Thermoresponsive Polymers of Poly(2-(N-alkylacrylamide)ethyl acetate)s

Thermoresponsive poly(2-(N-alkylacrylamide) ethyl acetate)s with different N-alkyl groups, including poly(2-(N-methylacrylamide) ethyl acetate) (PNMAAEA), poly(2-(N-ethylacrylamide) ethyl acetate) (PNEAAEA), and poly(2-(N-propylacrylamide) ethyl acetate) (PNPAAEA), as well as poly(N-acetoxylethylacrylamide) (PNAEAA), were synthesized by solution RAFT polymerization. Unexpectedly, it was found that there are induction periods in the RAFT polymerization of these monomers, and the induction time correlates with the length of the N-alkyl groups in the monomers and follows the order of NAEAA < NMAAEA < NEAAEA < NPAAEA. The solubility of poly(2-(N-alkylacrylamide) ethyl acetate)s in water is also firmly dependent on the length of the N-alkyl groups. PNPAAEA including the largest N-propyl group is insoluble in water, whereas PNMAAEA and PNEAAEA are thermoresponsive in water and undergo the reversible soluble-to-insoluble transition at a critical solution temperature. The cloud point temperature (Tcp) of the thermoresponsive polymers is in the order of PNEAAEA < PNAEAA < PNMAAEA. The parameters affecting the Tcp of thermoresponsive polymers, e.g., degree of polymerization (DP), polymer concentration, salt, urea, and phenol, are investigated. Thermoresponsive PNMAAEA-b-PNEAAEA block copolymer and PNMAAEA-co-PNEAAEA random copolymers with different PNMAAEA and/or PNEAAEA fractions are synthesized, and their thermoresponse is checked.


Introduction
Stimuli-responsive polymers have gained great attention due to their promising applications in drug delivery, separations, filtration, smart surfaces, and regulating enzyme activity [1][2][3][4][5]. The stimuli can be classified as chemical stimuli, e.g., pH [6][7][8][9][10][11][12][13][14] and chemical agents [15][16][17], and physical stimuli including temperature [8,10,12,[18][19][20][21][22][23][24][25], electric or magnetic fields [26][27][28], and mechanical stress [29][30][31]. In all these stimuli, temperature is one of the most commonly used because of its easy control [19,32,33]. Five kinds of thermoresponsive polymers owing to a lower critical solution temperature (LCST) in water, e.g., poly(meth)acrylamides [19,34], poly(aminoethyl methacrylate)s [11,23,[35][36][37], poly[oligo(ethylene glycol) (meth)acrylate]s [38][39][40][41][42], poly(2-alkyl-2-oxazoline)s [18,[43][44][45], and poly(vinyl methyl ether)s [20,46], are the most popularly studied. These LCST-type polymers are soluble in water below the cloud point temperature (T cp ) on account of hydrogen bonding between polymer chains and the surrounding water molecules. At a temperature above T cp , hydrogen bonding between polymer chains and water is destroyed and intra-and inter-molecular hydrogen bonding/hydrophobic interactions within the polymer chains dominate, which results in polymer insolubility accompanied by visible turbidity. This process of ethylacrylamide and anhydrous methanol (70 mL) were added into another flask with a magnetic bar, and then TMSCl (about 0.44 mL) was added dropwise to the flask under argon atmosphere at room temperature with pH at 3-4. After 2 h, the reaction mixture was concentrated under reduced pressure and the crude product was purified with column chromatography using ethyl acetate/methanol to afford a colorless product of N-methyl-N-2-(1-hydroxy) ethylacrylamide with 52% yield (4.50 g). Into a flask with a magnetic stirrer, N-methyl-N-2-(1-hydroxy) ethylacrylamide (4.00 g, 30.97 mmol), DCM (56 mL), potassium carbonate (6.85 g, 49.55 mmol), and acetic anhydride (3.48 g, 34.06 mmol) were added. The reaction mixture was filtered after it was stirred for 5 h at ambient temperature. The filtrate was concentrated by rotary evaporation under reduced pressure and the crude product was purified with column chromatography using ethyl acetate/light petroleum to obtain a colorless NMAAEA product with a yield of 78.5% (4.16 g). 1 Figure S1. As shown by the 1 H NMR spectra, imine-enamine tautomerism in the NMAAEA occurs similarly to those described elsewhere [72].  Figures 1 and 2 show the 1 H NMR spectra and GPC traces of the three polymers. For PNMAAEA198 and PNEAAEA194, the signal of the CTA moieties at about the chemical shift of 0.83 ppm can be discerned, and therefore, the molecular weight Mn,NMR can be calculated by comparing the chemical shifts δ at 0.83 and 3.84-4.48 ppm. As summarized in Table S1, for PNMAAEA198, its Mn,NMR is very close to the theoretical molecular weight Mn,th determined by Equation (S1). This indicates high introduction of the CTA moieties in the polymer chain end in the RAFT polymerization. For PNEAAEA194, Mn,NMR is lower than Mn,th, indicating that the chain transfer agent efficiency in the synthesis of PNEAAEA194 is not as high as that in the synthesis of PNMAAEA198. As indicated by Scheme 1, NEAAEA and NMAAEA have the same chemical composition, and a similar chain transfer agent efficiency in the RAFT polymerization was expected. However, the fact is much different and the exact reason needs further study. For PNPAAEA190, it is difficult to discern the δ signal at 0.83 ppm assigned to the CTA moieties, and therefore, Mn,NMR is not calculated. By GPC analysis, the molecular weight Mn,GPC and Đ were obtained. It is found that all three polymers have a narrow molecular weight distribution, as indicated by Đ below 1.2, and Mn,GPC is lower than Mn,th. The underestimation of Mn,GPC is possibly ascribed to the interaction between N-containing polymers and GPC columns, which is widely reported elsewhere [25,71]. Besides, the underestimation of Mn,GPC is also due to the poly(methyl methacrylate) calibration standards used in GPC analysis.  quenched by putting the flask in ice water. The monomer conversion was detected by NMR analysis by comparing the integral areas at δ = 5.49-5.52 ppm (C=C-H) assigned to the monomer with those at δ = 5.15 ppm assigned to the internal standard 1,3,5-trioxane. After precipitation in ice diethyl ether, the synthesized PNMAAEA was dried in vacuum at ambient temperature overnight.
