Characterization of degradation products from a hydrolytically degradable cationic flocculant

Journal Pre-proof Characterization of degradation products from a hydrolytically degradable cationic flocculant Derek A. Russell, Louise Meunier, Robin A. Hutchinson PII:

S0141-3910(20)30029-X

DOI:

https://doi.org/10.1016/j.polymdegradstab.2020.109097

Reference:

PDST 109097

To appear in:

Polymer Degradation and Stability

Received Date: 3 October 2019 Revised Date:

17 January 2020

Accepted Date: 27 January 2020

Please cite this article as: Russell DA, Meunier L, Hutchinson RA, Characterization of degradation products from a hydrolytically degradable cationic flocculant, Polymer Degradation and Stability (2020), doi: https://doi.org/10.1016/j.polymdegradstab.2020.109097. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.

Characterization of Degradation Products from a Hydrolytically Degradable Cationic Flocculant Derek A. Russell, Louise Meunier*, Robin A. Hutchinson* Department of Chemical Engineering, Queen’s University, Kingston, Canada K7L 3N6 [email protected], [email protected]

Abstract: The cationic flocculant poly(lactic acid) choline iodide ester methacrylate, poly(PLA4ChMA), is a promising candidate for dewatering oil sands mature fines tailings (MFT) that enhances solids consolidation as it undergoes partial hydrolysis. However, the degradation products have yet to be fully characterized. In the present work, proton nuclear magnetic resonance (1H-NMR) spectrometry is used to study the rate of hydrolytic degradation of poly(PLA4ChMA) and its precursor macromonomer in aqueous solution by tracking the release of and fully identifying the products. Although a higher temperature and a higher initial pH enhance the macromonomer degradation rate, the chemical identity and relative amounts of the degradation products are the same as those released at milder conditions. The majority of macromonomer molecules degrade at 50 °C to a compound containing a single lactate unit attached to the methacrylate functional group, with 10 – 20% degrading completely to yield methacrylic acid. The polymer releases the same choline and lactic acid oligomer species, but at a much faster rate, over 1 – 2 weeks at 50 °C vs. 8 – 11 weeks for the macromonomer. At the end of degradation, the partially degraded water-insoluble polymer, ~30 wt.% of the original mass, contains on average approximately two lactate units attached to each methacrylate unit of the backbone, explaining the increase in hydrophobicity that is beneficial to the dewatering of MFT.

Keywords: degradable polymer, dewatering, flocculation, hydrolysis, hydrolytic degradation Page 1 of 29

1. Introduction Poly(lactic acid) (PLA) and poly(ɛ-caprolactone) (PCL) are biodegradable polyesters important for a range of functions, including biomedical applications.1 Ring-opening polymerization (ROP) is a common method to produce high-molecular weight PLA and PCL polymers.2 PLA is produced from renewable resources,3 whereas PCL is produced from ɛ-caprolactone, which comes from petrochemical resources.4 PLA is used as raw material for packaging and bottles.5,6 PCL is used in the production of biocompatible nanocomposite materials.4 The Moscatelli group demonstrated that the rate of polymer hydrolytic degradation can be tuned by attaching the polyester units to methacrylate double bonds to produce macromonomers that are radically polymerized to produce nanoparticles.2,7–9 PLA and PCL polymers undergo degradation in aqueous solution by hydrolysis of the ester bonds.7,10 While random chain scission of PLA is known to occur, the degradation rates involving chain ends are higher11,12,13 and dependent on solution pH.14,15,16 In an acidic environment (pH < 4), PLA degradation occurs mainly through chain-end scission, i.e., the sequential release of lactate units.14,15,17,18 However, under basic conditions (pH > 7), the degradation also occurs through backbiting, an intramolecular cyclization step to produce lactide (in cyclic dimer form) that hydrolyzes to lactyl lactate and then to lactic acid.12,14,17 A similar chain scission mechanism for the hydrolytic degradation of PCL polymers is proposed in literature.10,19,20,21 Recently, partially degradable flocculants have been synthesized by radical polymerization of cationic macromonomers containing a quaternary ammonium group attached to either PLA or PCL units, and investigated for applications in the treatment of mature fine tailings (MFT), waste

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materials arising from oil extraction of oil sands.22,23 Compared with non-biodegradable commercial polyacrylamide (PAM) flocculants, PLA- and PCL-based cationic flocculants become more hydrophobic with partial hydrolytic degradation, as indicated by increased dewaterability of oil sands sludge by 50 vol% using the cationic flocculants versus negligible amounts when using PAM flocculants.23 Both PLA- and PCL-based polymers undergo degradation by hydrolysis of the ester functional groups from each repeat unit of the polymer chain, resulting in mass loss and the release of LA or CL oligomers.2,9 However, further research work is required to completely characterize the hydrolytic degradation products from the cationic version of these polymers used as flocculants.23 Ferrari et al. and Agostini et al. originally developed synthetic PCL-based nanoparticles for biomedical applications.2,24 Ferrari et al. synthesized polymer from macromonomer based on hydroxyethyl methacrylate (HEMA) units grafted with various numbers of caprolactone units (i.e., poly(HEMA-g-CLk) where k is between 1 and 10) whereas Agostini et al. added a cationic end group to the macromonomer through condensation with choline. Both investigations carried out degradation studies. Ferrari et al. concluded that the poly(HEMA-g-CLk)-based nanoparticles undergo hydrolytic degradation, releasing water-soluble acidic products such as 6 hydroxycaproic acid to yield a completely water-soluble poly(HEMA)-based backbone.2 Similarly, Agostini et al. reported that the PCL-based nanoparticles synthesized with cationic functionality release choline and PCL-based species including succinic acid and 6 hydroxycaproic acid to also leave a water-soluble poly(HEMA) backbone.24 These are indications that the synthesized PCL-based cationic nanoparticles degrade to yield completely water-soluble products.

