Polyacrylates Derived from Bio-Based Ethyl Lactate Solvent via SET-LRP

: The precise synthesis of polymers derived from alkyl lactate ester acrylates is reported for the first time. Kinetic experiments were conducted to demonstrate that Cu(0) wire-catalyzed single electron transfer-living radical polymerization (SET-LRP) in alcohols at 25 ºC provides a green methodology for the LRP of this forgotten class of bio-based monomers. The acrylic derivative of ethyl lactate (EL) solvent and homologous structures with methyl and n- butyl ester were polymerized with excellent control over molecular weight, molecular weight distribution, and chain end functionality. Kinetics plots in conventional alcohols such as ethanol and methanol were first order in monomer with molecular weight increasing linearly with conversion. However, aqueous EL mixtures were found to be more suitable than pure EL to mediate the SET-LRP process. The near quantitative monomer conversion and high bromine chain-end functionality, demonstrated by MALDI-TOF analysis, further allowed the preparation of innovative bio-based block copolymers containing rubbery poly(ELA) sequences. For instance, poly(ELA-b-poly(glycerol acrylate) block copolymer self-assembled in water to form stable micelles with chiral lactic acid-derived block forming micellar core as confirmed by pyrene-probe-based fluorescence technique. Dynamic light scattering (DLS) and transmission electron microscopy (TEM) measurements revealed nanosize spherical morphology for these bio-based aggregates. and MHz 25 . All chemical shifts are quoted on the δ scale in ppm using the residual solvent as internal standard ( 1 H NMR: CDCl 3 = 7.26 and 13 C NMR: CDCl 3 = 77.16). Infrared (IR) spectra were recorded on a FTIR-680PLUS spectrophotometer with a resolution of 4 cm −1 in the transmittance mode. An attenuated total reflection (ATR) devise with thermal control and a diamond crystal (Golden Gate heated single-reflection diamond ATR, Specac-Teknokroma) was used. Absorption maxima (ν max ) are reported in wavenumbers (cm –1 ). Fluorescence spectra were obtained on an RF-5301 PC Shimadzu fluorescence spectrometer with a RFPC software with emission using excitation slit widths of 5 nm. Supercritical fluidic chromatography (SFC) analysis was performed on a supercritical CO 2 chromatograph UPC2 from Waters equipped with Chiralpak IC (100x4.6 mm, 3 µm) column coupled with a DAD detector. CO 2 /2-PrOH (98:2) was used as eluent at a flow rate of 3.0 mL/min with the control ABPR pressure set at 1500 psi. ESI MS analysis were run on a chromatographic system Agilent G3250AA liquid chromatography coupled to 6210 time of flight (TOF) mass spectrometer from Agilent Technologies with an electrospray ionization (ESI) interface. Nominal and exact m / z values are reported in Daltons (Da). Optical rotations measurements were conducted on a Perkin-Elmer 241 MC polarimeter with a path length of 10 cm and are reported with implied units of 10 –1 deg cm 2 g –1 . Molecular weight analysis was performed via gel permeation chromatography spectrometer The contactangle of deionised water against polymer ºC, the contact angle


INTRODUCTION
Naturally occurring lactic acid (2-hydroxypropionic acid) was first isolated from sour milk by the Swedish chemist Scheele in 1780. Later on, this hydroxycarboxylic acid progressively became an industrial important product due to its versatile functional properties as a flavor agent, pH regulator, and preservative. 1 Currently, about 90% of the enantiomerically pure lactic acid is produced by the fermentation of refined carbohydrates with appropriate microorganisms. 2 However, more convenient bioprocessing technologies based on lignocellulosic raw materials are already consolidated. 3 In recent years, the derivation of polymeric materials from sustainable and annually renewable resources, such as vegetable oils, sugars, terpenes, polysaccharides, rosins and lignin, among others, has attracted increasing interest due to dwindling of fossil oil resources and environmental impact of petroleum manufacturing. 4,5 To this end, lactic acid has shown particular promise in production of poly(lactic acid) (PLA), either by its own polycondensation or ring-opening polymerization (ROP) of its cyclic dimer lactide. 