PNEAAEA and PNPAAEA were synthesized also by solution RAFT polymerization similar to PNMAAEA. However, for PNPAAEA, it was precipitated in ice n-hexane. The detailed synthesis of polymers is shown in Table S1.

Synthesis of Thermoresponsive Random Copolymers
The random copolymer PNMAAEA-co-PNEAAEA was synthesized by solution RAFT polymerization. Herein, the typical synthesis of PNMAAEA 100 -co-PNEAAEA 100 under The monomer conversions for both NMAAEA and NEAAEA determined by 1 H NMR were 100%. The synthesized PNMAAEA 100 -co-PNEAAEA 100 was purified by three precipitation-filtration cycles in cold diethyl ether and was dried under vacuum at room temperature overnight.

Characterization
The 1 H NMR analysis was accomplished on a Bruker Avance III 400 MHz NMR spectrometer using CDCl 3 with the proton signal at δ = 7.26 ppm as solvent. The polymer molecular weight (M w ) and polydispersity (Ð, Ð = M w /M n ) were determined by gel permeation chromatography (GPC) equipped with a Waters 600E GPC system at 50 • C employing THF as eluent, in which a series of poly(methyl methacrylate)s (PMMAs) with narrow-polydispersity molecular weight were used as calibration standards. Turbidity analysis was monitored by a Varian 100 UV-vis spectrophotometer equipped with a thermo-regulator (±0.1 • C) at 500 nm with a heating/cooling rate of 1.0 • C min −1 , in which the transmittance was normalized with deionized water and samples were tested three times in order to obtain a standard deviation analysis as error. T cp was determined by a 50% change in the transmittance. The differential scanning calorimetry (DSC) analysis was performed on a TA DSC Q100 differential scanning calorimeter under nitrogen atmosphere at 0.15 MPa. All the samples were initially heated to 160 • C at a heating rate of 10 • C min −1 , then cooled to -60 • C and kept for 5 min, and finally heated to 160 • C at a heating rate of 10 • C min −1 . Dynamic light scattering (DLS) analysis was carried out on a NanoBrook Omni laser light scattering spectrometer (Brookhaven, GA, USA) at the wavelength of 659 nm at 90 • angle.
Initially, three polymers with similar DP, e.g., PNMAAEA 198 , PNEAAEA 194 , and PNPAAEA 190 , were synthesized under [monomer] 0 :[ECT] 0 :[AIBN] 0 = 1000:5:1 at an almost 100% monomer conversion to check the polymerization conditions. Figures 1 and 2 show the 1 H NMR spectra and GPC traces of the three polymers. For PNMAAEA 198 and PNEAAEA 194 , the signal of the CTA moieties at about the chemical shift of 0.83 ppm can be discerned, and therefore, the molecular weight M n,NMR can be calculated by comparing the chemical shifts δ at 0.83 and 3.84-4.48 ppm. As summarized in Table S1, for PNMAAEA 198 , its M n,NMR is very close to the theoretical molecular weight M n,th determined by Equation (S1). This indicates high introduction of the CTA moieties in the polymer chain end in the RAFT polymerization. For PNEAAEA 194 , M n,NMR is lower than M n,th , indicating that the chain transfer agent efficiency in the synthesis of PNEAAEA 194 is not as high as that in the synthesis of PNMAAEA 198 . As indicated by Scheme 1, NEAAEA and NMAAEA have the same chemical composition, and a similar chain transfer agent efficiency in the RAFT polymerization was expected. However, the fact is much different and the exact reason needs further study. For PNPAAEA 190 , it is difficult to discern the δ signal at 0.83 ppm assigned to the CTA moieties, and therefore, M n,NMR is not calculated. By GPC analysis, the molecular weight M n,GPC and Ð were obtained. It is found that all three polymers have a narrow molecular weight distribution, as indicated by Ð below 1.2, and M n,GPC is lower than M n,th . The underestimation of M n,GPC is possibly ascribed to the interaction between N-containing polymers and GPC columns, which is widely reported elsewhere [25,71]. Besides, the underestimation of M n,GPC is also due to the poly(methyl methacrylate) calibration standards used in GPC analysis.  Figures 1 and 2 show the 1 H NMR spectra and GPC traces of the three polymers. For PNMAAEA198 and PNEAAEA194, the signal of the CTA moieties at about the chemical shift of 0.83 ppm can be discerned, and therefore, the molecular weight Mn,NMR can be calculated by comparing the chemical shifts δ at 0.83 and 3.84-4.48 ppm. As summarized in Table S1, for PNMAAEA198, its Mn,NMR is very close to the theoretical molecular weight Mn,th determined by Equation (S1). This indicates high introduction of the CTA moieties in the polymer chain end in the RAFT polymerization. For PNEAAEA194, Mn,NMR is lower than Mn,th, indicating that the chain transfer agent efficiency in the synthesis of PNEAAEA194 is not as high as that in the synthesis of PNMAAEA198. As indicated by Scheme 1, NEAAEA and NMAAEA have the same chemical composition, and a similar chain transfer agent efficiency in the RAFT polymerization was expected. However, the fact is much different and the exact reason needs further study. For PNPAAEA190, it is difficult to discern the δ signal at 0.83 ppm assigned to the CTA moieties, and therefore, Mn,NMR is not calculated. By GPC analysis, the molecular weight Mn,GPC and Đ were obtained. It is found that all three polymers have a narrow molecular weight distribution, as indicated by Đ below 1.2, and Mn,GPC is lower than Mn,th. The underestimation of Mn,GPC is possibly ascribed to the interaction between N-containing polymers and GPC columns, which is widely reported elsewhere [25,71]. Besides, the underestimation of Mn,GPC is also due to the poly(methyl methacrylate) calibration standards used in GPC analysis.   