segments or different hard segments is slow, except for the PPA-based polymers. The rate and degree of biodegradation appears to depend on the polarity, crystallinity and the pendant groups of the soft segments. TPUs based on the novel polyester PDMEA combined with the conventional, petrochemical hexamethylene diisocyanate (HDI) and diamine or water chain-extenders exhibit satisfactory thermal and mechanical properties. Therefore, it was expected that PDMEA is also a suitable precursor for fully biobased TPUs. Starting from PDMEA, a biobased diisocyanates (L-lysine diisocyanate LDI) and biobased diamine chain-extenders (putrescine DAB and diaminoisoidide DAII), fully biobased TPUs were obtained. It appears that LDI, which is an asymmetric DI, forms part of the soft segments in the resulting TPUs. Therefore, the actual hard segment contents (HSCs) are too low to achieve good phase separation and mechanical properties. An efficient way to increase the HSCs and therefore to improve the phase separation and mechanical properties in the TPUs is to introduce a urea-containing diamine chain-extender (di(4-aminobutyl)urea, DABU). Comparing the DABU-based TPUs to the DAB- or DAII-based polymers, the E-moduli are much higher (approx. 20 MPa for the former vs. about 1 MPa for the latter), while the Tfl increases from 80 °C to 150 °C or even 200 °C. Alternatively, a more symmetric biobased DI, such as isoidide diisocyanate IIDI, can be introduced instead of LDI to increase the HSCs and improve the properties of the resulting TPUs. Both methods were verified to lead to enhanced thermal and mechanical properties of the TPUs. In addition to the isocyanate-based route, an isocyanate-free route was studied to synthesize TPUs. In this isocyanate-free route, non-toxic dicarbamates were used as the starting materials instead of the toxic diisocyanates. The dicarbamates were synthesized through efficient TBD-catalyzed reactions using (potentially) biobased compounds, such as urea, putrescine, dimethylcarbonate, etc. The forming pure dicarbamates were polymerized with diamino-terminated PPG to obtain TPUs, in which TBD were also used as the catalyst. The hard segments of the prepared TPUs are well-defined and of uniform length. As a result, the TPUs obtained via the isocyanate-free route have very promising properties, such as sharp thermal transitions and temperature-independent rubbery plateau. By changing the length of the dicarbamates or the length of the PPG entries, the formed TPUs can be either plastics or elastomers, and they flow at either 90 °C or 130 °C or 175 °C.