Harvesting the benefits of inherent reactive functionalities in fully bio

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Harvesting the benefits of inherent reactive functionalities in fully bio-sourced isomeric benzoxazines Nagarjuna Amarnath, Swapnil Shukla, and Bimlesh Lochab ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b03631 • Publication Date (Web): 02 Oct 2018 Downloaded from http://pubs.acs.org on October 2, 2018

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ACS Sustainable Chemistry & Engineering

Harvesting the benefits of inherent reactive functionalities in fully bio-sourced isomeric benzoxazines Nagarjuna Amarnath, Swapnil Shukla and Bimlesh Lochab* Materials Chemistry Laboratory, Department of Chemistry, School of Natural Sciences, Shiv Nadar University, Gautam Buddha Nagar, Uttar Pradesh 201314, India. *[email protected]

ABSTRACT

Isomerization of double bonds from an allylic to propenyl position is generally mediated by expensive metal catalysts, demanding an additional synthetic step, thereby reducing sustainability of the reaction. However, such functionalities are inherent in naturallyoccurring compounds, enabling a versatile protocol for their industrial utility. Herein, we report the synthesis of benzoxazine monomers based on bio-sourced isomeric phenols, eugenol (E) and isoeugenol (IE), and biobased amine, furfurylamine (fa) to form E-fa and IEfa monomer, respectively. The structural variation in the phenols revealed a differential chemical reactivity, both during the synthesis of the monomer and polymerization reaction, confirming a significant influence of isomerism. The monomers only differ in the position of the double bond in the p-substituted propylene unit forming non-conjugated vs conjugated alkylene chain with the benzene ring containing benzoxazine in E-fa and IE-fa, respectively. The structure of the monomers was confirmed by 1H-NMR, 13C-NMR, FTIR, XRD, and mass spectrometry. The high purity of monomer was further affirmed by HPLC and DSC, to demonstrate the effect of isomerization on polymerization behaviour. The extended conjugation of the double bond in IE-fa with the proximal benzoxazine ring showed a higher reactivity toward ring opening polymerization, polymer conversion, and crosslinking reactions as supported by FTIR, NMR, and DSC-based kinetic studies. Thermal stability, mechanical properties, and adhesive analysis by TGA, DMTA and lap shear strength

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measurements further supported the effect of structural isomerism of monomers, with higher potential of PIE-fa over PE-fa network. Current work illustrates economic, one-step, microwave assisted, VOC's- and catalyst-free synthesis with a simultaneous solventless processing of synthesized monomers using renewable materials as feedstocks for high performance polymers. KEYWORDS: renewable, isoeugenol, sustainable benzoxazine, eco-friendly, isomerism

Introduction

Polymer industry is highly dependent on petroleum sourced feed-stocks to meet the ever-increasing demands of the society. The non-renewable nature and increasing costs of petrol demand an exploration of sustainable intermediates and raw materials, which are both efficacious and economic. Amongst thermosets, polybenzoxazines (PBzs) have emerged out as an attractive choice due to their superior physico-chemical and thermo-mechanical properties as compared to conventional resins.1 The synthetic requirement for benzoxazine (Bz) monomers is an aromatic phenol (with at least one free o-position), amine and formalin. The condensation reaction occurs at o-position to phenolic -OH with amine and formaldehyde to form 1, 3-benzoxazine ring in the monomer. Thermally assisted cleavage of the benzoxazine ring in monomer mediates ring opening polymerization (ROP) to form the PBz resin. Recently, replacement of conventional corrosive, toxic, and known endocrine disruptor phenolic sources viz. substituted phenols and bisphenol-A (BPA)2 with agrosourced cardanol,3-7 eugenol,8 guaiacol,9 vanillin10 has come up as an encouraging trend in benzoxazine chemistry. PBzs thus prepared from renewable phenols exhibited promising polymer properties enabling it as an acceptable and favorable strategy. In general, petroorigin amines are used for the synthesis of monomers due to the sparse availability of naturally occurring amines, limiting the realization of fully bio-based PBzs. So far, furfuryl amine (fa), and stearylamine (sa) have been mainly explored and incorporated to enhance the ensuing sustainable aspect.8,9,11-13 However, a lower degree of crosslinking ability, especially, in case of fully bio-sourced Bz monomers is still a pressing issue. To address this concern, a profoundly beneficial strategy relates to the introduction of additional crosslinking functionalities in the raw materials and hence, formed Bz monomer. Chemically tethered additional functionalities such as allyl,14,15 propargyl,16 ethynyl,17,