Figure 3 shows that the N-alkyl group in poly(2-(N-alkylacrylamide) ethyl acetate)s firmly correlates to the glass transition temperature (T g ), which is determined at the middle point in the step transition, as indicated by the intersection point in the DSC thermograms. The bigger N-alkyl group leads to a lower T g . That is, T g is in the order of PNMAAEA 198 > PNEAAEA 194 > PNPAAEA 190 . For comparison, poly(N-acetoxylethylacrylamide) (PNAEAA 193 ) with a similar structure but with the N-alkyl group replaced with a H atom was synthesized as reported previously [73]. It is found that PNAEAA 193 has a much higher T g of 43.34 • C than the other three homopolymers. It is thought that Polymers 2020, 12, 2464 6 of 15 due to the N-alkyl group being replaced by H, there exists hydrogen bonding or strong intermolecular interaction in the PNAEAA chains, and therefore, PNAEAA has a higher T g .  Figure 3 shows that the N-alkyl group in poly(2-(N-alkylacrylamide) ethyl acetate)s firmly correlates to the glass transition temperature (Tg), which is determined at the middle point in the step transition, as indicated by the intersection point in the DSC thermograms. The bigger N-alkyl group leads to a lower Tg. That is, Tg is in the order of PNMAAEA198 > PNEAAEA194 > PNPAAEA190. For comparison, poly(N-acetoxylethylacrylamide) (PNAEAA193) with a similar structure but with the Nalkyl group replaced with a H atom was synthesized as reported previously [73]. It is found that PNAEAA193 has a much higher Tg of 43.34 °C than the other three homopolymers. It is thought that due to the N-alkyl group being replaced by H, there exists hydrogen bonding or strong intermolecular interaction in the PNAEAA chains, and therefore, PNAEAA has a higher Tg.  Figure 4, for N-acetoxylethylacrylamide (NAEAA), there is almost no induction period in RAFT polymerization, and with the N-alkyl group becoming larger, the induction period increases from 30 to 150 min. RAFT polymerization undergoing an induction period is widely observed in solution RAFT formulation [72,[74][75][76][77], and it is ascribed to the slow fragmentation of the initiator to produce leaving group radicals, the slow re-initiation by the expelled radicals, the increased stability of the intermediate radicals, and/or the low efficiency of chain transfer agent in capturing radicals. After the induction periods, all RAFT polymerizations follow pseudofirst-order kinetics with close to the same polymerization rates as indicated by the linear ln([M]0/[M])time plots. As clearly shown in Figure 4B, the induction periods increase with the increase in the length of the N-alkyl substituents (carbon number) in the 2-(N-alkylacrylamide) ethyl acetate monomers in the following order: NAEAA < NMAAEA < NEAAEA < NPAAEA. This unexpected effect indicates that the structure of the N-alkyl group in the monomers plays a significant role in the processes leading to the induction periods, and investigating this phenomenon in more detail will be the subject of future investigations. The synthesized PNMAAEA, PNEAAEA, and PNPAAEA were characterized by NMR (herein, the 1 H NMR spectra are not shown) and GPC (Figures 4C and S2). As summarized in Figure 4D, all polymers have a narrow molecular weight distribution, as indicated by   Figure 3 shows that the N-alkyl group in poly(2-(N-alkylacrylamide) ethyl acetate)s firmly correlates to the glass transition temperature (Tg), which is determined at the middle point in the step transition, as indicated by the intersection point in the DSC thermograms. The bigger N-alkyl group leads to a lower Tg. That is, Tg is in the order of PNMAAEA198 > PNEAAEA194 > PNPAAEA190. For comparison, poly(N-acetoxylethylacrylamide) (PNAEAA193) with a similar structure but with the Nalkyl group replaced with a H atom was synthesized as reported previously [73]. It is found that PNAEAA193 has a much higher Tg of 43.34 °C than the other three homopolymers. It is thought that due to the N-alkyl group being replaced by H, there exists hydrogen bonding or strong intermolecular interaction in the PNAEAA chains, and therefore, PNAEAA has a higher Tg.  