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The synthetic cationic polymeric flocculant poly(caprolactone) choline iodide ester methacrylate, poly(PCL3ChMA), was developed through micellar radical polymerization of the cationic polyester macromonomer.22 The macromonomer synthesis strategy differed from that used by Agostini et al.,24 such that complete cleavage of the polyester groups from the resulting polymer would yield a poly(methacrylic acid) (poly(MAA)) rather than a poly(HEMA) backbone.22,23 After 10 days of degradation at 85 °C, the mixture of water-insoluble polymer and water-soluble degradation by-products of this poly(PCL3ChMA) material indicated only partial hydrolysis of the polymer chain.22 This partial degradation releases acidic oligomers and choline degradation products into the aqueous solution, and was hypothesized to leave a single hydrolyzed caprolactone ester functional group attached to the hydrocarbon chain, making it water insoluble.22 Younes et al. developed several variants of the cationic polymeric flocculants for the treatment of oil sands MFT by radical polymerization of macromonomers that contained either PCL and PLA units between the cationic functionality and the methacrylate or acrylate double bond.23 As they degrade, these cationic flocculants further expel water after the treated MFT sludge settles.23 Younes et al. also demonstrated that the partially degradable flocculant synthesized from the LA - based methacrylate version of the macromonomer results in greater than 50% further compaction of MFT sediments at ambient temperature (~20 °C) after initial settling over a 12 week period, making it the most promising compound for further study.23 The chemical structure of poly(lactic acid) choline iodide ester methacrylate (PLA4ChMA) macromonomer, containing on average 4 lactate units between the quaternary ammonium-iodide group and the reactive double bond, and the resulting polymer produced by radical polymerization23 are shown as Scheme 1.

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I-

Radical Polymerization

I-

H2O

Scheme 1. Radical polymerization of the cationic PLA4ChMA macromonomer to poly(PLA4ChMA) polymer. Subscript m represents the number of repeat units in the polymer chain. The PLA4ChMA macromonomer including iodine counterion has a number-average molecular weight, Mn, of 548 g/mol and weight-average molecular weight, Mw, of 592 g/mol, with a charge density of 1.70 mmol/g.23

It is the partial hydrolytic degradation of these novel cationic polyester-based compounds that offer advantageous dewatering capabilities. However, the characteristics of the water-soluble degradation products from these cationic flocculants as well as the structure of the remaining partially degraded polymer chain have yet to be investigated.23 In the present work, degradation products of the PLA4ChMA macromonomer are first completely identified using proton nuclear magnetic spectroscopy (1H-NMR) techniques, with the rates of degradation followed at two different temperature and initial pH values. The 1H-NMR peak assignments are then used to study the degradation rate of the cationic flocculant, poly(PLA4ChMA), by following the release of degradation products while also tracking the structural change of the partially degraded polymer.

2. Materials and methods 2.1 Materials Choline chloride ((2-hydroxyethyl)trimethylammonium chloride, > 98%), lactic acid solution (ACS reagent, > 85 wt.% in water), deuterium oxide (D2O, 99.9% D), sodium deuteroxide Page 5 of 29

(NaOD, 40 wt.% in D2O), and 2,2’-azobis(2-methylpropionamidine) dihydrochloride (C8H20Cl2N6, denoted V-50, 97%) were purchased from Sigma-Aldrich and used as received in the preparation of samples for 1H-NMR analysis. Poly(methacrylic acid) powder (poly(MAA), 100%) was purchased from Scientific Polymer Products and used as received in the preparation of samples for 1H-NMR analysis. Deuterated dimethyl sulfoxide (DMSO-d6, 99.5%) was purchased from Cambridge Isotope Laboratory and used as received in the preparation of partially degraded poly(PLA4ChMA) and poly(MAA) samples for 1H-NMR analysis. Hydrochloric acid (HCl, ACS reagent, 12.06 M) was purchased from Fisher Scientific and used, along with NaOD, to adjust aqueous pH as necessary. Distilled water (produced on-site, ~200 – 300 µS/cm) was used in the polymerization of macromonomers. 2.2 Synthesis of PLA4ChMA macromonomer and polymer The poly(lactic acid) choline iodide ester methacrylate (PLA4ChMA) macromonomer was synthesized according to the procedure described previously23 and reacted via batch radical polymerization to full conversion at 70 °C in a 10 wt.% aqueous solution to produce polymer with weight-average molecular weights in the range of 5-10 × 105 Da.23,25 The resulting polymers were then freeze-dried to produce a powder of poly(PLA4ChMA) ready for use. In addition, a similar batch radical polymerization process was conducted using approximately 0.5 wt.% of PLA4ChMA macromonomer in aqueous solution, followed by freeze - drying of the resulting polymers. 2.3 Macromonomer degradation A specified amount of PLA4ChMA macromonomer was added to 10 mL of D2O in a glass vial to prepare a ~0.5 wt.% aqueous solution. Preliminary experiments showed that the degradation rate

