María
Pin-Nó
ac,
Philippa L.
Jacob
a,
Vincenzo
Taresco
a,
Maud
Kastelijn
b,
Tijs
Nabuurs
b,
Chandres
Surti
c,
John
Bilney
c,
John
Daly
c,
Daniel J.
Keddie
a,
Steven M.
Howdle
*a and
Robert A.
Stockman
*a
aSchool of Chemistry, University of Nottingham, University Park, Nottingham, NG7 2RD, UK. E-mail: robert.stockman@nottingham.ac.uk; steve.howdle@nottingham.ac.uk
bCovestro (Netherlands) BV, Sluisweg 12, 5145PE, Waalwijk, The Netherlands
cCornelius Specialties Limited, 5c Rookwood Way, Haverhill, Suffolk CB9 8PB, UK
First published on 22nd July 2024
An improved synthesis of the pinene-derived monomers (3-pinanyl acrylate 1 and 3-pinanyl methacrylate 2), replacing hazardous and/or expensive reagents from established methods with cheaper, more innocuous and sustainable reagents, is reported; the monomers of high purity are obtained at up to 160 g scale, without the need for chromatographic separation. Subsequently, these monomers (1 and 2) were successfully copolymerized with n-butyl acrylate/methacrylic acid or styrene/methacrylic acid using a radical semi-batch emulsion copolymerization process. For comparison, materials incorporating the more established terpene-derived monomer iso-bornyl methacrylate 3 were also prepared in an analogous fashion. The obtained polymer latexes had particle sizes between 65 and 90 nm and very low polydispersities (<0.08) and were stable for several years without any coagulum formation. Gradient liquid chromatography indicated that all copolymers had relatively uniform chemical composition distributions. The n-butyl acrylate containing copolymers (P1–P3) were obtained with high molar masses (Mn > 40000 and Mw > 400
000), very high dispersities (Ð > 9.5), and low glass transition temperatures (Tg < −5 °C). The styrene-based copolymers (P4–P6) had slightly lower molar masses (Mn > 40
000 and Mw > 150
000), lower dispersities (Ð > 3) and high glass transition temperatures (95 °C < Tg < 120 °C). Preliminary testing of the n-butyl acrylate-based materials demonstrated the potential of these copolymers for use in coating applications. The poly(n-butyl acrylate)/pinanyl methacrylate copolymer P2 was found to be harder (König hardness) and had better stain resistance properties towards water-based substances than the analogous n-butyl acrylate-based copolymers containing 3-pinanyl acrylate (P1) or iso-bornyl methacrylate (P3). Through further refinement of the copolymerization process we expect that the properties of these polymers may be further tailored towards a range of coating applications.
Sustainability spotlightThis study investigates the scale up of the synthesis of α-pinene-derived monomers and their subsequent application as high Tg components in copolymer coatings. The newly developed, upscaled syntheses of these terpene-derived monomers employ less hazardous and/or toxic reagents and lead to new materials and coatings that have a very high biobased content. This work aligns with UN Sustainable Development Goal 12 (Responsible Consumption and Production). |
Currently, most acrylic-based commercial resins are prepared from petrochemical sources.2 There is a growing global concern about carbon emissions linked to fossil fuel consumption.3 Alarmingly, it has been predicted that polymer production will account for 20% of global fossil fuel consumption by 2050.4 In response, the chemical industry is intensifying its efforts to find more sustainable, biobased chemicals to produce high quality synthetic materials.5 Clearly, this motivation needs to be balanced against the often-competing pressures of cost and performance. However, it has been shown that market penetration of bioderived products is possible where the product brings specific advantages, such as avoiding toxicity and facilitating degradability (e.g. epoxidized soybean oil (ESO), isosorbide and poly(lactic acid) (PLA)).4,6–8
One strategy used to enhance the green credentials of polymers for coating technology focusses on the use of plant-based building blocks containing a reactive double bond that can be exploited via radical polymerization.9–12 There are numerous examples of the application of biobased monomers in radical polymerization13–16 and of these, terpenes appear as a versatile pool of molecules whose production and extraction do not compete with food production.17 In this regard, we have recently focused on developing biobased radically polymerizable monomers derived from terpenes.18–22
Terpenes and terpenoids are found abundantly in plant oils and many of these molecules are considered to be industrial by-products from processes including citrus processing as well as the pulp and paper industry.23,24 Turpentine, a by-product of the paper and pulp industry is produced on a scale of 330000 tons per year and is composed of mostly α- and β-pinene.25
From a coating standpoint, monomers derived from α-pinene, namely 3-pinanyl acrylate (PA, 1) and 3-pinanyl methacrylate (PMA, 2), appear particularly promising, due to the bicyclic ring present as a pendant moiety. (Meth)acrylic polymers derived from these compounds have relatively high glass transition temperatures (Tg),19 suggesting a potential role as ‘hard’ components in copolymer formulations.26 Previous work has demonstrated their compatibility with ‘soft’ monomers such as limonene acrylate in the synthesis of ABA block copolymers.19,26
Herein, we investigate the utility of the α-pinene derived monomers PA 1 and PMA 2 in this context. We describe both their improved, scaled-up synthesis and their use as a component in binder formulations prepared by semi-batch aqueous emulsion copolymerization. Importantly, we directly examine the effect of incorporation of the pinene-based monomers (PA 1 and PMA 2) into the final materials, comparing them to analogous polymers derived from the more established high Tg monomer, isobornyl methacrylate (iBoMA, 3).