Figure 4, for N-acetoxylethylacrylamide (NAEAA), there is almost no induction period in RAFT polymerization, and with the N-alkyl group becoming larger, the induction period increases from 30 to 150 min. RAFT polymerization undergoing an induction period is widely observed in solution RAFT formulation [72,[74][75][76][77], and it is ascribed to the slow fragmentation of the initiator to produce leaving group radicals, the slow re-initiation by the expelled radicals, the increased stability of the intermediate radicals, and/or the low efficiency of chain transfer agent in capturing radicals. After the induction periods, all RAFT polymerizations follow pseudofirst-order kinetics with close to the same polymerization rates as indicated by the linear ln([M]0/[M])time plots. As clearly shown in Figure 4B, the induction periods increase with the increase in the length of the N-alkyl substituents (carbon number) in the 2-(N-alkylacrylamide) ethyl acetate monomers in the following order: NAEAA < NMAAEA < NEAAEA < NPAAEA. This unexpected effect indicates that the structure of the N-alkyl group in the monomers plays a significant role in the processes leading to the induction periods, and investigating this phenomenon in more detail will be the subject of future investigations. The synthesized PNMAAEA, PNEAAEA, and PNPAAEA were characterized by NMR (herein, the 1 H NMR spectra are not shown) and GPC ( Figures 4C and S2). As summarized in Figure 4D, all polymers have a narrow molecular weight distribution, as indicated by  Figure 4, for N-acetoxylethylacrylamide (NAEAA), there is almost no induction period in RAFT polymerization, and with the N-alkyl group becoming larger, the induction period increases from 30 to 150 min. RAFT polymerization undergoing an induction period is widely observed in solution RAFT formulation [72,[74][75][76][77], and it is ascribed to the slow fragmentation of the initiator to produce leaving group radicals, the slow re-initiation by the expelled radicals, the increased stability of the intermediate radicals, and/or the low efficiency of chain transfer agent in capturing radicals. After the induction periods, all RAFT polymerizations follow pseudo-first-order kinetics with close to the same polymerization rates as indicated by the linear ln([M] 0 /[M])-time plots. As clearly shown in Figure 4B, the induction periods increase with the increase in the length of the N-alkyl substituents (carbon number) in the 2-(N-alkylacrylamide) ethyl acetate monomers in the following order: NAEAA < NMAAEA < NEAAEA < NPAAEA. This unexpected effect indicates that the structure of the N-alkyl group in the monomers plays a significant role in the processes leading to the induction periods, and investigating this phenomenon in more detail will be the subject of future investigations. The synthesized PNMAAEA, PNEAAEA, and PNPAAEA were characterized by NMR (herein, the 1 H NMR spectra are not shown) and GPC ( Figure 4C and Figure S2). As summarized in Figure 4D, all polymers have a narrow molecular weight distribution, as indicated by Ð below 1.17. M n,GPC and M n,NMR of the synthesized polymers linearly increase with monomer conversion, M n,NMR is very close to M n,th , and M n,GPC is smaller than M n,th . The underestimation of M n,GPC is as discussed above.

Thermoresponse of Poly(2-(N-alkylacrylamide) ethyl acetate)s
In this section, the thermoresponse of the aqueous solution of poly(2-(N-alkylacrylamide) ethyl acetate)s, as well as PNAEAA, is investigated. It is found that PNPAAEA190 is insoluble in water, and therefore, the further investigation is focused on PNAEAA193, PNMAAEA198, and PNEAAEA194.
As shown in Figure 5, PNAEAA193, PNMAAEA198, and PNEAAEA194 underwent a soluble-toinsoluble transition when the temperature increased above Tcp and a reversible insoluble-to-soluble transition when the temperature decreased below Tcp. Note that, herein, Tcp is determined at a 50% change in the transmittance, where the temperature increases from below Tcp to above Tcp. Readers can also refer to the Tcp determined at the inflection points in the heating process (Table S2). From However, the fact is inconsistent with the expectation. It is thought that intermolecular hydrogen bonding in PNAEAA chains depresses the hydrogen bonding between PNAEAA and water, and therefore decreases the Tcp of PNAEAA. Second, the phase transition takes place within a narrow temperature window of 1-3 °C. Third, there is almost no hysteresis in the cooling process, indicating a fully reversible solubility transition. For PNMAAEA198 and PNEAAEA194, this is reasonable, as the absence of a proton donor in PNMAAEA and PNEAAEA, leading to the weak intermolecular

Thermoresponse of Poly(2-(N-alkylacrylamide) ethyl acetate)s
In this section, the thermoresponse of the aqueous solution of poly(2-(N-alkylacrylamide) ethyl acetate)s, as well as PNAEAA, is investigated. It is found that PNPAAEA 190 is insoluble in water, and therefore, the further investigation is focused on PNAEAA 193 , PNMAAEA 198 , and PNEAAEA 194 .