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was independent of initial macromonomer concentration, in agreement with the first-order hydrolysis reaction observed for similar PLA-based polymers.14,16 The glass vial was inserted into a beaker filled with water and surrounded by a silicone oil bath at 50 °C or 85 °C to initiate degradation of macromonomer. An aliquot (approximately 1 mL) of this macromonomer solution was transferred to an NMR tube for analysis using a 400 MHz Bruker Advance instrument at 25 °C to obtain the proton (1H) spectrum. The 1H-NMR analysis was performed 28 times over 75 days for material degraded at 50 °C, and 12 times over 23 days at 85 °C. Pure choline chloride and lactic acid (< 5 wt.%) were dissolved in D2O separately for NMR analysis to aid in peak assignments, and for comparison with the spectra of the supernatant solutions containing degraded PLA4ChMA macromonomer and the degradation products. In another series of experiments, PLA4ChMA macromonomer was added to a 10−5 M NaOD solution with a pH initially adjusted to 9.0 ± 0.5. This pH reflects the environmental condition of the oil sands tailings pond in the bitumen extraction industry.22 For the degradation at 50 °C, 72 aliquots were taken over the 46-day experiment, and 7 aliquots were taken over the 13 - day degradation at 85 °C. Among these, 2 to 4 aliquots were selected to obtain averaged results from 1

H-NMR spectra at various days of the pH-adjusted PLA4ChMA macromonomer degradation at

50 °C, whereas one aliquot was used to obtain result from 1H-NMR spectrum at 85 °C. A separate vial containing PLA4ChMA macromonomer was prepared in parallel to monitor the change in pH with degradation time at 50 °C using a Fisher Scientific (Accumet AE150) pH probe meter. 2.4 Polymer degradation Freeze-dried poly(PLA4ChMA) was added to D2O in a glass vial to prepare a mixture with ~0.5 wt.% polymer. The vial was then inserted into a water beaker, inside a silicone oil bath, to Page 7 of 29

initiate the degradation process of the polymer at 50 °C. After 36 h, the solution was taken out of the oil bath, allowed to cool on the bench, and the water-insoluble polymer settled to the bottom of the vial. An aliquot (approximately 1 mL) of the supernatant solution was transferred to an NMR tube for 1H-NMR analysis. Seven aliquots were similarly taken at various intervals of degradation over 14 days. The experiment was repeated at 50 °C in a pH-adjusted solution (initial pH of 9.0 ± 0.5) using the same procedure. 2.5 Separation of water-insoluble polymer from degradation products Two additional polymer degradation tests were conducted at 50 °C and natural pH (without the addition of NaOD) to isolate the water-insoluble polymer from the supernatant solution containing water-soluble degradation products and traces of the polymer after centrifugation. In addition, the mass of water-insoluble polymer was determined. In duplicate glass vials, approximately 0.5 wt.% of poly(PLA4ChMA) was added to D2O to initiate degradation at 50 °C. After both 7 and 14 days, the solution from one vial was centrifuged at 3000 rpm (with relative centrifugal field of 1771 g, where g is the acceleration due to gravity) for 12 minutes using an IEC Centra CL 3R (Thermo Electron Corporation) centrifuge. The water-insoluble polymer was rinsed with D2O and re-suspended in centrifuge tubes to repeat the centrifugation process two more times to further remove the adsorbed water-soluble degradation products from the water-insoluble polymer. The supernatant solutions obtained after the second and third stage of centrifugation were discarded. The supernatant solution obtained after the first stage of centrifugation was analyzed by 1H NMR, and the water-insoluble polymer (obtained after the third stage of centrifugation) was rinsed, dried at 50 °C (in a New Brunswick Scientific, Excella E24 Incubator Shaker Series) and

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then weighed (Mettler Toledo, MS-TS Analytical Balances). The dried solids were then dissolved in DMSO-d6 for 1H-NMR analysis. In addition, freeze-dried undegraded poly(PLA4ChMA) was added to D2O and to DMSO-d6 separately for 1H-NMR analysis. Lastly, poly(MAA) powder (< 5 wt.%) was dissolved in DMSO-d6 to compare the resulting NMR spectrum with that measured for the partially degraded poly(PLA4ChMA) samples.

3. Results and Discussion Experiments to monitor the degradation of PLA4ChMA macromonomer and of poly(PLA4ChMA) polymer in aqueous solution were carried out to examine the physico chemical characteristics, degradation compounds released over time, and effects of temperature and pH. Results are compared between experiments conducted at 50 and 85 °C, and with a solution initially adjusted to pH 9.0 ± 0.5. Findings are compared to previously published experimental results on the degradation of similar compounds. 3.1 Physico-chemical characteristics of macromonomer and polymer degradation Variations in pH of solutions containing the macromonomer and polymer in D2O as a function of degradation time at 50 °C are shown in Fig. 1. The solution pH decreases from initial values of 4.5 (macromonomer) and 4.0 (polymer) to 3.1 during the first day, followed by a smaller rate of pH change after day 2 to stabilize at a value of 2.6 ± 0.2 after day 7. The addition of NaOD to adjust the initial solution pH to 9.0 ± 0.5 does not change the observed trend versus degradation time, with pH settling to a near-constant value after day 4 for these four cases. The final pH values are approximately the same for all experimental conditions except for the polymer with an initial pH of ~9, which levels to a slightly higher value of ~4. These results suggest that both macromonomer and polymer release similar acidifying compounds during degradation regardless Page 9 of 29

of the initial pH (i.e., natural pH and initially adjusted pH), a hypothesis that will be examined in detail below using 1H-NMR.

10

Polymer, Initial pH ~9 Macromonomer, Initial pH ~9 Macromonomer Polymer

9

8

pH

7

6

5

4

3

2 0

1

2

3

4

5

6

7

8

9

10

11

12

Days

Fig. 1. Change in pH of PLA4ChMA macromonomer and poly(PLA4ChMA) polymer in D2O during degradation at 50 °C over time. Lines are drawn between points for visual reference. Variations in pH are similar for the macromonomer and polymer, suggesting that degradation products are similar for both compounds. The addition of NaOD to adjust the initial pH to 9.0 ± 0.5 results in no further change in pH after day 4. There is a difference in final pH between each experiment for adjusted initial pH of 9.0 ± 0.5 and natural pH conditions.