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trityl19 etc. are known to enhance the crosslink density and altering the subsequent thermomechanical properties of the polymer. In general, allyl functionalities are attractive due to their crosslinking ability at a lower temperature,20, 21 compatibility with many resins, ability to act as reactive diluents, crosslinking tendency,22-24 ease of functionalization with nanomaterials,25 and further versatile chemical modifications.26,27 Therefore, introduction of allyl functionalities and their modifications in Bz monomers is promising as they further advance their scope to a variety of applications. However, inclusion of such functionalities in starting raw materials demands an exclusive designing of synthetic scheme with extra synthetic steps. In addition, increased dependence on petroleum reserves, explosive nature during scale-up, requirement of toxic, and expensive metal (Sn, Mg, Pd) catalysts further affects their relevance to industry. However, this can be tackled using inherent allyl functionalities that are present in naturally occuring compounds. Additionally, current regulations imposed by Environmental Protection Agency (EPA, USA), and Registration Evaluation Authorization and Restriction of Chemicals (REACH, Europe) encourage the utility of naturally occurring raw materials as safer feedstocks for the development of biobased polymers. This approach is favoured and supported by both academic and industrial community worldwide. In the current study, we have utilized bio-based phenols, eugenol and its structural isomer, isoeugenol due to three reasons. Firstly, both are sourced from the naturally occurring abundantly available phenylpropanoids class of chemical compounds. Clove (Eugenia aromaticum or Eugenia carophyllate) oil is one of the primary source of eugenol with variation in relative content as 45-90 % depending on the geographical resource. Besides clove, it can be extracted from several other plants,28,29 namely African basil (Ocimum gratissimum), cinnamon (Cinnamomum verum), holy basil (Ocimum teuiflorum), nutmeg (Myristica fragrans) etc. and can also be potentially obtained from widely available natural lignin polymer.30 Isoeugenol also occurs naturally and interconversion of eugenol to isoeugenol is well documented in the literature.31,

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Secondly, the presence of in-built

phenolic-OH functionality and free o-position makes it a viable phenolic raw material for the synthesis of Bz monomer. Thirdly, presence of propylene substitution at p-position, with variation in position of the double bond is worth exploring as a maverick tool to alter PBz properties. In eugenol, propylene chain possesses an isolated terminal double bond, while double bond is in conjugation with benzene ring in case of isoeugenol (Figure S1). The introduction of such functional variant via chemical transformation is synthetically tedious and demands expensive toxic metal (Grubbs) catalysts thereby making the process cost3 ACS Paragon Plus Environment


ineffective.33,

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Olefin bonds are generally (especially in the monomer) desired as they

provide effective sites for modification and extension of network during polymerization.15 Therefore, natural occurrence of structural isomers of allyl group may provide an additional understanding to design the future Bz monomers, and in general, chemical intermediates for polymer industry. Amongst naturally sourced amines, furfurylamine (fa) is chosen as it imparts higher rigidity owing to the furan ring and is known to reinforce the PBz network.35 Moreover, fa is traditionally derived from furfural, a pentose sugar which is present in corn cobs, oat and rice hulls, bagasse, cotton seeds, olive husks and wood chips. The worldwide industrial production of furfural is expected to reach to > 600 kilotons by the year 2020 with an estimated attractive cost of ∼ 861 €/ton36 enabling fa as a better raw material for substituting amines sourced from petroleum origin in polymers including PBzs. Herein, we synthesized fully bio-based Bz monomers by condensation reaction of eugenol or isoeugenol with fa via conventional and microwave heating protocols. The monomers are structurally characterized by nuclear magnetic resonance (1H-NMR, and

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C-

NMR), Fourier transform infrared (FTIR), X-ray diffraction (XRD), and mass spectrometry. The purity of monomers is additionally determined by high pressure liquid chromatography (HPLC) and differential scanning calorimetry (DSC). The monomers are thermally polymerized using catalyst-free conditions and associated structural changes are monitored by FTIR, and 1H-NMR spectroscopy to understand the role of propenyl/allyl chain in crosslinking reactions. The kinetic study of polymerization reaction is determined by FTIR, NMR, and DSC to unravel the effect of isomerization on the Bz properties. The thermal behaviour, mechanical and adhesion properties are determined by DSC, thermogravimetry analysis (TGA), dynamic mechanical thermal analysis (DMTA), and lap shear strength (LSS) measurements respectively.

