Preparation of dual stimuli-responsive block copolymers based on different activated esters with distinct reactivities

Accepted Manuscript Preparation of dual stimuli-responsive block copolymers based on different activated esters with distinct reactivities Lirong He, Jiaojiao Shang, Patrick Theato PII: DOI: Reference:

S0014-3057(15)00058-0 http://dx.doi.org/10.1016/j.eurpolymj.2015.01.045 EPJ 6740

To appear in:

European Polymer Journal

Received Date: Revised Date: Accepted Date:

20 December 2014 22 January 2015 29 January 2015

Please cite this article as: He, L., Shang, J., Theato, P., Preparation of dual stimuli-responsive block copolymers based on different activated esters with distinct reactivities, European Polymer Journal (2015), doi: http://dx.doi.org/ 10.1016/j.eurpolymj.2015.01.045

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Preparation of dual stimuli-responsive block copolymers based on different activated esters with distinct reactivities Lirong He, Jiaojiao Shang, Patrick Theato* 1

Institute for Technical and Macromolecular Chemistry, University of Hamburg Bundesstrasse 45, D-20146 Hamburg, Germany

Abstract: In the present study, we described a facile method for introducing functional moieties into block copolymer structures by sequential aminolysis of pentafluorophenyl acrylate (PFPA) ester and methyl salicylate acrylate (MSA) ester. For the first time, reversible addition-fragmentation transfer (RAFT) polymerization of MSA yielding block copolymers poly(MSA)-b-poly(PFPA) was reported. The yielded block copolymers were subsequently modified step-by-step using amino moieties, affording polymers with bespoke functionality. By exploring the reactivity difference of two activated esters (PFPA and MSA) toward amines, the installation of amine functionalities on polymer backbones while maintaining a block copolymer architecture was realized in a controlled manner. The selective reactivity of these two esters (PFPA and MSA) towards aliphatic amines, such as isopropylamine, cyclopropylamine and N,N-diethylethylenediamine, provided a novel synthetic approach in making temperature and pH dual responsive polymers. Finally, the self-assembly behavior of the obtained block copolymers was investigated.

Keywords: methyl salicylate acrylate ester; pentafluorophenyl acrylate ester; post-polymerization modification; RAFT polymerization; stimuli responsive polymers; block copolymers 1

TOC

Introduction Polymers with defined architectures and well-controlled functionalities are currently of great interest in academic research[1][2][3]. The straightforward way to prepare functional polymers is the direct polymerization of the respective functional monomer. However, in some cases functional monomers are incompatible with the available polymerization technique or inhibit a precise characterization[4][5]. In such cases, the polymerization of reactive polymer precursors, followed by a post-polymerization modification step provides an alternative approach for the synthesis of functional polymeric materials. Polymers featuring reactive groups allow the installation of various functional groups and hence preparation of novel materials, which find wide application in smart materials[6][7] and biomedical research. Such reactive polymers provide powerful synthetic platforms for the preparation of new materials for bioconjugation[8], drug carriers[9][10][11], nano thin film[12] as well as photovoltaic devices[13]. Various preformed polymer scaffolds based on reactive groups allowing for efficient chemical reactions were developed in recent years, affording versatile tools to tailor polymer architectures. 2

Post-polymerization modifications using efficient click type chemistries, including azide-alkyne and diene-dienophile cycloadditons, Diels-Alder, thiol-yne and thiol-ene addition, azlactone addition, Kabachnik-Fields reaction, mercaptoacetic acid locking imine reaction, Biginelli reaction [14][15][16][17] and substitution reactions based on activated esters laid the foundation to prepare functional polymer libraries[18][19][20][21][2]. The combination of multi-reactive groups in the same polymer chain provides a convenient way to synthesize complex macromolecule architectures. For example, Haddleton et. al. [22]described the preparation endfunctionalized glycopolymers by sequential functionalization using Huisgens cycloaddition and thiol-ene double click reactions. Tunca et. al. [23]reported a facile way in synthesizing star-block copolymers utilizing double click chemistry via a one-pot technique. As shown by several groups, preformed polymer precursors featuring more than one single type of reactive group afford a facile approach in preparing multi-functional polymers[24]. Among these chemistry tools, activated ester-amine chemistry has many characteristics of click type chemistries, especially featuring metal free and mild reaction conditions[25]. The advantages of this chemistry are (i) precursor polymers are easy to synthesize and purify, (ii) reactive components including activated ester polymers and amine species are inert to air, providing a facile way in terms of storing, (iii) the aminolysis reaction is easy to be monitored by FT-IR spectroscopy, (iv) numerous functional amines are commercially available, (v) the compatibility of activated ester based monomer with modern controlled radical polymerization methods, such as reversible addition fragmentation chain transfer (RAFT) polymerization and nitroxide-mediated polymerization (NMP)[1][26][27][2][28][29][8]. Because of these features, activated ester-amine chemistry has become a popular method allowing the easy synthesis of reactive polymer scaffolds. Even though N-hydroxysuccinimide derivaties have been reported as 3