It is the release of PLA-based oligomer degradation products during the partial hydrolysis of macromonomer and polymer that acidifies the aqueous solution, as was concluded in previous research on poly(PLA4ChMA) and the corresponding poly(acrylate) structure.23 The degradation products released from similar LA-based materials have also been shown to be independent of the initial pH of the polymer solution,11,12,17,22 and a similar profile in pH versus degradation time for polyester-based (ɛ-caprolactone) nanoparticles in solution was observed in previous work.2 Page 10 of 29

Yu et al. also found that the concentration of lactic acid (LA) species reaches a plateau after 5 days of poly(lactic acid)-based nanoparticle degradation,7 a result that is consistent with the trend observed in Fig. 1. Thus, degradation products released in MFT ponds (pH of ~9) will be identical to those of macromonomer and polymer studied under lab conditions. This release would lower the pH of the MFT environment, as previous work reports a drop in pH by 2 – 2.5 units for similar materials degraded in alkaline buffered solution.12,22 3.2 Degradation studies of the macromonomer 1

H-NMR allows for the complete quantification of the degradation process as the PLA4ChMA

macromonomer releases choline, lactic acid, and lactyl lactate (a dimer acidic species containing two lactate units), because the chemical shifts of the macromonomer protons and these degradation products are unique. The degradation mechanism shown as Scheme 2 is proposed based on previous studies of CL-based cationic methacrylate macromonomers and polymers,22,24,25 as well as the extensive literature on PLA degradation.11–18 The choline unit is released from the end of the macromonomer via hydrolysis (Scheme 2(A)), followed by the sequential release of lactate units (Scheme 2(B) – (D)) through acid-catalyzed chain-end scission; cleavage of an internal lactate group by random chain scission to release lactyl lactate (Scheme 2(B)) is also possible.14,16–18 The cleavage of these groups yields either a monomer containing a single lactate unit (product structure of reaction 2(C)), or methacrylic acid as the product of reaction 2(D). When the initial solution pH is increased to 9.0 ± 0.5, base-catalyzed backbiting can also release lactyl lactate7,11,12,14 that further hydrolyzes to form two molecules of lactic acid through a reversible reaction (Scheme 2(E)).

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(A) I-

H2O

+

I-

aq

+

(B) +

+

aq

H2O

+

aq

H2O

+

Or

+

aq

H2O

+

(C) +

H2O

aq

+

(D) O O OH

+

H2O

aq

+

O

(E) 2

aq

+

H2O

Scheme 2. Mechanism for the hydrolytic degradation of PLA4ChMA macromonomer to produce choline, lactic acid, lactyl lactate, and methacrylic acid. The orange squares represent the site of cleavage, and the blue circles represent the number of remaining lactate units, which reduces with hydrolytic degradation. Lactic acid undergoes a reversible dimerization to lactyl lactate.

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The 1H-NMR spectrum of the undegraded macromonomer in D2O shown in Fig. 2(A) is similar, with slightly shifted peak positions, to that reported by Younes et al. for the macromonomer dissolved in deuterated chloroform.23 The relative peak areas correspond closely to that expected for PLA4ChMA with (on average) 4 lactate units and a single quaternary ammonium cation unit attached. All peak assignments are summarized in Table 1, including those identified in Fig. 2(B) and 2(C), which arise from degradation products. As detailed in the supplementary information (SI), these assignments are confirmed by 1H-NMR spectra of choline chloride (Fig. S1) and lactic acid containing trace amounts of lactyl lactate (Fig. S2) in D2O; the shifts for the methine (d' located at 4.30 ppm) and methyl (e' located at 1.33 ppm) groups of lactic acid species are assigned according to literature.26 Fig. S3 demonstrates that the peaks arising from the methacrylic acid protons are shifted from those attached to the macromonomer double bond. With all peaks identified, the 1H-NMR analysis confirms that choline, lactic acid, lactyl lactate, and methacrylic acid species are the only species released from the degradation of macromonomer, as proposed in Scheme 2. The peak assignments in Table 1 are also used for the study of the partial hydrolysis of the polymer described in the following section. The peaks from the protons corresponding to cleaved species (degradation products), circled with dashed ellipses in Fig. 2(B), grow in relative intensity as macromonomer degradation continues (Fig. 2(C)). The release of choline is tracked by comparing the relative contributions of the peaks labeled as h and h' that arise from uncleaved (at 3.14 ppm) and cleaved (at 3.12 ppm) choline units.22,25 Peak d (solid dark blue ellipse) from internal lactate units of the macromonomer and partially degraded macromonomer disappears completely after 22 days; correspondingly, the amount of lactyl lactate relative to cleaved lactate units decreases from a fraction of ~0.25 at 5 days (Fig. 2(B)) to a negligible amount after 22 days (Fig. 2(C)). This latter

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finding is in agreement with the low concentrations of lactyl lactate in aqueous solution reported by Yu et al. after the degradation of PLA-methacrylate based polymer nanoparticles for ten days at 50 °C.7 By day 22, nearly all the choline units and ~85% of the lactate units are cleaved from the macromonomer, but only 40% of the methacrylate units have been degraded to methacrylic acid, as seen by the relative areas of peaks a, b, c to the corresponding a', b', c' peaks of methacrylic acid. HOD