Results and discussion

Synthesis of fully biobased monomers. A greener methodology was adopted for the synthesis of fully bio-origin Bz monomers, E-fa and IE-fa, Figure 1a. The monomers were synthesized with microwave energy accounting to reduced reaction time (4.5 h, conventional heating vs 15 min. in MW) with the protocol using naturally-occurring raw materials, via solventless, one-step, and substantially atom economized reaction with water as the only by-product. The liquid nature of both amine and phenol at room temperature, lower viscosity, and high 4 ACS Paragon Plus Environment

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miscibility assisted an ease in solventless synthesis as revealed by good synthetic yields. As a purification protocol, washing with an alkaline solution, to remove acidic unreacted impurities from synthesized Bz monomers was followed as a generalized aqueous work-up step. Surprisingly, unlike E-fa, treatment of IE-fa with base resulted in the formation of a white precipitate (Figure 1b), suggesting substantial difference in reactivity of two monomers due to the variation in structural features at p-position. This differential reactivity with base is also reconfirmed by the corresponding phenols, Figure S2. Amongst other probable reactions of aqueous sodium hydroxide on IE-fa, base mediated ring opening reaction followed by oligomerization is apparent as bases are known to assist ROP of benzoxazines.26, 27 Probable reaction of base with IE-fa is shown in Figure 1c. The phenoxide ion formed upon reaction of base with IE-fa and E-fa, is more stabilized due to extended resonance along the alkylene chain in the former structure.

Figure 1. a) Solventless synthesis of Bz monomers; b) effect of treatment of base on monomer (1 mL aqueous sodium hydroxide solution added to 100 mg/mL monomer solution



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in chloroform): (i) E-fa, and (ii) IE-fa; c) probable base mediated ring opening reaction of benzoxazine. The structural characterization and purity of monomers is analyzed by FTIR, 1H-, 13CNMR spectroscopy, HPLC, mass spectrometry, X-ray spectroscopy and DSC. FTIR spectra (Figure S3) confirmed the disappearance of phenolic O-H (~ 3500 cm-1) and N-H (~ 3300 cm-1) stretch suggesting absence of phenolic and amine impurities in Bz monomers. The condensation reaction of phenolic O-H with fa to form the Bz ring is suggested due to appearance of C-O-C asymmetric (1220 cm-1), and symmetric (1090 cm-1) stretches. The characteristic peaks at 1590 and 1000 cm-1 corresponds C- N-C due to amine and furan ring confirming its incorporation in the Bz monomer. The peak at 3005 cm-1 and ~ 915 cm-1 is assigned to the C-H stretch and C-H bending vibrations of the allylic bond respectively.37 1

H-NMR (Figure 2a-b) showed the characteristic signals at 3.98 (s, ArCH2N), and ~ 4.96

ppm (s, ArOCH2N) confirming successful formation of Bz ring in monomers. Other signals consistent in the spectra of both E-fa and IE-fa owing to high structural similarities include methoxy at 3.86 ppm (s, -OCH3), methylene bridge at 3.93 ppm (s, -CH2-) between furan and benzoxazine, and furan ring at 6.24, 6.31 and 7.39 ppm respectively. An obvious difference in NMR signals of the two benzoxazine moieties was noticed due to p-substituted functionalities. In E-fa, additional peaks centred at 3.28, 5.06 and 5.94 ppm are assigned to (CH2-CH=CH2) allylic protons. While IE-fa exhibited a different set of signals, in upfield region at 1.84 ppm (s, -CH3) and in downfield region at 6.08-6.12 (m, CH=CH-CH3) and 6.26 (d, CH=CH-CH3) due to conjugation of double bond of propenyl chain with the aromatic benzene ring. The absence of signals in the region 2.5 - 4.0 and 4.2 - 4.8 ppm suggests that the synthesized Bz monomers are devoid of any oligomeric impurities.

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C-NMR spectra,

Figure 2c-d, showed Bz carbon resonances centred at 55.85 and 82.37 (E-fa), 55.84 and 82.55 (IE-fa) due to ArCH2N and NCH2O in the ring respectively. Mass spectrometry showed a nearly similar molecular ion, [M + H]+ peak at 286.1456, consistent with the isomeric structure, Figure S4.


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Figure 2. 1H- and 13C-NMR spectra of Bz monomers: E-fa (a, c) and IE-fa (b, d) in CDCl3.