the first activated ester precursors in the 1970s[30], the library of activated ester precursors has been immensely developed afterwards. This library is mainly based on versatile activated ester polymers,

for

example

featuring

N-hydroxysuccinimide,

thiazolidine-2-thione,

pentafluorophenol[29][27][31], hexafluoroisopropanol[26], azlactone[32][33] and salicylate derivatives[25], affording a variability in terms of reactivity, polarity and biocompatibility. This diversity provides efficient and powerful tools in manufacturing a wide spectrum of functional materials applicable ranging from drug delivery and biosensors to photovoltaic devices[9][13]. In addition, the reactivity difference of selected activated esters enables the fabrication of polymers with defined architectures and well-controlled functionalities by sequential aminolysis of precursor copolymers. Theato et. al. described successfully a sequential conversion of block copolymers bearing PFP 4-vinylbenzoate and PFPA esters using aniline and aliphatic amines, respectively[3]. Later on, Davis et. al. reported a selective post-polymerization modification of copolymer backbones featuring PFPA and vinyl dimethyl azlactone using a library of amines[33]. Both reports focused on the selectivity towards difference in amines, i.e. aromatic amines vs. aliphatic amines, limiting their application in terms of manufacturing functional responsive polymers. Among these activated ester building blocks, the latest reported activated esters[25] based on salicylate derivatives, featuring the characteristics of lower cytotoxicity and cheap prices, are of great interest, especially when it comes to biological applications. Our previous work also demonstrated that the reactivity of PFPA ester is higher than activated esters derived from salicylate derivatives[25]. These positive features motivated us to investigate the difference in amine reactivity, especially aliphatic amines, between PFPA and methyl salicylate acrylate. To prove the practical applicability of this sequential aminolysis approach, block copolymers 4

derived from MSA and PFPA were synthesized for the first time. A sequential postpolymerization modification using amines will be conducted, yielding block copolymers whose self-assembly behavior will be investigated.

Experimental Section 1. Materials All chemicals were commercially available and used as received, unless otherwise stated. Dichloromethane (DCM), THF, and dioxane were dried using a solvent drying machine (Pure Solv™ from Innovative Technology) and used immediately. Triethylamine (Et 3N) was dried over molecular sieve (4Å). Azobisisobutyronitrile (AIBN) was recrystallized from methanol prior to use. Pentafluorophenyl acrylate (PFPA)[29] and methyl salicylate acrylate (MSA)[25] were synthesized according to previous reports. 2. Structural and chemical composition characterization 1

H-NMR spectroscopy was performed on a Bruker 300 MHz FT-NMR spectrometer in

deuterated solvents. The chemical shifts (δ) are given in ppm relative to a standard, tetramethylsilane (TMS). The molecular weight and corresponding molecular weight distribution (Mw/Mn) were determined by gel permeation chromatography (GPC) using polystyrene standards. Unless otherwise stated, the measurements were conducted in THF solution with a flow rate of 1 mL*min-1 at 25 °C. Infrared spectroscopy was performed on a Thermo Fisher Scientific Nicolet iS10 using ATR unit. Kinetic IR measurements were conducted on a Reactive IRTM 45m from Mettler Toledo. 5