(A)

b

1.00

0.99

(B)

b b' 1.00

(C)

f

d

c

c c'

h

a

e

3.02

12.5

g 10.1

2.21

4.25

e' e''

d'''

d

d'' d'

f'

0.42 1.28

0.18

g

e

g'

e'''

Y o u

a' 1.26

1.72

h'

0.54

e'

Y o u d' b b' 1.00

c

f'

c'

d'''

0.62

0.88

a

g'

d''

e'''

s

0.04 4.84 3.17

3.17

13.7

2.91

3.0416.7

Fig. 2. 1H-NMR spectra of PLA4ChMA macromonomer in D2O at day 0 (A), day 5 (B), and day 22 (C) of degradation at 85 °C. f1 is the chemical shift relative to tetramethylsilane. Numbers represent the area under a peak relative to peak b, with labels corresponding to the structures summarized in Table 1. Ellipses with solid lines highlight protons attached to functional groups of uncleaved choline, lactate, and methacrylate units, whereas dashed ellipses of the same colour identify the corresponding protons attached to carbon atoms of cleaved species. Peaks (labeled as d''' and e''') arise from protons of the terminal lactate unit of the partially degraded macromonomer or from cleaved lactyl lactate species. Page 14 of 29

Table 1. Structures and 1H-NMR peak assignments of PLA4ChMA macromonomer, the partially degraded macromonomer, and cleaved species (see Scheme 2). Letter Labels, Chemical Shift (ppm) a, 1.86 b, 6.15 c, 5.73 d, 5.19 e, 1.50 f, -b g, 3.70 h, 3.14

Number of Protons 3 1 1 1 3 2 2 9

e''', 1.46d d''', 5.02d

3 1

f', 3.97 g', 3.43 h', 3.12

2 2 9

d', 4.30 e', 1.33

1 3

Cleaved Lactyl Lactate

e'', 1.37 d'', 4.44

3 1

Methacrylic Acid

a', 1.82 b', 6.04 c', 5.65

3 1 1

Compound

a

Structure

Macromonomer (day 0) I-

Partially Degraded Macromonomerc

Cleaved Choline

I-

Cleaved Lactic Acid Y

a

The corresponding peak for the proton of –OH group in a chemical scheme is not observed due to the rapid proton exchange with D2O (HOD). bThe peak corresponding to the two protons attached to choline unit in the macromonomer overlaps with the HOD peak. cSubscript y represents the number of internal lactate units (1 to 3). dPeak positions of protons from the terminal lactate unit of the partially degraded macromonomer overlap with those of the corresponding protons of lactyl lactate species. Page 15 of 29

Similar spectra to those in Fig. 2 were measured throughout the 22-day study of macromonomer degradation at 85 °C, and the same methodology was used to study degradation at 50 °C for 75 days (sample spectra presented in SI). The fractional amount of cleaved choline and lactate species can be calculated through the ratio of relative peak intensities, as detailed in the SI. In addition, the fraction of the macromonomer that degrades completely to methacrylic acid is calculated from the spectra. The corresponding fractions of uncleaved units (i.e., still attached to macromonomer) as a function of degradation time at the two temperatures are shown in Fig. 3. The final fraction of uncleaved lactate units (0.20) and choline units (0.05) are slightly higher at 50 °C than that at 85 °C (0.15 and ~0, respectively). Approximately 20% of the lactate units remained attached to the macromonomer at the end of degradation for both cases, showing that only partial hydrolysis occurs regardless of the temperature at which degradation takes place. The incomplete release of lactate units is in agreement with the fraction of the macromonomer

Fraction of Uncleaved Units

1.0 0.9

1.0 0.9

0.8

0.8

0.7

0.7

0.6

0.6

0.5

0.5

0.4

0.4

0.3

0.3

0.2

0.2

0.1

0.1

0.0

0.0 0

10

20

30

40

Days

50

60

70

80

B1.1

1.1

1.0

1.0

0.9

0.9

0.8

0.8

0.7

0.7

0.6

0.6

0.5

0.5

0.4

0.4

0.3

0.3

0.2

0.2

0.1

0.1

Fraction of Uncleaved Units

1.1

Uncleaved Choline Uncleaved Lactate Methacrylic Acid (MAA)

Fraction of MAA Species

A 1.1

0.0

Fraction of MAA Species

that degrades entirely to methacrylic acid, ~0.20 at 50 °C and 0.40 at 85 °C.

0.0 0

5

10

Days

15

20

25

Fig. 3. Partial hydrolysis of PLA4ChMA macromonomer at 50 °C (A) and 85 °C (B) vs. degradation time, followed by tracking the fraction of uncleaved lactate and choline units attached to the partially degraded macromonomer, as well as the fraction of the macromonomer that degrades completely to methacrylic acid (MAA).

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The results shown in Fig. 3 are for degradation of the macromonomer at the natural initial solution pH (~4.5, see Fig. 1). 1H-NMR was also used to follow degradation rates of macromonomer solutions at both temperatures with initial pH adjusted to ~9, with the same cleaved species identified (NMR spectra included as SI). As shown in Fig. 4, the macromonomer degraded more quickly with an initial pH of ~9 at both 50 and 85 °C. For example, although it took ~15 days for a fraction of 0.50 of choline units to be cleaved with the solution at an initial pH of 4.5, it took only ~5 days with the initial pH of 9; the rate of lactate unit release accelerated by a similar factor at the same temperature of 50 °C. However, although the rate of degradation was accelerated, the final mixture of cleaved species was independent of the initial pH: 0.90 – 0.95 of choline units and 0.80 of lactate units were cleaved at 50 °C. Similar results are observed at 85 °C: degradation occurred more quickly at an initial pH of 9, but the final mixture of cleaved species is similar to that obtained with an initial pH of 4.5. The results in Fig. 4 show that a higher temperature and a higher initial pH enhance the degradation rate, as reported in previous studies for PLA-based compounds with different end-group functionality..11,14,17,21,27 However, the chemical identity and the relative amounts of cleaved species from the cationic macromonomer are independent of degradation temperature and pH, an important result when considering the application of the corresponding polymer as a flocculating agent for oil sands tailings.