Entry | Sample label | Copolymera | M n (g mol−1) | M w (g mol−1) | Đ | Biobased contentc (wt%) | Particle sized (nm) | PDId | Solids contente (%) | T g (°C) | T g, predicted (°C) | MFFTh (°C) |
---|---|---|---|---|---|---|---|---|---|---|---|---|
a Monomer feed ratios (wt%) are given as subscripts in the name abbreviations. b From SEC data (NMP eluent). c For total solid content from radiocarbon analysis.30 d From DLS analysis. e Calculated from gravimetry. f From DSC analysis, the Tg is reported as the onset temperature. g Calculated from the Fox equation31 (using the following homopolymer Tg: PBA = −54 °C,32 PSt = 100 °C,32 PMAA = 228 °C,32 PiBoMA = 150 °C,33 PPA = 84 °C,19 and PPMA = 168 °C19) h The minimum film forming temperatures (MFFTs) are outside the temperature range of our tester (0–60 °C) and are hence presented as estimates only. | ||||||||||||
1 | P1 | P(BA63%-co-PA35%-co-MAA2%) | 43![]() |
417![]() |
9.69 | 27 | 67 | 0.06 | 44.6 | −13.5 | −19 | <0 |
2 | P2 | P(BA63%-co-PMA35%-co-MAA2%) | 52![]() |
1![]() ![]() |
20.55 | 26 | 87 | 0.03 | 43.4 | −2.0 | −7 | <0 |
3 | P3 | P(BA63%-co-iBoMA35%-co-MAA2%) | 45![]() |
494![]() |
10.77 | 25 | 88 | 0.01 | 44.1 | 4.0 | −9 | <0 |
4 | P4 | P(St63%-co-PA35%-co-MAA2%) | 47![]() |
186![]() |
3.94 | 27 | 87 | 0.03 | 44.1 | 97.5 | 96 | >60 |
5 | P5 | P(St63%-co-PMA35%-co-MAA2%) | 50![]() |
210![]() |
4.19 | 26 | 87 | 0.03 | 44.3 | 114.0 | 121 | >60 |
6 | P6 | P(St63%-co-iBoMA35%-co-MAA2%) | 47![]() |
158![]() |
3.31 | 25 | 94 | 0.08 | 44.4 | 113.5 | 116 | >60 |
Residual monomer concentrations were determined using head-space GC for butyl acrylate (BA), styrene (St) and isobornyl methacrylate (iBoMA, 3). High-performance liquid chromatography (HPLC) was used to determine residual monomer concentrations for methacrylic acid, PA 1 and PMA 2.
For scale up for potential commercial use, neither of the above protocols is ideal either from economic or sustainability standpoints.‡ As such, we sought to improve our syntheses in terms of cost and scale to improve industrial feasibility, whilst exploiting sustainable reactions and methods of product isolation and purification, where possible. We discovered that the combination of sodium borohydride/acetic acid mediated hydroboration29 and hydrogen peroxide induced oxidation delivered 3-pinanol in reasonable yield (58%) at increased lab scale following purification by distillation (>50 g) (see Scheme 1, left), directly from α-pinene. We expect that these yields could be improved with further optimisation. Notably, this method provides a more sustainable pathway to the alcohol, at a fraction of the cost of the initial synthesis.