3. Reactivity study of PPFPA and PMSA using reactive IR PPFPA (180 mg, 0.75 mM, 1 eq.) and poly(methyl salicylate acrylate) (PMSA) (156 mg, 0.75 mM, 1 eq.) in anyhydrous 3.5 mL THF were placed in two-neck flask at 25 °C. The IR probe was inserted into the solution. Once, stable IR spectra could be detected, hexylamine (80.4 mg, 110.4 µL, 0.75 mM, 1 eq.) and triethylamine (80.4 mg, 110.8 µL, 0.75 mM, 1 eq.) were introduced to the mixture. IR spectra were then recorded in time intervals of 30 seconds. After the 30 minutes, hexylamine (230 mg, 298 µL, 2.27 mM, 3 eq.) and triethylamine (230 mg, 326 µL, 2.27 mM, 3 eq.) were introduced to the mixture. IR spectra were recorded in time intervals of 1 minute and at 10-minute intervals after the first hour. Time-resolved conversion was calculated by the decrease of the carbonyl peak in the IR spectrum. The integration of the peak at 0 min was defined as 0 % conversion. 4. Kinetic analysis of PMSA RAFT polymerization A schlenk tube equipped with a stirring bar was loaded with MSA (50 eq., 1 g, 4.84 mM), 2(dodecylthiocarbonothioyl-thio)-2-methylpropionic acid (DDMAT) (1 eq., 32 mg, 0.0968 mM) and AIBN (0.1 eq., 1.6 mg, 0.00968 mM). All compounds were dissolved in 1 mL anhydrous dioxane. After three freeze-pump-thaw cycles, the tube was placed in a 65 °C preheated oil bath. At each time point a small amount of crude solution was taken under the protection of nitrogen atmosphere. The samples were analyzed by 1H NMR and GPC in THF. The monomer conversion was calculated by comparing the signal integral of the monomer CH2=CH- protons  from ppm to 6.11 ppm to the broad polymer backbone protons signal from 3.11 ppm to 2.05 ppm. Additionally, the molecular weight could also be determined by comparing the integral ratio of methyl protons (-COOCH3) on the side chain of PMSA at ppm

6

to 2-(dodecylthiocarbonothioyl-thio)-2-methylpropionic acid end group (methyl protons –CH3) at ppm.

5. RAFT polymerization of homo polymer PMSA or PPFPA As an example the synthesis of P4 is described in the following. For a typical RAFT polymerization, a schlenk tube equipped with a stirring bar was loaded with MSA (100 eq., 500 mg, 2.42 mM), 2-(dodecylthiocarbonothioyl-thio)-2-methylpropionic acid (DDMAT) (1 eq., 8.9 mg, 0.0242 mM) and AIBN (0.1 eq., 0.4 mg, 0.00242 mM). All compounds were dissolved in 1 mL anhydrous dioxane. After three freeze-pump-thaw cycles, the tube was placed in a 65 °C preheated oil bath for 6 h. The reaction was quenched by cooling the reaction vessel in liquid nitrogen and exposed to air. The conversion rate was determined by taking a small amount of crude mixture in CDCl3 and comparing the signal integral of the monomer CH2=CH- protons  from ppm to 6.11 ppm to the broad polymer backbone protons signal from 3.11 ppm to 2.05 ppm. The product was purified by three times precipitation from THF to hexane. The precipitated polymer was dried at 40 °C vacuum oven for 24 h and affording yellow powder polymer (220 mg, yield 44%). H NMR (300 MHZ, δ, ppm, CDCl3): 7.73 (d, 1H), 7.10 (m, 3H), 3.78 (m, 3H), 3.30 (s, 1H), 2.30 (t, 2H);

1

IR (ATR mode): 1757.8 cm-1 (Methyl salicylate ester C=O), 1723.6 cm-1 (methyl ester C=O). GPC (PS standard, THF): Mn=7.783×103 g/mol, Mw/Mn= 1.25. 6. RAFT block copolymerization of poly(MSA)-b-poly(PFPA) or poly(PFPA)-b-poly(MSA)

As an example the synthesis of P12 is described. A schlenk tube equipped with a stirring bar was loaded with PFPA (100 eq., 238 mg, 1 mM), Macro CTA P1 (1 eq., 75.2 mg, 0.001 mM) and AIBN (0.1 eq., 0.2 mg, 0.0001 mM). All compounds were dissolved in 1 mL anhydrous dioxane. After three freeze-pump-thaw cycles the tube was placed in a 65 °C preheated oil bath for 6 h. The reaction was 7

quenched by cooling the reaction vessel in liquid nitrogen and exposed to air. The conversion rate was determined by taking a small amount of crude mixture in CDCl3 and comparing the signal integral of the monomer CH2=CH- protons  from ppm to 6.16 ppm to the polymer backbone proton (-CH2-CH-) signal  = 3.10 ppm. The product was purified by three times precipitation from THF to hexane. The precipitated polymer was dried at 40 °C vacuum oven for 24 h and affording yellow powder polymer (94 mg, yield 30%). H NMR (300 MHZ, δ, ppm, CDCl3): 7.73 (d, 1H), 7.10 (m, 3H), 3.78 (m, 3H), 3.30 (s, 1H), 2.53 (s, 1H,