6,7,8 The preparation of well-defined ABA thermoplastic elastomers illustrates an example on how recent advances in living radical polymerization has started a new era in the preparation of biomass-derived polymers with advanced properties and functions. 9,10,11,12 In this regard, single electron transfer living radical polymerization (SET-LRP) has gained great popularity as a facile tool for precision macromolecular engineering. 13 , 14 , 15 , 16 , 17 , 18 , 19 For example, when conducted in reaction media that facilitates disproportionation of Cu(I)Br into Cu(0) and Cu(II)Br2 20,21,22 this method enables the synthesis of vinylic polymers with nearly 100% chain end functionality at complete conversion. 23,24,25,26 This has been demonstrated to be feasible even in "programmed" biphasic SET-LRP systems (i.e. aqueous-organic solvent mixtures based on both disproportionating 27 , 28 , 29 and non-disproportionating organic solvents). 30,31,32,33 Consequently, benefiting from this and other inherent attributes (e.g. facile setup, ambient temperature, oxygen tolerance, compatibility with water and biological media), SET-LRP is an Materials. The following chemicals were purchased from Sigma-Aldrich and were used as received: Tris(hydroxymethyl)ethane was received from Alfa Aesar. Acrylic acid (stabilised with hydroquinone monomethyl ether, for synthesis), 2,2,2-trifluoroethanol ( 99%) and HPLC grade acetonitrile were obtained from Merck. HPLC grade methanol (MeOH) and ethanol (96%) were purchased from Scharlab and VWR Chemicals, respectively. Acetone (synthesis grade) was also purchased from Scharlab. The radical inhibitor of methyl acrylate (MA, 99%, Sigma Aldrich) was removed by passing the monomer through a short column of basic Al2O3 prior to use. Deuterated chloroform (CDCl3) was purchased from Eurisotop. Ethyl lactate (EL, natural, 98%), methyl L-lactate (ML, 98%) and butyl L-lactate (BL, 99%) were purchased from Sigma-Aldrich and distilled prior to use. Triethylamine (TEA, 99%, Merck) and dichloromethane (DCM, reagent grade, Scharlab) were distilled from CaH2. Propan-2-ol (2-PrOH, >97.7%) was passed through a short column of basic Al2O3 and freshly distilled before to use. Ethylene glycol (99%, Sigma-Aldrich) was dried by azeotropic distillation before to use and stored under inert atmosphere. Ethylene bis(2-bromoisobutyrate) (bisEBiB) 49 and ethane-1,2-diyl bis(2-bromo-2methylpropanoate) ((OH)2EBiB) 50 initiators and both solketal 51 and -pinene 52 acrylates (SA and PA, respectively) were prepared according to literature procedures. Copper(0) wire 99.9% pure of 20 gauge diameter, received from Creating Unkamen, was activated using hydrazine following a procedure developed in our laboratory. 53 Methods. Proton ( 1 H NMR) and carbon ( 13 C NMR) nuclear magnetic resonance spectra were recorded on a 400 MHz (for 1 H) and 100.6 MHz (for 13 C) Varian VNMR-S400 NMR instrument at 25 ºC in CDCl3.
All chemical shifts are quoted on the δ scale in ppm using the residual solvent as internal standard ( 1 H NMR: CDCl3 = 7.26 and 13 C NMR: CDCl3 = 77.16). Infrared (IR) spectra were recorded on a FTIR-680PLUS spectrophotometer with a resolution of 4 cm −1 in the transmittance mode. An attenuated total reflection (ATR) devise with thermal control and a diamond crystal (Golden Gate heated single-reflection diamond ATR, Specac-Teknokroma) was used. Absorption maxima (νmax) are reported in wavenumbers (cm -1 ). Fluorescence spectra were obtained on an RF-5301 PC Shimadzu fluorescence spectrometer with a RFPC software with emission using excitation slit widths of 5 nm. Supercritical fluidic chromatography (SFC) analysis was performed on a supercritical CO2 chromatograph UPC2 from Waters equipped with Chiralpak IC (100x4.6 mm, 3 µm) column coupled with a DAD detector. CO2/2-PrOH (98:2) was used as eluent at a flow rate of 3.0 mL/min with the control ABPR pressure set at 1500 psi. ESI MS analysis were run on a chromatographic system Agilent G3250AA liquid chromatography coupled to 6210 time of flight (TOF) mass spectrometer from Agilent Technologies with an electrospray ionization (ESI) interface.