It must be noted even though NMR spectra of monomers showed respective integration confirming the formation of monomer, still it is not sufficiently sensitive technique to 8 ACS Paragon Plus Environment

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comment on the purity of monomer. The high purity of Bz monomers is crucial to determine the effect in properties of polymers due to isomerization, as the presence of phenolic, amine and oligomeric impurities in Bz is known to catalyse ROP reactions.38, 39 Therefore, both the monomers were further purified by various techniques to allow better comparison in properties and to elucidate the role of double bonds in alkenyl chain in polymerization, if any. Purification of IE-fa is relatively easy and recrystallization is successful to gain extra pure crystals as confirmed by X-ray diffraction studies. Monoclinic structure (Figure 3, Table S1) and absence of impurities is confirmed by crystal analysis.

Figure 3. Ortep diagram of IE-fa.

On the contrary, single crystals of E-fa could not be obtained despite variations of solvent and storage conditions to grow crystals. Flash chromatography system is a widely accepted purification strategy in industry with a potential to purify milligrams to grams of pure compound from complex mixtures, easily and rapidly. Moreover, there are no reports so far on its utility in benzoxazine chemistry for purification of monomers. Therefore, flash chromatography technique was explored for its potential in the purification of E-fa. Surprisingly, flash chromatography on C-18 column led to the removal of inseparable impurities in E-fa, which were difficult to remove after repetitive purification by traditional silica/alumina column. Simultaneous detection of eluted fractions containing impurities were rejected as recognized by highly sensitive ELSD and UV detectors for noticing non-UV- and UV-sensitive compounds respectively. Such impurities are not detected with similar proficiency on a conventional thin layer chromatography plate analysis. The fractions containing pure E-fa monomer were separated as shown Figure S5.



Figure 4. Stacked HPLC chromatograms: a) phenols (Eugenol, Isoeugenol) and Bz monomers (E-fa and IE-fa); b) effect of NaOH (3N, aqueous) treatment on IE-fa monomer. The samples were eluted in water-acetonitrile mixture containing 1% formic acid (λ = 297 nm) at 37 °C.

HPLC was utilized for the determination of purity of monomers quantitatively. HPLC traces of phenol and Bz monomers, Figure 4a, revealed elution time of E, IE, E-fa, and IE-fa as 13.7, 13.9, 9.5, and 9.4 min. respectively. The absence of peaks due to phenols confirmed their absence in purified E-fa and IE-fa monomers. Furthermore, a lower retention time of Bz monomers than the corresponding phenols supported their relative difference in polarity. The absence of fa impurity was also re-confirmed by HPLC (Figure S6). Additionally, HPLC analysis revealed the effect of alkaline sodium hydroxide solution on IE-fa (during aqueous work-up). In addition to the peak due to monomer at 9.4 min., a broad peak at lower retention time of 4.9 to 5.7 min. appeared in base treated IE-fa, Figure 4b, supporting our earlier observations during synthesis (Figure 1b). This peak at lower retention time can be attributed to the oligomeric impurities. The relative percentage of monomer decreased substantially by 86% and a further increase in treatment time to 12 h led to a significant broadening and shifting of peak to lower retention time indicating formation of more heterogeneous structures.

It is reported that DSC studies can also be used as a tool to determine the purity of Bz monomer from the curing exotherm. As the percentage purity increases by recrystallization, the Tp of curing exotherm in Bz monomer was shifted to a higher temperature and became


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constant suggesting occurrence of highly pure form of monomer.40 Both E-fa and IE-fa were purified by a series of recrystallization procedures after initial column chromatography purification to allow determination of pure monomer by DSC (Figure S7a-c). Surprisingly in our case, the polymerization temperature remained nearly unaffected after each recrystallization step of the monomer. However, ∆H value of curing reaction showed significant variations. To ascertain this behaviour, simultaneous HPLC (Figure S7b-d) was recorded, that is suggestive of existence of impurities after each purification stage. The purification was continued until a single Gaussian peak corresponding to the monomer was detected in HPLC. Purification by recrystallization was not achieved as indicated from the broadening of DSC thermograms (Figure S7a) in case of E-fa, while sample purified by flash chromatography showed a prominent curing exotherm, with subsequent confirmation by HPLC (Figure S7b). On the contrary, IE-fa monomer is purified successfully by recrystallization, which is also in congruence with the XRD data.

Curing Behaviour and Polymerization of the Bio-based Benzoxazine Monomers. The oand p-blocked Bz monomers suffer from the issue of lower crosslink density as these positions are unavailable for polymerization reactions.41 A lower extension of polymer network is anticipated as both E-fa and IE-fa have substituents at o- and p-positions in the benzene ring. Alternatively, in our approach existence of fa and propenyl/allyl at p-position in both the monomers are expected to be involved in development of the polymer network due to their reactive nature.