1

-CH- PFPA main block), 2.30 (t, 2H); IR (ATR mode): 1757.8 cm-1 (Methyl salicylate ester C=O), 1723.6 cm-1 (methyl ester C=O). 1787 cm-1 (PFP ester C=O). GPC (PS standard, THF): Mn=1.25×104 g/mol, Mw/Mn= 1.32.

7. Sequential polymer analogous reaction of block copolymers with various amines Poly(MSA)-b-poly(PFPA) 120 mg (equivalent to 0.229 mM MSA units and 0.33 mM PFPA units, respectively) was dissolved in 1 mL THF; then desired first amine A (0.33mM) and 0.33 mM triehthylamine were added into the solution. The mixture was stirred at 25 °C for 3 h and afterwards a small amount of sample was taken for ATR-IR test. Continuing with the prior reaction mixture, 0.687 mM of the desired second amine B and 0.687 mM triethylamine were added to the solution and stirred at 50 °C for 6h. A test amount of sample for analyzing by ATR-IR was taken before purifying by dialysis against acetone for 2 days and then against water for 1 day with MWCO 3500. Finally, the polymers were analyzed by 1H NMR.

8. Self-assembly of copolymers As an example, 8 mg of P161 were dissolved in 1 mL acetone and stirred overnight. 2 mL 0.1 mol/L HCl solution (or 0.1 mol/L HCl NaOH solution) was dropped to the solution under stirring. The opaque solution was then dialyzed against distilled water, including at least 3 times of water 8

changes. The final concentration of self-assembled solution was determined by measuring the solution final volume.

Results and Discussions Our objective in this work was first to explore the feasibility of selective functionalization, exploring the differing reactivity of different activated ester functionalities toward amines. Specifically, the synthetic strategy was to explore the difference in amine reactivity between the two activated esters: pentafluorophenyl ester (PFP) and methyl salicylate (MS). As reported in our previous study[25], PFPA has a much higher reactivity towards hexylamine than methyl salicylate acrylate[25]. This motivated us to further explore the reactivity difference of these two esters. Hexylamine was used as a reference amine in this study because of its known reactivity. In order to eliminate the side reaction of Michael-amine addition reaction[34][19] between carbon double bond and hexylamine, homopolymers - PPFPA and PSMA, were used to investigate the differing reactivity instead of the respective monomers. Both activated ester polymers were mixed together and simultaneously reacted with hexylamine using the following feeding ratio [PPFPA]:[PSMA]:[hexylamine]=1:1:1. The reaction was monitored via Online IR analysis (Figure S1). Conversions of both ester groups as a function of reaction time were plotted and are shown in Figure 1. As shown in Figure S1, the PPFPA ester (C=O band at 1785 cm-1) was consumed rapidly upon the addition of quantitative amount of hexylamine, while both the methyl salicylate acrylate ester signal (C=O band at 1756.5 cm-1) and methyl ester signal (C=O band at 1723.6 cm-1) remained almost the same (less than 5%). As proved by the disappearing of peak at 1785 cm-1 (Figure S1) and the absence of PFP ester groups (-153.20 ppm, 9

-156.46 ppm and -162.43 ppm) in the 19F NMR spectrum (Figure S2), hexylamine has been fully consumed after 1 hour by selectively reacting with the PFP ester. As shown in Figure 1, at least 95% of methyl-salicylate ester groups were kept intact, while the PFP ester group was fully consumed. At this point, the second feed of hexylamine was introduced to allow the consumption of the methyl-salicylate acrylate ester. Note the slower reaction of the MS ester with the second batch of hexylamine compared to the first reaction with the PFP ester.