Nominal and exact m/z values are reported in Daltons (Da). Optical rotations measurements were conducted on a Perkin-Elmer 241 MC polarimeter with a path length of 10 cm and are reported with implied units of 10 -1 deg cm 2 g -1 . Molecular weight analysis was performed via gel permeation chromatography (GPC) using an Agilent 1200 series system equipped with three serial columns (PLgel 3 m MIXED-E, PLgel 5 m MIXED-D and PLgel 20 m from Polymer Laboratories) and an Agilent 1100 series refractive-index detector. THF (Panreac, HPLC grade) was used as eluent at a flow rate of 1.0 mL/min. The calibration curves for GPC analysis were obtained with poly(methyl methacrylate) (PMMA) standards purchased from PSS Polymer Standards Service GmbH. The molecular weights were calculated using the universal calibration principle and Mark-Houwink parameters. MALDI-TOF analysis was performed on a Voyager DE (Applied Biosystems) instrument with a 337-nm nitrogen laser (3-ns pulse width). For all polymers, the accelerating potential was 25 kV, the grid voltage was 93.5%, the laser power was 1700 units, and a positive ionization mode was used. The analysis was performed with trans-2- [3-(4-tert-butylphenyl)-2-methyl-2-propenylidene]malononitrile as matrix. THF solutions of the matrix (30 mg/mL), KTFA as cationization agent (10 mg/mL), and polymer (10 mg/mL) were prepared separately.
The solution for MALDI-TOF analysis was obtained by mixing the matrix, polymer and salt solutions in a 9/1/1 volumetric ratio. Then 1 µL portions of the mixture were deposited onto three wells of a sample plate and dried in air at room temperature before being subjected to MALDI-TOF analysis. Differential scanning calorimetry (DSC) measurements were carried out on a Mettler DSC3+ instrument using N2 as a purge gas (50 mL/min) at scanning rate 20 C/min in the -80 to 150 C temperature range. Calibration was made using an indium standard (heat flow calibration) and an indium-lead-zinc standard (temperature calibration). Thermal stability studies were carried out on a Mettler TGA2 /LF/1100 with N2 as a purge gas at flow rate of 50 mL/min. The studies were performed in the 30-600 ºC temperature range at a heating rate of 10 ºC/min. Transmission electron microscopy (TEM) was performed using a JEOL JEM-1011 TEM microscope. Before the measurement, a drop of solution was placed on a copper grid which was allowed to dry at room temperature. Dynamic light scattering (DLS) measurements were carried out at room temperature using Zetasizer Nano ZS (Model ZEN3500) from Malvern Instruments equipped with a He-Ne laser. Chiral polymers were characterized on a Chirascan circular dichroism spectrometer from Applied Photophysics. The contactangle of deionised water against polymer surfaces was measured by the water drop method (3 L) at 25 ºC, using the OCA15EC contact angle setup (Neurtek Instruments).   1 H NMR. After 48 h, the reaction was diluted with water (30 mL) and the aqueous phase was extracted with diethyl ether (3x30 mL). The combined organic layers were rinsed with aqueous HCl 1 M (30 mL), saturated aqueous solution of NaHCO3 (30 mL), brine (20 mL) and finally dried with MgSO4. The resulting solution was concentrated under reduced pressure, and the residue was purified by column chromatography (9:1 hexanes/ethyl acetate) to afford ELA (0.9 g, 60%) as a colorless liquid.

Preparation and Characterization of Amphiphilic Block Copolymer poly(ELA)-b-poly(GA) micelles.
Polymer micelles were prepared by nanoprecipitation as follows: 1 mg of poly(ELA)-b-poly(GA) copolymer was first dissolved in acetone (1 mL). This solution was added dropwise into 10 mL of deionized water via a syringe. The colloidal dispersion was sonicated for 4 h at room temperature to remove the organic solvent. The critical micelle concentration (CMC) was determined by using pyrene as a fluorescence probe by monitoring the emission peaks at 382 and 372 nm. The concentration of block copolymer was ranging from 1.0 x 10 -9 to 1.0 x 10 -3 g L -1 and the pyrene concentration was fixed at 6.0 x 10 -7 M.

RESULTS AND DISCUSSION
Synthesis of Ethyl Lactate Acrylate. As illustrated in Scheme 2a, the acryloyl polymerizable functionality was installed on EL, commercially produced from sugarcane by fermentation, by acylation with acryloyl chloride in the presence of trimethylamine (TEA) using dichloromethane (DCM) as solvent.
ELA was isolated as a colorless liquid after work up and vacuum distillation in the presence of hydroquinone to minimize auto-polymerization (70% yield). The synthesis of the monomer was confirmed by NMR and FTIR spectroscopy ( Figures S1-3). The acrylic protons appear in the 1 H NMR spectrum between 6.50 and 5.88 ppm, whereas the four characteristic signals of the vinylic and carbonyl carbons appear in the 13 C NMR spectrum at 170, 165 and 131, 127 ppm, respectively.