As shown in Figure 5, PNAEAA 193 , PNMAAEA 198 , and PNEAAEA 194 underwent a soluble-to-insoluble transition when the temperature increased above T cp and a reversible insoluble-to-soluble transition when the temperature decreased below T cp . Note that, herein, T cp is determined at a 50% change in the transmittance, where the temperature increases from below T cp to above T cp . Readers can also refer to the T cp determined at the inflection points in the heating process (Table S2). From Figure 5, three conclusions are made. First, T cp firmly correlates to the N-alkyl group in poly(2-(N-alkylacrylamide) ethyl acetate)s. In the case of the H atom, PNAEAA 193 has a moderate T cp of 50.4 • C; in the case of the N-ethyl group, PNEAAEA 194 has the lowest T cp of 20.5 • C; and in the case of the N-methyl group, PNMAAEA 198 has the highest T cp of 57.5 • C. It was expected that PNAEAA 193 should have the highest T cp as the N-H group is more hydrophilic than either the N-methyl or N-ethyl group. However, the fact is inconsistent with the expectation. It is thought that intermolecular hydrogen bonding in PNAEAA chains depresses the hydrogen bonding between PNAEAA and water, and therefore decreases the T cp of PNAEAA. Second, the phase transition takes place within a narrow temperature window of 1-3 • C. Third, there is almost no hysteresis in the cooling process, indicating a fully reversible solubility transition. For PNMAAEA 198 and PNEAAEA 194 , this is reasonable, as the absence of a proton donor in PNMAAEA and PNEAAEA, leading to the weak intermolecular interaction, accounts for the reversible solubility transition without hysteresis. For PNAEAA, it seems a little surprising due to the strong intra-and inter-molecular hydrogen bonding ascribed to the N-H group in the polymer chains. Herein, it is thought that the inter-and/or intramolecular hydrogen bonding in PNAEAA chains dominates over the intermolecular hydrogen bonding between the polymer chains and water molecules, which therefore leads to no or very slight hysteresis in the cooling process.
Polymers 2020, 12, x FOR PEER REVIEW 8 of 15 8 interaction, accounts for the reversible solubility transition without hysteresis. For PNAEAA, it seems a little surprising due to the strong intra-and inter-molecular hydrogen bonding ascribed to the N-H group in the polymer chains. Herein, it is thought that the inter-and/or intra-molecular hydrogen bonding in PNAEAA chains dominates over the intermolecular hydrogen bonding between the polymer chains and water molecules, which therefore leads to no or very slight hysteresis in the cooling process. It is known that for the typical thermoresponsive PNIPAM, both the polymer concentration and the polymer DP exert almost no or a slight influence on the LCST of PNIPAM, when the DP of PNIPAM is above a given critical point [78]. The thermoresponse of PNMAAEA and PNEAAEA in aqueous solution is shown in Figure S3 and Figure S4, respectively. As summarized in Figure 6, Tcp of PNEAAEA keeps almost a constant of 20-21 °C, indicating that it is independent of DP in the case of DP above 80 and is independent of polymer concentration (C) in the case of C > 0.5 wt %. However, PNMAAEA is slightly different. Its Tcp decreases from 67.  The influences of inorganic sodium salts on the thermoresponse of PNMAAEA198 and PNEAAEA194, as well as PNAEAA193, are checked. Two typical sodium salts, NaSCN (chaotrope) and It is known that for the typical thermoresponsive PNIPAM, both the polymer concentration and the polymer DP exert almost no or a slight influence on the LCST of PNIPAM, when the DP of PNIPAM is above a given critical point [78]. The thermoresponse of PNMAAEA and PNEAAEA in aqueous solution is shown in Figures S3 and S4, respectively. As summarized in Figure 6, T cp of PNEAAEA keeps almost a constant of 20-21 • C, indicating that it is independent of DP in the case of DP above 80 and is independent of polymer concentration (C) in the case of C > 0.5 wt %. However, PNMAAEA is slightly different. Its T cp decreases from 67.6 to 61.5 • C when the DP increases from 65 to 151, and T cp decreases from 71.3 to 55.4 • C with the increase in polymer concentration from 0.1 to 2.0 wt %, respectively. PNEAAEA and PNMAAEA exhibit different thermoresponsive phenomena, although they have very similar structure, differing in the N-alkyl groups of N-ethyl and N-methyl. The exact reason leading to the different thermoresponse needs further study. interaction, accounts for the reversible solubility transition without hysteresis. For PNAEAA, it seems a little surprising due to the strong intra-and inter-molecular hydrogen bonding ascribed to the N-H group in the polymer chains. Herein, it is thought that the inter-and/or intra-molecular hydrogen bonding in PNAEAA chains dominates over the intermolecular hydrogen bonding between the polymer chains and water molecules, which therefore leads to no or very slight hysteresis in the cooling process. It is known that for the typical thermoresponsive PNIPAM, both the polymer concentration and the polymer DP exert almost no or a slight influence on the LCST of PNIPAM, when the DP of PNIPAM is above a given critical point [78]. The thermoresponse of PNMAAEA and PNEAAEA in aqueous solution is shown in Figure S3 and Figure S4, respectively. As summarized in Figure 6, Tcp of PNEAAEA keeps almost a constant of 20-21 °C, indicating that it is independent of DP in the case of DP above 80 and is independent of polymer concentration (C) in the case of C > 0.5 wt %. However, PNMAAEA is slightly different. Its Tcp decreases from 67.  The influences of inorganic sodium salts on the thermoresponse of PNMAAEA198 and PNEAAEA194, as well as PNAEAA193, are checked. Two typical sodium salts, NaSCN (chaotrope) and The influences of inorganic sodium salts on the thermoresponse of PNMAAEA 198 and PNEAAEA 194 , as well as PNAEAA 193 , are checked. Two typical sodium salts, NaSCN (chaotrope) and NaH 2 PO 4 (kosmotrope) [22,61,64,72,79,80], are selected. As shown in Figure 7, the T cp s of the three thermoresponsive homopolymers increase with NaSCN concentration and decrease with NaH 2 PO 4 concentration. Following the calculations as reported previously [73], Equation (S2) is obtained, Polymers 2020, 12, 2464 9 of 15 from which the T cp of the thermoresponsive polymers at a given salt concentration can be determined. The detailed values for the three polymers are summarized in Table S3.