Addition of hexylamine

Conversion

1.0 0.8 0.6 0.4 0.2

PFP ester MSA ester MS ester

0.0 0 5 10 15 20 25 30

75

150 225 300

Time (min) Figure 1. Kinetics of PPFPA (black solid squares) and PSMA (red hollow triangles) in the presence of hexylamine in THF. Note: in the first step of reaction, [PPFPA]:[PSMA]:[hexylamine]=1:1:1. After 1 hour, an additional mixture of hexylamine and triethylamine was added to the reaction mixture (final ratio: [PPFPA]:[PSMA]:[hexylamine]=1:1:4). Note: PPFPA and PSMA conversions were determined by reactive IR analysis using the characteristic absorptions at 1780 cm-1 and 1763 cm-1, respectively.

10

Recently, the use of MSA as an activated ester has been reported by our group for the first time[25]. Due to the concern about the stability of labile activated ester group under high polymerization temperature, the thermal stability of MSA was investigated via 1H NMR. For this, MSA was dissolved in DMSO-d6 and heated at 70 °C for 8 hours. As shown in Figure S3, after heating the 1H NMR spectra (II) and (I) exhibit exactly the same peaks. Especially, no new peak appeared at 10.52 ppm and around 6.9 ppm, which would belong to the free phenol proton and aromatic protons of methyl salicylate (III), respectively. Hence, we believe that the methyl salicylate ester groups are kept intact during the polymerization process and no thermal degradation occurred. Since trithiocarbonates such as 2-(dodecylthiocarbonothioyl-thio)2-methylpropionic acid (DDMAT) possess high chain transfer constant, is more hydrolytically stabile, and causes less retardation, it has been reported as a suitable CTA for the polymerization of acrylates[35]. Thus, RAFT polymerization of MSA was explored DDMAT as the chain transfer agent, AIBN as the radical source and dioxane as the solvent (Scheme 1). To confirm that the polymerization reaction conditions result in a first-order monomer conversion, we analyzed the polymerization kinetics via 1H NMR in CDCl3 and GPC in THF. By carefully taking small amount of samples out of a RAFT polymerization reaction every half an hour under nitrogen protection, we were able to monitor the polymer chain propagation and the monomer conversion. The kinetic plot of the polymerization (Figure 2a) at the feeding ratio of [MSA]:[CTA]:[AIBN]= 50:1:0.1 indicated that after an induction period of ~ 100 minutes, the monomer polymerized smoothly following a first-order kinetics. The molecular weight (Figure 2b) shows a linear increase with conversion, while PDI levels off around 1.19. All GPC traces (Figure 2c) were unimodal, with no evidence of high molecular weight species arising from termination products. All these information confirmed the controlled radical polymerization. By increasing the feeding ratio of radical sources ([MSA]:[CTA]:[AIBN]= 50:1:0.2), the induction time could be reduced to 20 minutes (Figure 2d). The plot of ln[M0]/[Mt] as a function of time suggested a pseudo-first-kinetic as well. Additionally, plotting molecular weight against conversion, as shown in Figure 2e and Figure 2f, the molecular weight showed a linear growth with conversion, a narrow PDI (1.19) and all GPC traces 11

were unimodal. Since the inhibition step had very little effect on the characteristics of the final product, [CTA]:[AIBN]=1:0.1 was applied for the following polymerizations.

Scheme 1. RAFT homo and block copolymerization of methyl salicylate acrylate (MSA). (a)

(b)

(c)

2.1 n (kg/mol)

0

1.4

0.7

PDI

6

100

200 time (min)

300

0 0

400

(d)

1.2

6.5

Mn (kg/mol)

0.72 0.48 0.24 0.00 0

20

40 60 Conversion (%)

80

0.8

8.1

5.2

9.0 9.9 Time(min)

10.8

(f) Mn (SEC) Mn theo PDI

30 min 120 min

1.4

1.2

3.9 2.6

60 min 150 min

90 min

PDI

ln[M]0/[M]

0.96

210 min

1.0

(e)

1.20

270 min 120 min

M n, theo

3

0

345 min 150 min

1.4

M n (SEC)

PDI

ln[M] /[M]

9

1.0

1.3

30

60 90 120 time (min)

150

0.0 0

20 40 Conversion (%)

60

0.8

9.0

9.9 Time(min)

10.8

Figure 2. RAFT polymerization kinetics for poly(methyl salicylate acrylate). [MSA]:[CTA]:[AIBN]= 50:1:0.1, C[MSA] = 2 mol/L in dioxane at 65 °C: (a) pseudo-first-kinetic plot; (b) evolution of molecular weights and dispersities as a function of conversion and (c) SEC profiles in THF for the homopolymerization of methyl salicylate acrylate; [MSA]:[CTA]:[AIBN]= 50:1:0.2; C[MSA] = 2 mol/L in 12

dioxane at 65 °C: (d) pseudo-first-kinetic plot; (e) evolution of molecular weights and dispersities as a function of conversion and (f) SEC profiles in THF for the homopolymerization of methyl salicylate acrylate.