Figure 5 shows FTIR analysis of ring opening polymerization reaction in Bz monomers. The variation in absorbance intensity in E-fa and IE-fa was studied by nonisothermal curing studies. To assist a comparative study, absorbance was normalized using the band of asymmetric deformation of methyl group in -OCH3 as a reference (~2850 cm-1) which remained unaffected during the curing reaction. Both the monomers exhibited similar curing behaviour with the disappearance and broadening of characteristic Bz peaks at 1225 and 1090 cm-1 progressively with an increase in temperature. The variation in alkene C-H stretch and bending vibrations (due to alkenyl group) at 3005 and ~ 915 cm-1 is indicative of their involvement in the crosslinking reactions. The % decrease in normalized intensity of alkene double bond and C-O-C stretch of benzoxazine ring with temperature was found to be relatively faster in IE-fa than E-fa, Figure 5c. A concomitant decrease in relative intensities of both the reactive functionalities is suggestive of their involvement in ring opening reaction 11 ACS Paragon Plus Environment


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and/or crosslinking reaction via alkene bond. The double bond may act as an electrophile and undergo electrophilic reactions with the generated iminium ion intermediate from benzoxazine ring opening reaction and furan moiety.

Figure 5. Normalized transmission IR absorption spectra of non-isothermally cured: a) E-fa; b) IE-fa; and c) Relative change in intensity due to alkene double bond C=C-Hstretch and C=CHbend. The blocked o-position or sterically hindered Bz monomer affects the formation of ring opened structures via true Mannich linkages. The presence of o-methoxy group in both E-fa and IE-fa is reflected on mechanism of ROP reaction and related kinetics. Time dependent normalized NMR studies of both monomers revealed vanishing of O-CH2-N signal to form initially phenoxy-type and furfural-type linkages, while former linkages rearranged to phenolic-type with the advancement of polymerization reaction at high temperature,42,

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Figure 6a. The signals between 4.3 to 4.4 ppm are apparent during 0.5 h of polymerization, which are attributed to the phenoxy structures, but with an increase in time, signals at 2.7 4.4 ppm became apparent supporting ROP to form thermally stable phenolic structures via rearrangement of thermally labile phenoxy structures, Figure S8. The evaluation of % ROP supported a faster ROP with relatively higher monomer conversion in case of IE-fa, Figure 6b as evident from faster disappearance of characteristic O-CH2-N signal at ~ 4.7 ppm (Figure S8c). The rate of disappearance of characteristic benzoxazine signals are faster in case of IE-fa than E-fa, Figure 6c, further confirming the assistance provided by extended conjugation along the alkenyl chain in former monomer to favour ROP.


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Figure 6. a) Probable polymer structures formation in o-blocked furfurylamine based benzoxazine monomer; b) Percentage ROP; and c) decrease in characteristic benzoxazine signals as determined from NMR kinetics. Interestingly, a significant increase in integration of methyl protons in IE-fa polymerization was noticed (Figure S8c and S9), suggesting involvement of propylene units in crosslinking reaction. However, no signals due to newly formed methylene units (0.8-2.0 ppm) owing to crosslinking reaction of allyl chains in E-fa is apparent by NMR spectroscopy, Figure S9b. Figure S10 showing the DSC profiles for the monomers, E-fa and IE-fa revealed a melting endotherm followed by a curing exotherm transition. E-fa and IE-fa showed the characteristic temperatures i.e. Tm, To , and Tp as 91, 224 and 241 oC, and 99, 215 and 238 oC respectively. A lower melting point of E-fa than IE-fa is ascribed to the better packing efficiency of the latter, in congruence with XRD. A single and broad curing exotherm was observed which may be attributed to simultaneous ROP and crosslinking reactions mediated by alkenyl chains. In addition, heat released during monomer to polymer conversion, ∆H, for 13 ACS Paragon Plus Environment


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E-fa and IE-fa was determined as 47 and 53 Jg-1 respectively. This variation in curing characteristics confirmed preferential participation of propenyl over allyl functionalities, in congruence with FTIR and NMR results. Both the monomers showed a wide processing window [Tm - To: 116 (IE-fa) and 133 oC (E-fa)] indicating an ease in processing of both the monomers industrially. It is important to understand and compare the thermal characterization results of both E-fa and IE-fa with other allyl and structurally related mono-benzoxazine monomers reported in the literature, Table 1. Table 1. Thermal characterization of allyl containing and structurally related monobenzoxazine monomers and polymers Tpolymerization (oC) Structure

Tm (oC)

To

Tp

Tdecomposition (oC) T10% T5%

E-fa

91

224

241

338

IE-fa

99

215

238

G-fa

91

228 (219)

P-fa

55

P-a

Char Yield (%)

LOI44

Tg

Ref.