Polymers 2020, 12, x FOR PEER REVIEW 9 of 15 9 NaH2PO4 (kosmotrope) [22,61,64,72,79,80], are selected. As shown in Figure 7, the Tcps of the three thermoresponsive homopolymers increase with NaSCN concentration and decrease with NaH2PO4 concentration. Following the calculations as reported previously [73], Equation (S2) is obtained, from which the Tcp of the thermoresponsive polymers at a given salt concentration can be determined. The detailed values for the three polymers are summarized in Table S3. The thermoresponse of PNAEAA, PNMAAEA, and PNEAAEA under different urea and phenol concentrations is shown in Figure S5-S7. As summarized in Figure 8A, urea can increase the Tcp of the aqueous solution of PNAEAA193, PNMAAEA198, and PNEAAEA194, and their Tcp increases with urea concentration. It is thought that the urea/poly(2-(N-alkylacrylamide) ethyl acetate) complexes formed via hydrogen bonding become more hydrophilic, as discussed previously [73], and therefore, they exhibit a higher Tcp than that of poly(2-(N-alkylacrylamide) ethyl acetate) aqueous solution in the absence of urea. Unlike urea, phenol exerts an opposite effect on Tcp (Figure 8B), as the phenol/poly(2-(N-alkylacrylamide) ethyl acetate) complexes become more hydrophobic.

Synthesis and Thermoresponse of Block Copolymer and Random Copolymer
As shown above, PNMAAEA and PNEAAEA have two much different Tcps. It is thought that the block copolymer may exhibit two separated Tcps. Following this concern, the PNMAAEA198-b-PNEAAEA80 block copolymer was synthesized employing PNMAAEA198 as the macro-CTA. Herein, we employ PNMAAEA but not PNEAAEA, as the macro-CTA is due to the high integrity of CTA moieties in PNMAAEA, as discussed above. Figure 9 shows the 1 H NMR spectra and GPC traces of  Figure 8A, urea can increase the T cp of the aqueous solution of PNAEAA 193 , PNMAAEA 198 , and PNEAAEA 194 , and their T cp increases with urea concentration. It is thought that the urea/poly(2-(N-alkylacrylamide) ethyl acetate) complexes formed via hydrogen bonding become more hydrophilic, as discussed previously [73], and therefore, they exhibit a higher T cp than that of poly(2-(N-alkylacrylamide) ethyl acetate) aqueous solution in the absence of urea. Unlike urea, phenol exerts an opposite effect on T cp (Figure 8B), as the phenol/poly(2-(N-alkylacrylamide) ethyl acetate) complexes become more hydrophobic.
Polymers 2020, 12, x FOR PEER REVIEW 9 of 15 9 NaH2PO4 (kosmotrope) [22,61,64,72,79,80], are selected. As shown in Figure 7, the Tcps of the three thermoresponsive homopolymers increase with NaSCN concentration and decrease with NaH2PO4 concentration. Following the calculations as reported previously [73], Equation (S2) is obtained, from which the Tcp of the thermoresponsive polymers at a given salt concentration can be determined. The detailed values for the three polymers are summarized in Table S3. The thermoresponse of PNAEAA, PNMAAEA, and PNEAAEA under different urea and phenol concentrations is shown in Figure S5-S7. As summarized in Figure 8A, urea can increase the Tcp of the aqueous solution of PNAEAA193, PNMAAEA198, and PNEAAEA194, and their Tcp increases with urea concentration. It is thought that the urea/poly(2-(N-alkylacrylamide) ethyl acetate) complexes formed via hydrogen bonding become more hydrophilic, as discussed previously [73], and therefore, they exhibit a higher Tcp than that of poly(2-(N-alkylacrylamide) ethyl acetate) aqueous solution in the absence of urea. Unlike urea, phenol exerts an opposite effect on Tcp ( Figure 8B), as the phenol/poly(2-(N-alkylacrylamide) ethyl acetate) complexes become more hydrophobic.