Based on these results, a series of homo poly(methyl salicylate acrylate) (PMSA) was prepared by varying the monomer:CTA ratio and the reaction time, which is summarized in Table 1. Molecular weights of PSMA in the range of 4.5-7.8 kg mol-1 and narrow dispersities from 1.19 to 1.25 were obtained for P1-P4. The trithiocarbonate containing polymers were then applied as macro-chain transfer agents for a RAFT block copolymerization of PFPA as a second monomer (Scheme 1). The length of both blocks can be easily tuned by adjusting the monomer:macro CTA ratio, allowing to tailor the weight fractions of both blocks (Table 2). As shown in Figure 4 and Figure S4 and S5, after chain extension with PFPA, the GPC showed a distinct shift of molecular weight to lower elution volumes, i.e. higher molecular weights, compared with the starting macro-CTA, demonstrating the successful chain extension. The copolymer molecular weights obtained by GPC were lower than those by NMR data. The absence of a shoulder peak at higher elution volumes indicates the complete block efficiency. When approaching high molecular weights, as in the sample P15, the increase of molecular weight distribution to 1.4 has been observed, suggesting to limit the synthesis of reactive blocks to a not too high molecular weight. In a similar way, PPFPA was used as polymeric CTA and MSA as a second monomer to prepare the diblock copolymer poly(PFPA-b-MSA) (Table 3 and Table 4). In this case, GPC showed a slight tailing in lower elution volumes after chain extension with MSA, suggesting some incomplete blocking efficiency. However, remaining homopolymer PPFPA could be removed efficiently by dialysis in acetone, resulting in well-defined diblock copolymers with molecular weight distributions around 1.25-1.30 (Figure 3). Overall, MSA can be polymerized as the first block or as the second block. However, the purification of polyMSA when polymerized as the first block can be simply done by precipitation instead of time 13

consuming dialysis, hence the recommendation to polymerize MSA as the first block. 1H NMR analysis showed the characteristic peaks of aromatic protons from 7.86 to 7.06 ppm, –CH3 at 3.65 ppm attributable to methyl salicylate acrylate and –CH– / –CH2– of PFPA/PMSA backbone (Figure S9a-11a). These signals allowed us to define the final copolymer composition. The advantage of using PFPA as a monomer is that it is easy tracked by 19F NMR. 19F NMR analysis showed the expected signals at -153.20 ppm, -156.46 ppm and -162.43 ppm of poly(PFPA). Utilizing 2,2,2-trifluoroethyl methacrylate as inner standard, we were able to determine the accurate composition of PPFPA by 19F NMR. ATR-FTIR of the block copolymers further confirmed the presence of PFP ester and MSA ester at 1785 cm-1 and 1756 cm-1, respectively (Figure S7). Table 1. Results of poly(methyl salicylate acrylate) RAFT polymerization

#

a

mono : CTA: AIBN t (h) T (°C) Ca (%) Mn,theo a

Mnb

Mw/Mn

P1

50 : 1 : 0.1

6

65

73

7210

4410

1.19

P2

100 : 1 : 0.1

4

65

66.6

13590

6910

1.22

P3

100 : 1 : 0.1

5

65

68.5

14000

7700

1.23

P4

100 : 1 : 0.1

6

65

77.7

15860

7780

1.25

Determined by 1H NMR (300 MHZ, CDCl3). b Determined by GPC (THF, PS-Std.)

Table 2. Results of block copolymer poly(MSA)-b-poly(PFPA) using PSMA macro-CTA.