Scheme 2. Synthetic Routes to ELA Starting from EL Solvent
FTIR spectroscopy showed characteristic absorptions of the two ester moieties at 1748 and 1726 cm -1 and the stretching of the acrylate C=C bond at around 1637 cm -1 . An additional structure confirmation was provided by high-resolution mass spectrometry (see experimental section). Supercritical fluid chromatography (SFC) was used for analytical chiral separation of the synthesized monomer ( Figure S4).
On the basis of this analysis and optical rotation measurements ([]D 25 = -53.9, c 1.0 mg/mL, MeCN), ELA employed was L-(-)-ELA with 96.7% enantiomeric excess. Being critical with the sustainability of the above described procedure, two alternative greener routes were explored in attempt to prepare ELA with the aid of acrylic acid (Scheme 2b and c). It is worth to mention that with the recent developments toward the commercial production of bio-acrylic acid and the cost-competitive production of bio-ethanol, ELA may be ultimately prepared entirely from biomass derived platform chemicals. 3 Unfortunately, the acrylic acid/EL acid-catalyzed esterification by azeotropic distillation in toluene was low-yield because extensive oligomerization of EL occurred at high temperature. 46 Conversely, the use of propylphosphonic anhydride (T3P ® ) under milder conditions gave an excellent result. 52 This ester coupling promoter lacks the toxicity and shock sensibility associated with other coupling agents (e.g. DCC and EDC). 54 Moreover, by-products from the coupling are H2O-soluble and therefore easily separated from the reaction mixture.  Moreover, it stabilizes the resulting colloidal Cu(0) particles and at the same time is also a good solvent for Cu(II)X2 ligand complex. 55 Figure 1a,b depicts kinetic plots and GPC analysis for the polymerization using the monofunctional initiators methyl -bromopropionate (MBP) and ethyl -bromoisobutyrate (EBiB) at a targeted degree of polymerization (DP) of 50 (entries 1 and 2 in Table 1). 1 Table 1 and Figure S5). However, we preferred using EBiB and other mono and bifunctional bromoisobutyrate derivatives, in absence of externally added deactivator, for the rest of this study.

Selection of Eco-Friendly Solvents for SET-LRP of Ethyl Lactate Acrylate
Ethanol and other Conventional Alcohols. A more environmentally friendly process for the SET-LRP of ELA was devised through the use of alcohols as solvents because they combine both acceptable levels of  As shown in Figure 2c, other conventional alcohols having similar solvent properties such as methanol (MeOH) could also be used to prepare well-defined poly(ELA) (entries 5 in Table 1). The reaction in propan-2-ol (2-PrOH) furnished a polymer with higher Mw/Mn (entries 6 in Table 1). However, in a fluorinated alcohol such as 2,2,2-trifluoroethanol (TFE), Mw/Mn was as low as 1.17 (entry 7 in Table 1 and Figure 2c). The kinetic plots for the polymerization in TFE also validates the use of fluorinated alcohols ( Figure S6). 61,62,63 Pushing the envelope of the ethanolic SET-LRP, we further investigated its potential in delivering welldefined poly(ELA) across a broad range of molecular weight while retaining control. Thus, a series of polymerizations were conducted varying the targeted DPs from 25 to 400 (entries 8-11 in Table 1). In all cases, SET-LRP smoothly proceeded at 25ºC to high monomer conversions (>90%), yielding polymers with controlled molecular weight up to 65,000 ( Figure 3).  Table 1 for polymerization conditions). The inset shows a digital image of the homogeneous reaction mixture after ethanolic SET-LRP at targeted DP=400. Numbers shown in black correspond to monomer conversion, Mn (GPC), and Mw/Mn respectively from the top to bottom.