Synthesis and Thermoresponse of Block Copolymer and Random Copolymer
As shown above, PNMAAEA and PNEAAEA have two much different Tcps. It is thought that the block copolymer may exhibit two separated Tcps. Following this concern, the PNMAAEA198-b-PNEAAEA80 block copolymer was synthesized employing PNMAAEA198 as the macro-CTA. Herein, we employ PNMAAEA but not PNEAAEA, as the macro-CTA is due to the high integrity of CTA moieties in PNMAAEA, as discussed above. Figure 9 shows the 1 H NMR spectra and GPC traces of

Synthesis and Thermoresponse of Block Copolymer and Random Copolymer
As shown above, PNMAAEA and PNEAAEA have two much different T cp s. It is thought that the block copolymer may exhibit two separated T cp s. Following this concern, the PNMAAEA 198 -b-PNEAAEA 80 block copolymer was synthesized employing PNMAAEA 198 as the macro-CTA. Herein, we employ PNMAAEA but not PNEAAEA, as the macro-CTA is due to the high integrity of CTA moieties in PNMAAEA, as discussed above. Figure 9 shows the 1 H NMR spectra and GPC traces of PNMAAEA 198 -b-PNEAAEA 80 and its precursor of the PNMAAEA 198 macro-CTA, and the results are summarized in Table S4. PNMAAEA198-b-PNEAAEA80 and its precursor of the PNMAAEA198 macro-CTA, and the results are summarized in Table S4.  Figure 10 shows the temperature-dependent transmittance of the aqueous solution of PNMAAEA198-b-PNEAAEA80, as well as the homopolymers of PNEAAEA79 and PNMAAEA198. As expected, the aqueous solution of PNMAAEA198-b-PNEAAEA80 has two Tcps, one (30.9 °C, Tcp1) is higher than that of PNEAAEA79 (20.9 °C) due to the hydrophilic PNMAAEA198 block, and the other (67.7 °C, Tcp2) is slightly higher than that of PNMAAEA198 (60.7 °C). The higher Tcp2 may be due to steric repulsion of the PNMAAEA198 block tethered on the dehydrated PNEAAEA80. It is thought that PNMAAEA198-b-PNEAAEA80 forms colloidal aggregates at temperatures above Tcp1 and Tcp2, which is indicated by the formation of colloidal dispersion, in which the hydrodynamic diameter (Dh) of the colloidal aggregates is confirmed by DLS analysis ( Figure S8). Besides, as shown in Figure 10, the soluble-to-insoluble transition of the PNMAAEA198-b-PNEAAEA80 block copolymer at Tcp1 or Tcp2 occurs within a broader temperature window in comparison with those of the corresponding homopolymers. This is due to the hydrophilic PNMAAEA block delaying the soluble-to-insoluble transition of the PNEAAEA block at Tcp1 and the dehydrated PNEAAEA block increasing the steric repulsion of the PNMAAEA block and hindering the soluble-to-insoluble transition of the PNMAAEA block at Tcp2. The much different Tcps of PNMAAEA and PNEAAEA causes the tuning of Tcp via synthesis of PNMAAEA-co-PNEAAEA random copolymers. These PNMAAEA-co-PNEAAEA random  Figure 10 shows the temperature-dependent transmittance of the aqueous solution of PNMAAEA 198 -b-PNEAAEA 80 , as well as the homopolymers of PNEAAEA 79 and PNMAAEA 198 . As expected, the aqueous solution of PNMAAEA 198 -b-PNEAAEA 80 has two T cp s, one (30.9 • C, T cp 1) is higher than that of PNEAAEA 79 (20.9 • C) due to the hydrophilic PNMAAEA 198 block, and the other (67.7 • C, T cp 2) is slightly higher than that of PNMAAEA 198 (60.7 • C). The higher T cp 2 may be due to steric repulsion of the PNMAAEA 198 block tethered on the dehydrated PNEAAEA 80 . It is thought that PNMAAEA 198 -b-PNEAAEA 80 forms colloidal aggregates at temperatures above T cp 1 and T cp 2, which is indicated by the formation of colloidal dispersion, in which the hydrodynamic diameter (D h ) of the colloidal aggregates is confirmed by DLS analysis ( Figure S8). Besides, as shown in Figure 10, the soluble-to-insoluble transition of the PNMAAEA 198 -b-PNEAAEA 80 block copolymer at T cp 1 or T cp 2 occurs within a broader temperature window in comparison with those of the corresponding homopolymers. This is due to the hydrophilic PNMAAEA block delaying the soluble-to-insoluble transition of the PNEAAEA block at T cp 1 and the dehydrated PNEAAEA block increasing the steric repulsion of the PNMAAEA block and hindering the soluble-to-insoluble transition of the PNMAAEA block at T cp 2.
Polymers 2020, 12, x FOR PEER REVIEW 10 of 15 10 PNMAAEA198-b-PNEAAEA80 and its precursor of the PNMAAEA198 macro-CTA, and the results are summarized in Table S4.  Figure 10 shows the temperature-dependent transmittance of the aqueous solution of PNMAAEA198-b-PNEAAEA80, as well as the homopolymers of PNEAAEA79 and PNMAAEA198. As expected, the aqueous solution of PNMAAEA198-b-PNEAAEA80 has two Tcps, one (30.9 °C, Tcp1) is higher than that of PNEAAEA79 (20.9 °C) due to the hydrophilic PNMAAEA198 block, and the other (67.7 °C, Tcp2) is slightly higher than that of PNMAAEA198 (60.7 °C). The higher Tcp2 may be due to steric repulsion of the PNMAAEA198 block tethered on the dehydrated PNEAAEA80. It is thought that PNMAAEA198-b-PNEAAEA80 forms colloidal aggregates at temperatures above Tcp1 and Tcp2, which is indicated by the formation of colloidal dispersion, in which the hydrodynamic diameter (Dh) of the colloidal aggregates is confirmed by DLS analysis ( Figure S8). Besides, as shown in Figure 10, the soluble-to-insoluble transition of the PNMAAEA198-b-PNEAAEA80 block copolymer at Tcp1 or Tcp2 occurs within a broader temperature window in comparison with those of the corresponding homopolymers. This is due to the hydrophilic PNMAAEA block delaying the soluble-to-insoluble transition of the PNEAAEA block at Tcp1 and the dehydrated PNEAAEA block increasing the steric repulsion of the PNMAAEA block and hindering the soluble-to-insoluble transition of the PNMAAEA block at Tcp2. The much different Tcps of PNMAAEA and PNEAAEA causes the tuning of Tcp via synthesis of PNMAAEA-co-PNEAAEA random copolymers. These PNMAAEA-co-PNEAAEA random The much different T cp s of PNMAAEA and PNEAAEA causes the tuning of T cp via synthesis of PNMAAEA-co-PNEAAEA random copolymers. These PNMAAEA-co-PNEAAEA random copolymers with an almost constant DP at about 200 but different PNMAAEA and/or PNEAAEA fractions were synthesized, and were characterized by 1 H NMR ( Figure S9) and GPC ( Figure S10), and the results are summarized in Table S4. As shown in Figure 11, by finely tuning the PNMAAEA and/or PNEAAEA fractions, PNMAAEA-co-PNEAAEA random copolymers with a T cp ranging from 21.2 to 60.7 • C were obtained. It should be pointed out that PNMAAEA 100 -co-PNEAAEA 100 has a T cp of 36.5 • C, very close to human body temperature, which will have potential applications. copolymers with an almost constant DP at about 200 but different PNMAAEA and/or PNEAAEA fractions were synthesized, and were characterized by 1 H NMR ( Figure S9) and GPC ( Figure S10), and the results are summarized in Table S4. As shown in Figure 11, by finely tuning the PNMAAEA and/or PNEAAEA fractions, PNMAAEA-co-PNEAAEA random copolymers with a Tcp ranging from 21.2 to 60.7 °C were obtained. It should be pointed out that PNMAAEA100-co-PNEAAEA100 has a Tcp of 36.5 °C, very close to human body temperature, which will have potential applications.