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B1.1

50 Celsius 50 Celsius, Initial pH ~9 85 Celsius 85 Celsius, Initial pH ~9

1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 0

10

20

30

40

50

60

70

80

Fraction of Uncleaved Lactate Units

Fraction of Uncleaved Choline Units

A 1.1

1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0

Days

C 0.55

0

10

20

30

40

50

60

70

80

Days

Fraction of MAA Species

0.50 0.45 0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00 0

10

20

30

40

Days

50

60

70

80

Fig. 4. Partial hydrolysis of PLA4ChMA macromonomer at different temperatures and pH vs. degradation time, followed by tracking the fraction of uncleaved choline (A) and lactate units (B) attached to partially degraded macromonomer, as well as the fraction of macromonomer that degrades completely to methacrylic acid, MAA, (C). Experiments were carried out at 50 and 85 °C, and repeated with solutions initially adjusted to pH of 9.0 ± 0.5. 3.3 Degradation studies of the polymer The degradation of the polymer was expected to yield the same cleaved lactic acid and choline species as found in the study of macromonomer degradation. However, the verification by NMR analysis is more complex, as the polymer exists as a nano-suspension in aqueous solution and becomes more hydrophobic and agglomerates as its partial degradation occurs.22,23 Thus, the aqueous supernatant was separated from the water-insoluble polymer through centrifugation of aliquots after 7 and 14 days of degradation at 50 °C. 1H-NMR in D2O was then used to identify

Page 18 of 29

the water-soluble cleaved species and to compare their rates of formation to those identified for the macromonomer. The mass of the remaining water-insoluble polymer was recorded after drying, and then dissolved in DMSO-d6 for 1H-NMR analysis. The structure of the partially degraded polymer inferred from 1H-NMR analysis in DMSO-d6 was compared with the estimated relative amounts of cleaved (water-soluble) species to check closure of the total mass balance. The 1H-NMR spectrum of the original polymer in D2O is shown in Fig. 5(A). Even though the polymer is not water-soluble, the expected characteristic peaks are present, as confirmed by comparing the macromonomer spectrum in Fig. 2(A) to that for the polymer dispersion shown as Fig. 5(A). In particular, signals from the quaternary ammonium ion in Fig. 5(A) are located at the expected positions of 3.7 (g) and 3.2 (h) ppm, respectively, and in the expected ratio of ~2:9. However, the ratio of lactate to choline units estimated from the ratio of peak d to peak g is only 2:1, rather than the value of ~4 found for macromonomer. Thus, it is evident that the 1H-NMR analysis in D2O cannot fully capture the structure of the polymer, because peaks associated with protons of the terminal group of the repeat unit (i.e., the quaternary ammonium ion) are more readily detected than those of the internal lactate units. The g' signal associated with cleaved choline and the peaks attributed to cleaved lactic acid species (peaks d'', d' and e'', e') suggest that degradation of the polymer has already occurred to a small extent, likely during synthesis. In addition, peaks a, b, and c suggest that the polymer dispersion contains residual macromonomer. However, the amounts relative to polymer inferred from the peak areas cannot be trusted, due to the polymer insolubility in water, and this analysis must be supplemented and combined with analysis in another solvent. The peak labelled x is assigned to the protons of the two methyl groups attached to nitrogen in the intermediate product ( 2 - (N,N - dimethylamino)ethyl ester

Page 19 of 29

methacrylate) associated with macromonomer synthesis.23 Peak x is also visible in Fig. 2, but with much reduced intensity relative to the macromonomer peaks. The higher relative intensity of this peak in Fig. 5(A) is a further indication that 1H-NMR in D2O is not able to completely detect the structure of the water-insoluble polymer. Thus, the structure of partially degraded polymer was quantified by 1H-NMR in DMSO-d6, as described later. HOD

(A)

e

h

e'', e' j e'''

f b c

(B)

0.09

d

x

g

d'', d'

g'

f'

0.35

0.37 2.00 0.40 10.5

1.99

a

0.31 i

g'

f' d

d'''

d'', d'

0.06

e'', e' e'''

g h'

1.85 0.52 2.00

(C)

3.12 3.10 e'', e' e'''

f' d

0.05

d'''

d'', d'

g' g

1.87 0.33 2.00

8.85

2.20 3.41

Fig. 5. 1H-NMR spectra of poly(PLA4ChMA) polymer in D2O at day 0 (A), and cleaved species in the supernatant solution after the first stage of centrifugation at day 7 (B) and day 14 (C) of the polymer degradation at 50 °C. f1 is the chemical shift relative to tetramethylsilane. Numbers represent the area under a peak relative to peak g for day 0 (A) and gʹ for day 7 (B) and 14 (C), with labels corresponding to the structures summarized in Table 1 and 2. Ellipses with solid lines highlight protons attached to uncleaved choline, lactate, and the polymer backbone units, whereas dashed ellipses of the same colour identify the corresponding protons attached to carbon atoms of cleaved species. The peak for the CH2 functional group of the polymer backbone chain (labeled as i) is not detected due to incomplete solubility of the polymer in D2O and the overlap with the peak of the lactate unit methyl group (labeled as e). Page 20 of 29