364

52

38.3

162† 183‡

this work

375

395

60

41.5

164† 179‡

this work

241 (240)

296 (352)

-

54 (56)

39.1 (39.9)

(148)*

45, 9

233

241

336

382

53

30

315‡

35

n.d.

202

230

342

369

44

35.1

146† 161‡

15

P-aa

n.d.

145, 225

207, 259

348

374

44

35.1

285† 297‡

15

AP-a

n.d.

241

263

288

356

45

35.5

107†

15

Name

LOI: Limiting oxygen index, LOI =17.5 + 0.4 (Char yield, %); Tg values are reported from *DSC, loss modulus† and tanδ‡.

Earlier reported work on E-fa showed a Tm, To and Tp as 74, 198 and 222 oC respectively, which is substantially lower than our case.8 In their work, they have not considered and studied the purity of monomer and lowering in curing temperatures may be accounted to the residual starting materials and oligomeric impurities in the monomer. Therefore, it is worth


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pointing out that the properties determined may not be an ideal depiction of both the monomers and polymers. A control Bz monomer to E-fa and IE-fa, with no p-substitution i.e. guaiacol furfurylamine (G-fa) showed Tm, To, and Tp of 91, 228 and 241 oC respectively.45 While another work on G-fa reported a To value of 219 oC.9 In general, G-fa monomer showed a higher To and similar Tp as IE-fa suggesting prior involvement of double bonds to mediate ROP reaction of benzoxazine ring due to the ease in electronic availability of alkenyl

π −bonds. Phenol furfurylamine (P-fa)50 showed a similar Tp of 241 oC and a very high ∆H of 265 Jg-1 than IE-fa, again suggesting no influence on Tp due to the presence of o- and psubstitution in IE. A higher ∆H of 265 Jg-1 than both E-fa and IE-fa monomers is attributed to the free o- and p-positions in P-fa, which allowed for ease of extension of ROP reaction to form true thermally stable Mannich bridges accounting for a highly exothermic nature. The replacement of furfurylamine with aniline in P-a15 resulted in a lowering of cure temperatures which can be attributed to the resonance stabilization provided by the aromatic ring to the intermediate iminium ion.40 In cases, where allyl groups is on the amine counterpart are also worth comparing with the synthesized fully bio-based monomers. For petro-origin based phenol allylamine (P-aa) monomer, two exotherms are observed with Tp at 207 °C and 260 °C associated with the thermal cure of the allyl group (attached to N), and formation of PBz respectively.15 This confirms that allylic polymerization precedes benzoxazine ROP curing reaction. Both E-fa and IE-fa showed a very low Tp for ROP than P-aa, which can be attributed to the assistance provided by alkenyl groups (when present in phenolic ring) in ROP reaction. However, in case of allylphenol aniline (AP-a) monomer, where allyl group is at the o-position, a single curing exotherm is observed with To, and Tp at 241 °C, and 263 °C respectively. This suggests that the position of double bond in alkenyl chain of Bz monomer plays a crucial role in polymerization behaviour. The allyl group at o-position sterically hinders rather than providing assistance to polymerization. Furthermore, presence of allylic group at p-position is less sterically hindered than at o-position (as in AP-a) restricting their accessibility to react with other co-monomers for copolymerization. To summarize, both a very high To, and Tp is reflected in petro-sources mono-benzoxazine monomers confirming bio-based Bz monomers, IE-fa and E-fa as equally competitive alternatives for ROP reaction. Effect of free radical initiator, AIBN is worth exploring to understand the potential of E-fa and IE-fa monomers for free radical polymerization. Both E-fa and IE-fa showed a new exotherm starting at ~107 °C with maximum at 125 °C, inset Figure S10, along with a larger exotherm due to curing reaction by ROP. The appearance of a new exotherm at much lower