![]() | ||
Scheme 1 Synthesis of PA 1 and PMA 2 from α-pinene in two-steps via hydroboration/oxidation and (meth)acrylation. |
Next, we developed a new approach for the preparation of the methacrylate-based monomer PMA 2 from 3-pinanol, using methacrylic anhydride and catalytic copper(II) tetrafluoroborate,28 to install the ester functionality. This delivered the product 2 (>60 g) in excellent yield (90%) and high purity after distillation (see Scheme 1, right upper).
For the scaled-up synthesis of the acrylate monomer PA 1 from 3-pinanol we used a standard acryloyl chloride method adapted from that reported in our previous investigation,18 due to the lack of commercial availability of acrylic anhydride. This gave the desired monomer PA 1 (>160 g) in high purity (distillation) and high yield (87%) (see Scheme 1, right lower).§
![]() | ||
Scheme 2 Emulsion radical copolymerization of terpene derived monomers PA 1, PMA 2 or iBoMA 3 with BA or St (and MAA). |
With reasonable quantities of the monomers PA 1 and PMA 2 in hand, we were able to utilise them in copolymerization of industrial relevance as outlined below.
In general, the level of residual BA and terpene monomers (1, 2 or 3) slowly increases across the 120-min feed time, with the ratio between the monomers remaining fairly constant (see Fig. 1a). Following the ‘burn-up’ reaction, quantitative (>99.9%) monomer consumption was achieved for all samples.
![]() | ||
Fig. 1 (a) Plot of residual BA (open symbols) and PA 1, PMA 2 or iBoMA 3 (closed symbols) concentrations versus time (the grey dashed line indicates the semi-batch feed profile in cumulative % of the monomers with time); (b) evolution of PBA-based polymer particle size (closed symbols) and polydispersity index (open symbols) with total monomer conversion; (c) SEC traces of BA-based copolymers; and (d) instantaneous monomer conversion for BA (open symbols) and PA, PMA or iBoMA (closed symbols) versus cumulative monomer feed, for BA/PA/MAA (P1, red), BA/PMA/MAA (P2, blue) and BA/iBoMA/MAA (P3, black) semi-batch emulsion radical copolymerizations. Note that the data related to MAA are omitted for clarity. P1–P3 refer to the sample labels given in Table 1. |
The size of the BA-based copolymer particles increased with cumulative monomer feed (and therefore total monomer conversion), while the particle size polydispersity index (PDI) remained low (see Fig. 1b). The final obtained latexes (P1–P3) showed particle sizes of 65 to 90 nm and a very low PDI (<0.06) (see Table 1). All samples were highly stable with no sign of coagulum over several years.
In each case, the final polymers (P1–P3) have high molar mass (Mn > 40000, Mw > 400
000) and very high dispersity (Ð > 9.5) (see Table 1, entries 1–3). Additionally, bimodality is observed in each of the SEC traces (see Fig. 1c), with the high molar mass fraction being most prominent for the polymer (P2) made with the methacrylate co-monomer, PMA 1. The ‘burn up’ of the residual monomers present after the 120 min feed profile gives rise to the high molar mass peak in the molar mass distributions; the build-up of a higher concentration of residual BA in formation of P2 (see Fig. 1a, blue dashed line) than that observed for the P1 or P3 reactions results in a more prominent high molar mass fraction in the P2 sample.