Nevertheless, 1H-NMR analysis in D2O may be used to analyze the water-soluble cleaved species found in the supernatant solution separated from the water-insoluble polymer in aliquots centrifuged after day 7 (Fig. 5(B)) and day 14 (Fig. 5(C)) of degradation. The integrated areas in Fig. 5(B) and 5(C) are reported relative to peak g' from the CH2 group of cleaved choline species. The chemical shifts from cleaved choline, lactic acid, and lactyl lactate in Fig. 5 are the same as those summarized in Table 1 for cleaved species produced during the macromonomer degradation, with the chemical structures of a polymer repeat unit (before and after partial degradation) and corresponding peak assignments summarized in Table 2. Table 2. Structure and peak assignments for protons of poly(PLA4ChMA) polymer and the degraded polymer in DMSO-d6 and in D2O (in italics). Compound

Polymer(day 0)

Structure

I-

Partially Degraded Polymerb

Letter Labels, Chemical Shift (ppm)

Number of Protons

j, 1.21, 1.21 d, 5.20, 5.17 e, 1.50, 1.48 f, -a, 4.56 g, 3.73, 3.70 h, 3.16, 3.16

3 1 3 2 2 9

e''', 1.40 d''', 4.94

3 1

a

The peak corresponding to the two protons attached to choline units in the polymer overlaps with the HOD peak. bSubscript y represents the number of internal lactate units (1 to 3).

As indicated by the dashed ellipses in Fig. 5(B) and 5(C), the polymer degrades to release the same choline and lactic acid species found in the macromonomer degradation study. Calculated Page 21 of 29

ratios of cleaved lactate to cleaved choline units at day 7 and 14 of the polymer degradation are 2.1 and 1.9, respectively (see SI for details). Peak g arising from the polymer containing the choline unit is still observed in the supernatant solution, despite separation of the water-insoluble polymer fraction by centrifugation. However, the ratio of total cleaved to uncleaved choline units (and total cleaved to uncleaved lactate units) cannot be calculated without further information, as the majority of the water-insoluble polymer has been removed by centrifugation. As presented in SI, it is estimated that polymer particles with a diameter less than 1 µm would remain in suspension after centrifugation, accounting for the observation of these small peaks associated with the polymer structure. Additional NMR spectra of the polymer degraded over 14 days at 50 °C at both natural pH and with initial pH adjusted to ~9.0 are shown as Fig. S9 – S11. These spectra were recorded using the entire sample, without first removing the solids by centrifugation. Signals from both cleaved and uncleaved units were used to generate degradation profiles of uncleaved choline and lactate units. As shown in Fig. S12, the profiles are similar to those for the macromonomer case in Fig. 4. These estimates are only qualitative, as they do not account for water-insoluble polymer not detected by 1H-NMR analysis in D2O. However, it is clear that the rate of polymer degradation is much faster than that of the macromonomer, occurring over 1 to 2 weeks at 50 °C and natural pH compared to the 8 to 11 weeks required for the macromonomer degradation. For the case of an initial pH of 9.0 ± 0.5, the polymer partial degradation process is complete after 2 or 3 days, compared to the ~50 days required for the macromonomer under the same conditions. This result is consistent with findings from Younes et al. that oil sands MFT sludge treated with poly(PLA4ChMA) does not dewater further after five days of degradation at 50 °C.23

Page 22 of 29

To complete the analysis of the degradation process, the water-insoluble polymer isolated by centrifugation was solubilized in DMSO-d6 for NMR analysis. The spectrum of the undegraded polymer in DMSO-d6 is shown in Fig. 6(A), where areas reported relative to peak f are set to 2 for the CH2 adjacent to the quaternary ammonium group. The peak assignments, summarized in Table 2, resemble those from polymer/macromonomer analysis in D2O. Although broader than the peaks obtained from analysis in D2O, the relative areas match the expected structure of the undegraded polymer repeat unit (i.e., ~4 lactate units), verifying that the polymer is soluble in DMSO-d6. The weight fraction of water-insoluble polymer isolated by centrifugation after day 7 and day 14 of degradation was 0.31 and 0.28, respectively, as determined by gravimetry. Thus, the polymer does not substantially degrade further after 7 days at 50 °C. The partially degraded material was then dissolved in DMSO-d6, with the resulting 1H-NMR spectra shown in Fig. 6(B) and 6(C). Only trace levels of cleaved units are found, an indication that the water-soluble cleaved species were effectively removed by centrifugation. The integrated areas are reported relative to peak d, set to 1 for the CH group of an interior lactate unit. The signal from a hydrolyzed lactate unit, d''', not visible in the Day 0 polymer spectrum, is clearly detected in the degraded samples. Peak area of d''' relative to d (0.8 in Fig. 6(B) and 0.95 in Fig. 6(C)) indicate that there are, on average, ~2 lactate units remaining on each repeat unit of the partially degraded polymer. In addition, the spectra are very different from that of poly(MAA) in DMSO-d6 (Fig. S13), confirming that the polymer undergoes only partial hydrolytic degradation instead of complete degradation to a poly(MAA) backbone.22 The relative strength of signal f arising from choline units attached to the partially degraded polymer collected are 0.08 and < 0.02, after 7 and 14

Page 23 of 29

days of degradation. Thus, to a first approximation, all the choline units are cleaved from the polymer. (A)

h

DMSO

j e

d

4.07

f

g

2.00

2.23

HOD 3.29

9.64 i

(B)

e

HOD d

h d'''

1.00 0.80

f

e'''

g

j

0.08

(C) e d

d'''