temperature range revealed the catalytic effect of the free radical initiator to initiate the addition polymerization reaction of p-allyl group. A significantly low value of ∆H for E-fa and IE-fa as 13 Jg-1 and 7 Jg-1 respectively is determined. This is attributed to the steric hindrance in IE-fa as β-methyl substituted styrene structures are more stable and less prone to undergo spontaneous radical polymerization to form homopolymers.46 The predominance of degradative chain transfer in allylic polymerization in case of E-fa also led to the lowering of tendency to homo-polymerize. Surprisingly, an insignificant affect in ∆H value and curing temperatures of exotherm associated with benzoxazine polymerization is noticed. However, both E-fa and IE-fa have potential to undergo free radical co-polymerization with many monomers such as maleic anhydride to form effective thermally stable covalently linked blends.47, 48 The kinetic study of polymerization of E-fa and IE-fa was also performed using nonisothermal DSC analysis, to determine the apparent activation energy of the polymerization process. The activation energy (Ea) was determined using Kissinger49 and Flynn Wall Ozawa50 methods, Figures S11-S12, and Table S2. The Ea calculated from the slope of curing exotherm using Kissinger and Ozawa’s plots is evaluated as ~197 and ~134 kJ/mol respectively. Interestingly, Ea of IE-fa is ~ 63 units lower than E-fa which is attributed to the presence of more reactive conjugated double bond at p-position to assist crosslinking reactions than a terminal non-conjugated allyl group in E-fa. P-a monomer exhibited a lower Ea value of 84 kJ/mol51 than IE-fa which is due to its free o-position, while the Ea value of Efa is in good agreement to G-fa monomer (231 kJ/mol).52 The lower Ea of IE-fa than G-fa may be explained due to easier accessibility of propylene unit in crosslinking reactions because of its dangling and higher degree of freedom as compared to the rigid benzene ring in latter. The probable mechanism and structures associated with the ROP of E-fa and IE-fa is shown in Figure 7 as supported by FTIR, NMR and DSC studies. The benzoxazine ring opens up to form the intermediate iminium ion, which is more stabilized in IE-fa due to extended resonance across the propenyl chain, while it is restricted to the benzene ring in case of E-fa. Besides crosslinking reactions of iminium ion and furan to form N, O-acetal and general Mannich bridges, alkenyl chain may also participate in extending the crosslink network. The probability of their involvement led to the formation of longer unsubstituted alkyl chains vs shorter methyl-substituted ones within the PE-fa and PIE-fa matrix respectively.


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Figure 7. Probable mechanism of ring opening polymerization in Bz monomers a) E-fa and b) IE-fa. Heat induced intrinsic thermal polymerization as monomers are devoid of any impurities.



Thermal stability and mechanical properties of polymers. Thermal stability of monomers is another parameter important in determining the effect of molecular designing on resulting polymeric properties and ensuing industrial applications. TGA (Figure 8a) traces for both the polymers differ considerably. PIE-fa exhibited a higher thermal stability as compared to PEfa with a higher T5% (∆ = 37 oC), T10% (∆ = 31 oC) and char yield (∆ = 8%). The observed lower thermal stability of PE-fa may be explained by the formation of longer alkylene chain (formed during polymerization) and presence of residual un-crosslinked allyl chains which might not be the part network formation (Figure 7a). On the other hand, in case of PIE-fa, double bond is more reactive towards electrophilic addition reactions and involved to a higher extent in PBz network formation (Figure 7b). The DTG traces (Figure 8b) showed a maximum mass loss occurring at a lower temperature for PE-fa as compared to PIE-fa. In comparison to other bio-based origin polybenzoxazines, both PE-fa and PIE-fa exhibited explicitly a better and comparative stability towards thermal degradation, Table 1.

Figure 8. a) TGA and b) DTG analysis of PE-fa and PIE-fa.

PG-fa showed a lower thermal stability confirming role of alkenyl chains in imparting a network with higher crosslink density.9, 45 PP-a also revealed a lower thermal stability35 as furan-containing PBz extend Mannich bridges network to enhance crosslink density thereby contributing to a higher thermal stability with high char yields. In conclusion, both PE-fa and PIE-fa exhibited potentially higher thermal stability than other reported mono-benzoxazine based poly(allylbenzoxazines). Furthermore PBz resin based on bis-benzoxazine of eugenol and p-phenylenediamine, PE-pPDA, showed substantial thermo-degradation (mass loss ~30%) at 270 oC which is assigned to the decomposition of eugenol component.53 However, 18 ACS Paragon Plus Environment