#

Macro CTA

mono : macro-CTA: AIBN

t (h)

Mnb

Mw/Mn

fMSA

fPFPA

14

P11

P1

50 : 1 : 0.1

6

9640

1.30

45%

54%

P12

P1

100 : 1 : 0.1

6

12260

1.32

32%

68%

P13

P2

100 : 1 : 0.1

6

13980

1.32

55%

45%

P14

P2

150 : 1 : 0.1

6

14130

1.40

36%

64%

P15

P2

200 : 1 : 0.1

6

21230

1.40

22%

78%

P16

P4

100 : 1 : 0.1

15

18890

1.39

41%

59%

Table 3. Results of poly(PFPA) RAFT polymerization. mono : CTA: AIBN t (h) T (°C) Ca/% MNMR

#

a

Mnb

Mw/Mn

8610

1.18

P5

50 : 1 : 0.1

5

70

67.5

8240

P6

100 : 1 : 0.1

5

70

69.9

14540 14640 1.25

Determined by 1H NMR (300 MHz, CDCl3). b Determined by SEC (THF, PS standard) Table 4. Results of poly(methyl salicylate acrylate)-b-poly(PFPA)

#

P51

Macro

mono : Macro CTA:

CTA

AIBN

P5

50 : 1 : 0.1

t (h)

T (°C)

Mnb

Mw/Mn

fPFPA

fMSA

5.5

70

10310

1.32

58%

42%

15

P52

100 : 1 : 0.1

P5

21480

1.34

73%

9

10 11 Time(min)

12

7

27%

P2 P13 P14 P15

B

RI det. normalized

RI det. normalized

70

A

P1 P11 P12

8

5.5

8

9 10 Time(min)

11

12

Figure 3. A) GPC for P1 (Mn=4 410 g/mol, MW/Mn = 1.19) (black solid curve), P11 (red solid line) (Mn = 9 640 g/mol, Mw/Mn = 1.30) and P12 (blue solid line) (Mn = 12 260 g/mol, Mw/Mn = 1.32); B) GPC for P2 (Mn = 1240 g/mol, Mw = 7 700 g/mol, Mw/Mn = 1.25), P13 (Mn=10 980 g/mol, MW/Mn = 1.32), P14 (Mn=14 130 g/mol, MW/Mn = 1.40), P15 (Mn=21 230 g/mol, MW/Mn = 1.40)

Next, block copolymers were reacted sequentially with functional amines to obtain well-defined functional block copolymers. The conversion of both esters could easily be monitored by ATR-FT-IR. First, the reactivity of both esters in the block copolymers was analyzed by applying different amounts of cyclopropylamine to block copolymer P13 in THF. The conversion was monitored by ATR-FT-IR utilizing the characteristic signals at 1786, 1758 and 1724 cm-1, respectively (Figure 4). PFP ester reacted exclusively first, while the MSA ester groups remained untouched, which further confirmed our previous results obtained by conversion of a mixture of homopolymers PPFPA and PMSA.

16

Absorbance

Time P13 0.4 eq. 0.8 eq. 0.9 eq. 1.0 eq.

amide

MSA ester

PFP ester

1800 1700 1600 -1 Wavenumber (cm ) Figure 4. FT-IR spectra of poly(MSA)-b-poly(PFPA) block polymer P13 versus feeding ratio of cyclopropylamine.

Figure 5. a) 1H NMR (300 MHZ, CDCl3) of poly(MSA)-b-poly(PFPA) P14; b) 1H NMR (300 MHZ, CDCl3) of poly(MSA)-b-poly(PFPA) after post-polymerization modification with cyclopropylamine and N,N-diethylethylenediamine sequentially P142.

Subsequently, we decided to extend our synthetic approach by post-polymerization modification of the double reactive block copolymers with commercially available amines, such as benzylamine, N,N17