Conclusions
Thermoresponsive poly(2-(N-alkylacrylamide) ethyl acetate)s with different N-alkyl groups, including PNMAAEA, PNEAAEA, and PNPAAEA, as well as poly(N-acetoxylethylacrylamide) (PNAEAA), were synthesized by solution RAFT polymerization. Unexpectedly, it was found that these polymerizations proceed with significant induction periods, and these increase with the length of the N-alkyl group in the order of NAEAA < NMAAEA < NEAAEA < NPAAEA. The solubility of poly(2-(N-alkylacrylamide) ethyl acetate)s is firmly dependent on the N-alkyl groups. PNPAAEA including the largest N-propyl group is insoluble in water, whereas PNMAAEA and PNEAAEA are thermoresponsive in water. It is found that the Tcps of the thermoresponsive polymers with a very similar theoretical DP are in the order of PNEAAEA194 < PNAEAA193 < PNMAAEA198, and PNEAAEA194 has a much lower Tcp than PNMAAEA198. The parameters affecting the Tcp of the thermoresponsive polymers, e.g., DP, polymerization concentration, salt, urea, and phenol, are investigated. The PNMAAEA-b-PNEAAEA block copolymer and PNMAAEA-co-PNEAAEA random copolymers with different PNMAAEA and/or PNEAAEA fractions were synthesized. Due to the large difference in Tcps of PNMAAEA and PNEAAEA, the PNMAAEA-b-PNEAAEA block copolymer has two separate Tcps, and the PNMAAEA-co-PNEAAEA random copolymers have a Tcp ranging from 21.2 to 60.7 °C by tuning the PNMAAEA and/or PNEAAEA fractions. Interestingly, PNMAAEA100-co-PNEAAEA100 has a Tcp of 36.5 °C very close to human body temperature. The thermoresponsive poly(2-(N-alkylacrylamide) ethyl acetate)s with tunable thermal response are believed to have potential applications.

Conclusions
Thermoresponsive poly(2-(N-alkylacrylamide) ethyl acetate)s with different N-alkyl groups, including PNMAAEA, PNEAAEA, and PNPAAEA, as well as poly(N-acetoxylethylacrylamide) (PNAEAA), were synthesized by solution RAFT polymerization. Unexpectedly, it was found that these polymerizations proceed with significant induction periods, and these increase with the length of the N-alkyl group in the order of NAEAA < NMAAEA < NEAAEA < NPAAEA. The solubility of poly(2-(N-alkylacrylamide) ethyl acetate)s is firmly dependent on the N-alkyl groups. PNPAAEA including the largest N-propyl group is insoluble in water, whereas PNMAAEA and PNEAAEA are thermoresponsive in water. It is found that the T cp s of the thermoresponsive polymers with a very similar theoretical DP are in the order of PNEAAEA 194 < PNAEAA 193 < PNMAAEA 198 , and PNEAAEA 194 has a much lower T cp than PNMAAEA 198 . The parameters affecting the T cp of the thermoresponsive polymers, e.g., DP, polymerization concentration, salt, urea, and phenol, are investigated. The PNMAAEA-b-PNEAAEA block copolymer and PNMAAEA-co-PNEAAEA random copolymers with different PNMAAEA and/or PNEAAEA fractions were synthesized. Due to the large difference in T cp s of PNMAAEA and PNEAAEA, the PNMAAEA-b-PNEAAEA block copolymer has two separate T cp s, and the PNMAAEA-co-PNEAAEA random copolymers have a T cp ranging from 21.2 to 60.7 • C by tuning the PNMAAEA and/or PNEAAEA fractions. Interestingly, PNMAAEA 100 -co-PNEAAEA 100 has a T cp of 36.5 • C very close to human body temperature. The thermoresponsive poly(2-(N-alkylacrylamide) ethyl acetate)s with tunable thermal response are believed to have potential applications.