Early in the reaction (i.e. feed < 40%), lower overall monomer conversion for the PA 1 system (P1) is observed than that for either of the reactions incorporating the other co-monomers (see Fig. 1d). This is likely caused by differences in the copolymerization kinetics between the acrylate/methacrylate systems (P2 and P3) and the acrylate/acrylate system (P1).¶ The broad molar mass distributions observed in each case are attributed to changes in polymerization reaction kinetics over the course of the reaction due to the starved feed nature of the process, rather than any drastic composition drift caused by differences in comonomer reactivity. The relative chemical uniformity of the obtained materials is confirmed by GLC data, where each polymer (P1–P3) is observed as a single narrow peak (see Fig. S1, ESI†). Notably, both 3-pinanyl-containing polymers (P1 and P2) remained completely soluble, as indicated by the GLC (see Fig. S1, ESI†) and solution NMR analysis following freeze drying of the latexes (see Fig. S2–S5, ESI†). This confirmed that no significant crosslinking of the copolymers occurs under the current reaction conditions. This contrasts with our prior observations from solution polymerization, where polymers of PMA 2 were found to crosslink via H-abstraction from the tertiary carbon of the pinanyl moiety.35
All PBA-based emulsion polymers (P1–P3) had low Tgs (i.e. ∼−5 to −20 °C), which were very close to that predicted using the Fox equation31 (see Table 1, entries 1–3). This demonstrates that materials with predictable thermal properties can be readily prepared through this copolymerization process. As expected, based on the monomer feed compositions all BA-based polymer emulsions had ∼25% bioderived content based upon radiocarbon analysis of total solids (see Table 1).
![]() | ||
Fig. 2 (a) Plot of residual St (open symbols) and PA 1, PMA 2 or iBoMA 3 (closed symbols) concentrations versus time (the grey dashed line indicates the semi-batch feed profile in cumulative % of the monomers with time), and (b) SEC traces of St-based copolymers, for St/PA/MAA (P4, red), BSt/PMA/MAA (P5, blue) and St/iBoMA/MAA (P6, black) semi-batch emulsion radical copolymerizations. Note that the data related to MAA are omitted for clarity. P4–P6 refer to the sample labels given in Table 1. |
The Tgs of the St-based polymers (P4–P6) were relatively high (i.e. between 95 and 120 °C) (see Table 1, entries 4–6) Again, the Tgs were in agreement with those predicted using the Fox equation.31
As with the BA-copolymers, the St-based polymer emulsions had ∼25% bioderived content based upon total solids (see Table 1), in line with the monomer feed compositions.
Due to the presence of the acrylate co-monomer PA 1, the PBA-based polymer P1 had by far the lowest hardness on the König scale of the samples tested (7 s, see Table 2, entry 1); in view of their higher Tgs the PBA polymers (P2 and P3) containing methacrylate-based comonomers had much higher König hardness values as expected. Interestingly, polymer P2 which contains the pinene-based monomer (2) has the highest König value (22 s, see Table 1, entry 2), with the iBoMA (3) containing polymer P3 being slightly softer (14 s, see Table 1, entry 3). Hardness values can be further improved via the Tg by optimising the BA/P(M)A ratio in the copolymer.
Entry | Sample label | Copolymer | König hardnessa (s) | Stain resistanceb | |||
---|---|---|---|---|---|---|---|
Water (24 h) | 50% aq. ethanol (1 h) | Coffee | |||||
(1 h) | (16 h) | ||||||
a König hardness of copolymer films prepared on glass, and the values are an average of duplicate analyses, DFT = 40 μm. b Stain resistance of films prepared on cards, ranked by visual inspection on a scale from 0 to 5 (0 = no stain resistance; 5 total stain resistance), and the values in parentheses indicate exposure time. | |||||||
1 | P1 | P(BA63%-co-PA35%-co-MAA2%) | 7.0 ± 0.3 | 3 | 3 | 4 | 2 |
2 | P2 | P(BA63%-co-PMA35%-co-MAA2%) | 22.0 ± 0.9 | 4 | 2 | 4 | 3 |
3 | P3 | P(BA63%-co-iBoMA35%-co-MAA2%) | 14.0 ± 0.6 | 3 | 3 | 4 | 3 |
All three coating formulations had good to very good water stain resistance over a 24 h period, with the PMA 2 containing polymer P2 visually outperforming the other two formulations (P1 and P3) (see Table 2). In contrast, when exposed to 50% aqueous ethanol solution, P2 performed marginally poorer than P1 and P3 (see Table 2). All three polymer coatings (P1–P3) had excellent coffee stain resistance at 1 h, with performance decreasing as expected when exposed for 24 h (see Table 1). Notably, the polymers P2 and P3 which each contain a methacrylate-based comonomer behaved similarly, outperforming the acrylate-comonomer containing polymer P1.
