A potentially completely biobased highly thermal resistant polymer.

[NNadir]

The paper I'll discuss in this post is this one: Making Benzoxazine Greener and Stronger: Renewable Resource, Microwave Irradiation, Green Solvent, and Excellent Thermal Properties (Liu et al, ACS Sustainable Chem. Eng., 2019, 7 (9), pp 8715–8723

Although polymers represent a huge environmental problem, the bulk of this problem lies with single use plastics, mostly PE, PP, and PET, all of which are based on dangerous fossil fuels. I try, to the extent possible, to limit my use of single use plastics although, regrettably, I sometimes let future generations down on this score which is bad, since I know better.

To the extent that they can be reformed in supercritical water - if high temperatures can be approached cleanly - it may be possible to remediate this environmental mess over many generations, but this is neither here nor there.

Polymers however can play critical roles in many other systems for long term use, including structural materials, and if they are made from (reduced) carbon dioxide or biological materials, they may offer an opportunity to sequester significant quantities of carbon dioxide as value added materials.

For this reason this particular paper caught my eye.

From the introduction:

Polybenzoxazine, a relatively new kind of phenolic resin, not only inherits the merits of traditional phenolic resin, such as superior thermal and mechanical properties, but also possesses low surface energy, good dielectric, excellent dimensional stability, and molecular design flexibility. It has been a promising candidate in various high value-added applications, including electronic packing and aerospace.(1)

Benzoxazine can be readily synthesized via Mannich reaction from varied phenolic derivatives and primary amines. Currently, bisphenol A-based benzoxazine might be the most widely used product. However, the effect of bisphenol A on human endocrine system has attracted more and more attention.(2) In addition, concerns on the depletion of crude oil and environmental issues are driving us to develop polymeric materials from renewable feedstock. Therefore, a large quantity of naturally occurring phenolic compounds, such as cardanol,(3?5) eugenol,(6,7) vanillin,(8?10) sesamol,(11) catechol,(12) chavicol,(13) urushiol,(14) daidzein,(15) and guaiacol(16) have been tried as the raw materials for benzoxazine synthesis. However, most of them suffered from low cross-link density, low glass transition temperature (Tg), and poor thermal stability.(3?9,16?18) For instance, the benzoxazine derived from cardanol had a very low Tg due to the presence of long flexible alkyl chain.(3?5) The monofunctional benzoxazines synthesized from guaiacol(16) and arbutin(17) also showed low Tg because of the limited cross-linking sites. To increase the cross-link density, some multifunctional petroleum-based compounds are usually introduced, and it is the monobio-based (either phenol or amine is from bio-based feedstock), rather than fully bio-based benzoxazine, that has been developed.(13) On the basis of the literature survey results, it is concluded that significant efforts should be dedicated to the properties and bio-based content improvement of benzoxazine. And the exploration of biomass-derived phenols with multifunctional is worthy of expectation.

Magnolol, also known as 4-allyl-2-(5-allyl-2-hydroxy-phenyl) phenol, can be isolated from the stem bark of Magnolia officinalis. As a bioactive compound, magnolol has been used as neurologically active agent in traditional Chinese and Japanese medicine for centuries.(19,20) Besides that, magnolol is a highly functional compound containing both phenolic and allyl groups, which allows for high-performance polymer synthesis. However, it was seldom used for polymer synthesis(21) and never taken as the phenolic resource to prepare benzoxazine.

Here is the synthesis of the monomer, showing the structure of all three components:



The caption:

Scheme 1. Synthetic Scheme for Bisbenzoxazine M-fa

The furfurylamine is readily available from the reductive amination of furfural, for which many processes for production from biomass such as straw, corncobs, and wheat straw. Formaldehyde is available from the partial hydrogenation of carbon dioxide.

I am not about to recommend cutting down magnolia trees to get magnolol from the bark. However I would expect that it may be possible to insert genes to produce it into algae or plants other than trees, fast growing grasses like perhaps bamboo.

I only have time to post some pictures:

Yields:



The caption:

Figure 1. Yields of M-fa in different solvents as a function of reaction time (a) and yields of M-fa in PEG 600 as a function of reactant concentration (b).

Figure 2. (a) 1H NMR and (b) 13C NMR spectra of M-fa.

The monomer is highly pure based on the DSC:



The caption:

Figure 3. Non-isothermal DSC exothermic curves of M-fa at different heating rates.

The curing reaction followed by IR:



The caption:

Figure 4. FT-IR spectra of M-fa during the step-by-step curing reaction (cured at 160 °C for 2 h, 180 °C for 2 h, 200 °C for 2 h, 220 °C for 2 h, 240 °C for 2 h, and 250 °C for 1 h).

The caption:

Scheme 2. Proposed Curing Mechanism of M-fa

The remarkable thermal stability, shown by TGA (thermogravimetric analysis).



The caption:

Figure 6. Non-isothermal and isothermal TGA curves for the poly(M-fa).

Concluding remarks:

In this article, we have designed and successfully synthesized a high bio-based content bisbenzoxazine from magnolol, furfuryamine, and formaldehyde in PEG via microwave heating protocols. With low-dielectric PEG 600 as a solvent, the yield of M-fa reached 73.5% only within 5 min. The DSC and FT-IR results revealed relatively high reactivity of M-fa toward polymerization. M-fa also showed excellent processability with wide processing window. Both the non-isothermal and isothermal TGA results showed exceptionally good thermal stability of poly(M-fa). In particular, the Td5 of poly(M-fa) was as high as 440 °C, which was higher than almost all the previously reported values for bio-based polybenzoxazines. The Tg of poly(M-fa) was also remarkable, up to 303 °C. In summary, we have demonstrated the high possibility of developing natural renewable resources for the generation of high-performance resins under environmentally friendly conditions. The principles of green chemistry were fulfilled by renewable feedstock, green solvent, and energy-efficient heating technique in this work.

I like this kind of work quite a bit.

A great value added way to remove carbon dioxide from the air and put it away.

Esoteric I know, but interesting.