It is worth to mention that the SET-LRP at DP=400 was still homogeneous at high conversion, suggesting  Ethyl Lactate and Aqueous Ethyl Lactate Mixtures. Encouraged by these results, the polymerization of ELA was investigated in detail using its bio-sourced synthetic precursor EL as solvent. EL is an economically viable green solvent with effectiveness comparable to some petroleum-based solvents. 42,43,44,45 Replacing EtOH by EL, under identical conditions, also furnished poly(ELA) with narrow MWD (entry 1 in Table 2 and Figure 2c). However, despite the fact that poly(ELA) was also soluble in this solvent the reaction achieved lower monomer conversion (compare entry 1 in Table 2 with entry 4 in Table 1) (Figure 2b). After, the polymerization proceeded following a second kinetic domain with a significantly lower rate constant (kp 2app = 0.0098 min -1 ). According to previous reports, this result may be attributed to rapid activation combined with insufficient disproportionation, which favors bimolecular termination events between growing chains (i.e. loss of bromine chain ends). 66,67,68,69,70,71 It has been previously demonstrated that the addition of small amount of H2O to poor disproportionation reaction mixtures can dramatically improve its ability to produce reactive Cu(0) and the needed levels of Cu(II)X2 deactivator to prevent irreversible termination of chains in early stages of SET-LRP reactions. 66,67 Indeed, tuning EL with H2O and other co-solvents is a common strategy to create ideal conditions in organic synthesis. 72,73 Table 2). The control experiment in pure EL showed again limited monomer conversion and loss of livingness manifested as kinetic plots with two linear regimes ( Figure S8a). However, after the addition of 5% H2O to EL the polymerization rate of the second linear regime significantly increased (3x). An increase on kp 1app was also observed, but much lower (1.5x) than that determined for kp 2app . To our delight, increasing further the H2O content completely eliminated kp 2app and generated the characteristic first order kinetic of a LRP processes.  Consequently, in the presence of 10%, and even 5% H2O, better control over the MWD was obtained.
Although EL is a good solvent for poly(ELA) and EL/H2O mixtures are miscible at any composition, the SET-LRP reaction mixture of this series of experiments progressively transitioned from a one phase to a biphasic SET-LRP system by showing increasing turbidity ( Figure S10). However, higher loadings of H2O only slightly compromise initiator efficiency (Ieff) probably due to extremely fast activation and propagation in more polar media and not due to appearance of turbidity (Figure 5d, open symbols). Note that in the presence of 15% H2O, reaction rate was accelerated by 135% compared to kp 1app obtained in pure EL (compare entries 2 and 5 in Table 3). These results demonstrate that the judicious selection of solvent is critical to practice SET-LRP and highlight the importance of mixed solvent systems.  Figure S11. In both cases, the Cu (0) wire-catalyzed polymerization initiated by EBiB furnished well-defined polymers with high conversions (entries 1 and 2 in Table 3). No significant differences were found between the kinetic data in comparison with ELA (compare entry 2 in Table 3 with entry 4 in Table 1). Also in this case, the linear relationship of the semi-logarithmic kinetic plot and the linear increase of molecular weight values throughout the polymerization strongly support that the SET-LRP of these monomers follows a LRP mechanism. Further, the use of difunctional and hydoxyl-functional bromoisobutyrate-type initiators (bisEBiB and (OH)2EBiB, see Scheme 1) allowed the preparation of well-defined,-dibromo telechelic and -dihydroxy functional polymers (entries 3 and 4 in Table 3 and Figure 6a). MALDI-TOF analysis evidenced the very high end-group fidelity for the poly(BLA) functional polymer (Figure 6b). These materials could be interesting in the preparation of more complex polymer architectures based on alkyl lactate acrylic polymers including ABA triblocks and AB2 stars using LRP or other living polymerization reactions in a second step. Research in this line will be reported in a forthcoming publication.  Table 3 for polymerization conditions). Numbers shown in black above GPC traces correspond to monomer conversion, Mn (GPC), and Mw/Mn respectively from the top to bottom. (b) MALDI-TOF spectrum of poly(BLA) synthesized by SET-LRP using (OH)2EBiB initiator. Magnified region in (b) confirms the expected peak-to-peak spacing for BLA repeating unit and the near perfect bromine chain end functionality of the synthesized polymer.

Block Copolymerization of poly(ethyl lactate acrylate) with Bio-based -Pinene and Solketal
Acrylates. Poly(alkyl lactate acrylate)s are amorphous hydrophobic polymers with glass transition temperature (Tg) below ambient temperature and thermal stability comparable to conventional alkyl acrylates (see discussion in supplementary information, Figure S12a,b). Also appealing is the chiroptical activity of these bio-based polymers ( Figure S13 and Table S1). Boosted by the near-perfect retention of bromine chain-ends at high conversion in ethanolic SET-LRP, we investigated the block copolymerization of poly(ELA) by sequential addition of a second vinylic monomer. Two diblock copolymers of ELA were targeted using -pinene acrylate (αPA), 52  segments (green and black traces, respectively), suggesting immiscibility between the poly(ELA) segments with the bulky pol(αPA). The existence of microphase separated morphology in this system could be exploited in the preparation of innovative ABA sustainable thermoplastic elastomers. 9,10,11,12 Conversely, poly(ELA)-b-poly(GA) showed only one Tg at 2 ºC but the hydrolysis of the acetal protecting group of SA segments in acidic media afforded a novel block copolymer poly(ELA)-b-poly(GA) ( Figure   S18). 76,77,78