Key developments in monomer syntheses include the replacement of hazardous and expensive reagents for established transformations. The one-pot hydroboration/oxidation of α-pinene was readily achieved using NaBH4 and AcOH in THF, followed by H2O2 and NaOH, to give 3-pinanol in moderate yield (58%), while the reaction of 3-pinanol with methacrylic anhydride and catalytic Cu(BF4)2) provided PMA 2 in high yield (90%). Importantly, both of these syntheses are sustainable, cost efficient and scalable. Additionally, PA 1 was readily prepared at a >100 g scale through the reaction of 3-pinanol with acryloyl chloride in high yield (87%), demonstrating the ease of access to larger volumes of these materials.
Copolymerization performance of PA 1 and PMA 2 was assessed by preparing copolymers with BA or St (and MAA) by semi-batch emulsion copolymerization. Analogous materials from the more established monomer iBoMA 3 were prepared as a reference. Successful copolymerization of the monomers 1–3 with BA (and MAA) gave latexes (P1–P3) with particle sizes between 65 and 90 nm and with a very low PDI (<0.06). The BA-based copolymers (P1–P3) had high molar mass (Mn > 40000, Mw > 400
000) and very high dispersity (Ð > 9.5). The BA-based materials showed bimodal molar mass distributions and low Tgs (∼−5 to −20 °C). Throughout the reactions the monomer feed ratios remained relatively constant, particle size was found to increase with monomer conversion, and particle size polydispersity index (PDI) remained low (<0.2). Copolymerization of the monomers 1–3 with St (and MAA) behaved similarly, giving latexes (P4–P6) with particle sizes close to 90 nm and with a very low PDI (<0.08). The St-based copolymers (P4–P6) had high molar mass (Mn > 40
000 and Mw > 150
000) and reasonably high molar mass dispersity (Ð > 3). In contrast to the BA-cases, the St-based copolymers had monomodal molar mass distributions and relatively high Tgs (∼95 to 120 °C). Importantly, GLC indicated that all the copolymers (P1–P6) had relatively uniform chemical composition distributions.
Preliminary coating application testing was performed on the low Tg BA-based materials (P1–P3). Overall, the PMA-containing copolymer (P2) appeared to be the best performing coating material; in comparison to the iBoMA-containing copolymer (P3), P2 was harder and had slightly better stain resistance. The PA-containing copolymer (P1) was the softest coating, which is in line with it having the lowest Tg, and displayed similar stain resistance to the iBOMA-containing material (P3).
In summary, we have demonstrated the industrially relevant scale up of the monomers PA 1 and PMA 2 using sustainable and cost-efficient processes, successfully incorporated them into copolymers via emulsion polymerization, and finally demonstrated the potential of these new polymers as coating materials. We expect that through refinement of the copolymerization process the properties of the polymers derived from these renewable monomers may be improved further. Companies that are interested in steps to bring these pinene (meth)acrylates to the market are invited to contact the authors.
Footnotes |
† Electronic supplementary information (ESI) available: Experimental details for polymer syntheses, gradient liquid chromatographs of all copolymers, and 1H and 13C NMR spectra of polymers P1 and P2. See DOI: https://doi.org/10.1039/d4su00210e |
‡ BH3·SMe2 is toxic, flammable, and preventatively expensive. It also has limited commercial availability for use on larger scales. Both methacryloyl chloride and acryloyl chloride are also toxic highly reactive reagents, and thus would be best avoided from a sustainability standpoint. |
§ Previously we also reported this synthesis using the reagent propanephosphonic acid anhydride (T3P) and acrylic acid (see ref. 18). While this approach is more sustainable than the route reported here, it is currently not economically (and industrially) viable at the reported scale because of the current cost of T3P. |
¶ Clearly the MAA presence in each of these systems will also play a role in the copolymerization kinetics. Due to its low feed ratio MAA is omitted from the discussion here to aid in the direct comparison across the copolymerizations using the acrylate (PA) and methacrylate (PMA & iBoMA) terpene-derived comonomers. |
|| This is attributed to the differences in polymerization rates for the ‘burn up’ reactions of the two different copolymerization systems, (i.e. the BA-based high kp systems and the St-based low kp systems). Note that as a fresh initiator is added in the ‘burn-up’ step of the reactions, the kinetics differ from that of the main polymerization reactions. |
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