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PE-fa showed a meagre mass loss (2%) at 270 oC, while no such mass loss is observed in PIE-fa suggesting involvement of alkylene chain in polymerization leading to a more thermally stable fully bio-based polymer. In order to gain further insights in the structural performance of different isomers on mechanical properties, DMTA was performed (Figure S13). The initial storage modulus of PIE-fa is higher than PE-fa with a value 268 and 171 MPa respectively indicating a higher crosslink density of the former polymer. Interestingly, with an increase in temperature further ≥ 180 oC, a crossover in storage modulus values was observed. This suggested attainment of more rigidity in PE-fa network due to the crosslinking reactions which are mediated at higher temperature. A fairly well developed rubbery plateau was observed in both PE-fa and PIE-fa, unlike previously studied cardanol based PBz resins which directly highlights the positive effects of a reduced side chain and incorporation of fa vs benzene in the structure.6 A nearly similar Tg values of ~163 oC were deduced from loss modulus for both the polybenzoxazines. Even tanδ peak showed not much variation in Tg values among PIE-fa and PE-fa, 179 and 183 oC respectively, with a very broad full-width half maxima in case of latter network. Apparently a broader tanδ peak in PE-fa indicates formation of various types of linkages during curing reaction. The heterogeneous structural feature is associated with wide array of arrangements showing segmental mobility over a wide range of temperature. This variation is an indication of formation of longer aliphatic flexible segments, domains of lower and higher crosslink density, and presence of residual un-crosslinked allyl chains leading to an extra free volume within the framework. In addition to covalent crosslinks, associated hydrogen bonding extensions amongst ring opened structures also accounted for the variation in segmental mobility. All the above mechanical properties are attributed to the existing structural motifs in two networks propenyl vs allyl, as the extent of H-bonding interactions are expected to be of similar extent. Finally, the conversion of monomer to polymer occurred via an environmentally benign, solvent-, catalyst-, VOCs-free melt processing technique with application as adhesive for steel substrates. PIE-fa exhibited a lap shear strength 25.8 kg/cm2 (Figure S14) while samples based on PE-fa, reflected no adhesion strength, confirming a pivotal role played by alkenyl chain in governing the properties of polymers.

Conclusions A pair of isomeric fully bio-based Bz are synthesized based on eugenol and isoeugenol, and fufurylamine as the renewable phenolic and amine components respectively. In IE-fa, the 19 ACS Paragon Plus Environment


presence of side chain in extended conjugation with the aromatic ring is found to manifest its effects on the properties of monomers and polymers. The purity of monomers holds special relevance in polymerization behaviour. The participation of propenyl chain of IE-fa led to the attainment of faster ROP with higher polymer conversion and well defined crosslinked network as elucidated from FTIR, NMR, TGA, and DMTA results. A higher storage moduli, and good adhesion characteristics of PIE-fa are suggesting its suitability for applications demanding good mechanical properties. The above results, clearly point to the higher potential of naturally occurring raw materials as feedstock in polymers for controlling final properties and the resultant applications. In addition, it also open up the possibility of using their inbuilt functionalities efficiently avoiding toxic, lengthy, time-consuming, costineffective synthetic protocols on petro-sourced raw materials. This paper reinforces the efficacy of isomerization, conjugation and existence of structurally rigid linkages as strategies to modulate the properties of Bz monomers on the template of fully bio-based benzoxazines. ASSOCIATED CONTENT Supporting Information. Experimental section, structures eugenol and Isoeugenol; FTIR spectra of E-fa, IE-fa, E and IE; DSC scan of E-fa, IE-fa with and without free radical initiator; Viscoelastic analysis of PE-fa and PIE-fa; Effect of normality of aqueous NaOH on phenols Eugenol and Isoeugenol; Mass spectra of E-fa and IE-fa, Crystal data for compound IE-fa, figure of purification of E-fa monomer by flash chromatography; stacked HPLC plots of E, IE, fa, E-fa, IE-fa at λ= 224 nm; DSC and corresponding HPLC profiles of monomer after successive purification of E-fa and IE-fa, Normalized time dependent 1H-NMR spectra of E-fa and IE-fa; degree of conversion vs temperature; isoconversional plots; apparent activation energy (Eα) at different conversion; images of representative stress vs strain curve of PE-fa and PIE-fa on steel coupons.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Tel: (+91-120) 3819 100

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. 20 ACS Paragon Plus Environment

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Graphical abstract

Smartly designed fully bio-sourced isomeric benzoxazine monomers revealed a differential reactivity towards formation of polymers with excellent thermal stability


Harvesting the benefits of inherent reactive functionalities in fully bio

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