1.00 0.95

f

g

h

e'''

j

0.02

Fig. 6. 1H-NMR spectra of poly(PLA4ChMA) polymer in DMSO-d6 at day 0 (A), and the water insoluble polymer in DMSO-d6 collected after the third stage of centrifugation at day 7 (B) and day 14 (C) of the polymer degradation at 50 °C. f1 is the chemical shift relative to tetramethylsilane. The DMSO solvent peak is located at 2.5 ppm. Numbers represent the area under a peak relative to peak f for day 0 (A) and to peak d for day 7 (B) and 14 (C), with labels corresponding to the structures summarized in Table 2. Ellipses with solid lines highlight protons attached to functional groups of uncleaved choline, lactate, and the polymer backbone units. The peak for the CH2 functional group of the polymer backbone chain (labeled as i) is not clearly detected due to interference with the methyl groups of the lactate units (peaks labeled as e and e'''). Combining the 1H-NMR analyses of the degraded polymer (in DMSO-d6) with that of the watersoluble cleaved species (in D2O) provides a complete picture of the polymer degradation process, as summarized by Scheme 3. The partially degraded (water-insoluble) polymer repeat units Page 24 of 29

contain, on average, two lactate units attached to the backbone, with the quaternary ammonium group completely cleaved. This result is consistent with the analysis of water-soluble cleaved species, which indicates a ratio of ~2 to 1 of lactic acid to choline in solution. Furthermore, the number of polymer backbone units in the chain remains constant, as no further small molecule species (such as MAA) are detected. The expected mass fraction of the partially degraded polymer assuming two lactate units remain attached to the backbone is 0.40, in reasonable agreement with the ~0.30 mass fraction of water-insoluble polymer recovered. The difference in values can be attributed to incomplete separation; as mentioned previously, centrifugation does not remove the water-insoluble polymer contained in particles less than 1 µm in diameter and some evidence of the undegraded polymer is seen in the supernatant solution (Fig. 5). Finally, to verify that the partially degraded polymer does not degrade further under basic conditions (as might be encountered when the material is used as a flocculant), the waterinsoluble fraction was added to D2O with the initial pH adjusted to 9.0 ± 0.5 (see Fig. S14 in SI). While the pH decreased to ~6.5, indicating the release of a small amount of additional lactic acid, the polymer remained insoluble. Thus, the polymer degradation remains partial, in agreement with observations made when a similar material (poly(PCL3ChMA)) was held in a buffered solution at pH of ~9.22 O

aq

O

+

2

O

+

OH 2 H2 C CH3 m

Scheme 3. Proposed mechanisms for partial hydrolytic degradation of poly(PLA4ChMA) polymer in H2O. Subscript m represents the number of repeat units in the polymer chain. The partial hydrolytic degradation of the polymer was studied at 50 °C and natural pH of ~4 for 14 days. Page 25 of 29

4. Conclusions This study investigated the partial hydrolytic degradation of PLA4ChMA macromonomer and poly(PLA4ChMA) polymer at various temperature and pH conditions. Degradation of the macromonomer is first studied, in order to verify NMR procedures and peak assignments. Under the studied conditions (50 and 85 °C, acidic and basic initial pH), the macromonomer does not undergoes complete hydrolytic degradation to methacrylic acid as not all lactate units are cleaved. Although the higher temperature and initial pH enhance the degradation rate, the chemical identity of the cleaved species identified by the 1H-NMR analysis – choline, lactic acid, and lactyl lactate – remain the same, and the same species were identified from the study of polymer degradation. However, the polymer degrades at a much faster rate: 1 – 2 weeks for polymer vs. 8 – 11 weeks for the macromonomer at 50 °C. The partially degraded (waterinsoluble) polymer contains approximately two lactate units attached to each repeat unit of the backbone at the end of degradation, a structure inferred by mass balance as well as by the 1HNMR analyses of the water-insoluble polymer fraction (in DMSO-d6) and the water-soluble cleaved species (in D2O). This study demonstrates that both the macromonomer and polymer undergo only partial hydrolytic degradation, regardless of the temperature of degradation or the initial pH of the solution. This finding explains the increased hydrophobicity of the polymer inferred by the continued dewatering of oil sands MFT observed in previous studies over time at both elevated and room temperatures.23 Furthermore, identification of a consistent set of degradation products over a range of conditions is an important first step in the study of the possible effects of flocculant applications on the environment and human health. Further experiments will be conducted to better understand the interactions between degradation products and the Page 26 of 29

components of wastewater and receiving waters as part of the environment assessment of flocculants.

Acknowledgments The financial support from the Queen’s Chemical Engineering department, Natural Sciences and Engineering Research Council (NSERC) Discovery Grant, and Dean’s Graduate Studies Research Assistant Award are much appreciated. We want to acknowledge Kyle Lister for synthesizing the macromonomer and preparing the freeze-dried polymer materials for this study.

Appendix A. Supplementary data Supplementary data for discussions of results in this article can be found online at:

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A novel polycationic flocculant leads to increased dewatering of tailings sludge as it degrades. The degradation mechanism of the polymer and its precursor macromonomer are studied. Proton NMR is used to quantify the lactic acid and choline by-products released. The polymer only partially degrades, explaining the favorable dewatering performance.

Derek Russell: Methodology, Investigation, Validation, Writing- Original draft preparation Louise Meunier: Funding Acquisition, Conceptualization, Supervision, Writing- Reviewing and Editing Robin A. Hutchinson: Funding Acquisition, Conceptualization, Supervision, Writing- Reviewing and Editing

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

Derek Russell, Louise Meunier, Robin Hutchinson