diethylethylenediamine, isopropylamine and cyclopropylamine, to prepare several responsive functional block copolymers. Initially, poly(MSA)-b-poly(PFPA) copolymers were reacted with the first amine A for 3 h; then a second amine B was added to the mixture to react with the MSA activated ester. The sequential conversion of both activated esters with the two respective amines was monitored by FT-IR and 1H NMR. As shown in Figure S8, FT-IR confirmed the successive conversion of PFPA during the first aminolysis by the disappearing of its characteristic peak at 1785 cm-1. Noteworthy, the vanishing of methyl salicylate acrylate ester signals at 1758 and 1724 cm-1 was only observed after feeding of the second amine. The sequential aminolysis results are in very good agreement with our previous study using hexylamine and cyclopropylamine. The block copolymers were purified by dialysis against acetone after post-polymerization modification. The absence of MSA ester characteristic aromatic protons at 7.86 ppm, 7.17 ppm and 7.04 ppm further confirmed the successful aminolysis reaction. In addition, 1H NMR of the pure products allows the determination of the block copolymer compositions after two sequential steps of post-polymerization modification. For example, after post-polymerization modification with cyclopropylamine and N,N-diethylethylenediamine sequentially, the ratio of poly(N-cyclopropyl acrylamide) (PCPM) and poly(2-(diethylamino)ethyl acrylamide) (PDEEA) is 65%:35%, which is determined by the integral ratio of cyclopropane protons –CH2- belonging to poly(cyclopropyl acrylamide) at 1.04 ppm (Figure 4B-b) and –CH3 protons owing to N,N-diethylamino moieties from 0.54 ppm to 0.69 ppm (Figure 4B-a). The yielded block copolymer composition is almost identical to its parent poly(MSAb-PFPA) (66%:34%), which is calculated by the peak integration proportions of backbone –CH- protons of PPFPA at 3.10 ppm (Figure 4A-d) with PMSA backbone –CH- protons at 3.31 ppm (Figure 4A-c). In the other cases, the block copolymer compositions were also almost identical with the initial feeding ratio of their respective parent precursor polymers, indicating the quantitative aminolysis of both esters (Figure S9-11). In addition, GPC analysis of the block copolymers revealed unimodal traces, suggesting that the well-defined features of precursor reactive block copolymers were remained. Further, the intermediately

18

formed thiol – via aminolysis of the CTA end group – did not result in any polymer coupling but rather that the polymeric free thiol reacted with the added acrylamide (Figure S12).

Table 5. Summary of different amines used for post-polymerization modification.

Parent poly(MSA)-b-

Amine

Amine

f

f

F Amine-

F Amine-

poly(PFPA)

A

B

PFPA

SMA

A

B

Code

P121

P12

68%

32%

70%

30%

P141

P14

64%

36%

62%

38%

P151

P15

78%

22%

80%

20%

P161

P16

59%

41%

55%

45%

The block copolymers that were successfully modified with benzylamine and N,N-diethylethylenediamine have the potential to self-assemble into pH induced nanoparticles, due to the pH responsive property of 19

tertiary amino group. It is known that under 0.1 M HCl (pH = 1.4), the tertiary amine group of poly(2(diethylamino)ethyl acrylate) (PDEAEA) is protonated and renders PDEAEA soluble in water, and therefore acts as a permanent hydrophilic block[36]. Similarly, the homopolymer prepared by postpolymerization functionalization poly(2-(diethylamino)ethyl acrylamide) (PDEAEAm) is soluble under acidic conditions. In contrast to PDEAEA, PDEAEAm behaved differently under basic conditions (0.1 M NaOH, pH=13.2), when the tertiary amino groups is deprotonated. Under basic conditions PDEAEAm featured a lower critical solution temperature (LCST) with a very sharp transition at 32.4 ºC (Figure S13). Consequently, under acidic conditions (pH = 1.4), P161 should be an amphiphilic block copolymer, which can be expected to form self-assembled micelles in water. Under basic conditions (pH = 13.2), P161 is expected to form micelles below LCST, however, when the temperature is raised above LCST, it is expected to form precipitates. DLS showed that the size distribution of P161 in THF was matching to fully dissolved unimers (Dh = 5 nm) (Figure 6A). Under acidic conditions (pH = 1.4), a larger size distribution of Dh ≈ 19 nm with a dispersity of 0.2 was observed, which can be explained by the formation of micelles. Under basic conditions (pH=13.2) at 25 °C, larger micelles with a diameter of Dh ≈ 61 nm with a low dispersity of 0.043 were obtained, which is in good agreement with TEM data (Figure S14). When further heating the basic solution to 70 °C, larger aggregates with a size around 700 nm appeared

42

Intensity

36 30 24 18

A 0.1 M NaOH Size 61 nm PDI 0.043 25 ºC

THF Size 5 nm PDI 0.63 25 ºC

0.1 M NaOH Size 685 nm PDI 0.013 70 ºC

0.1 M HCl Size 19.30 nm PDI 0.231 25 ºC

12 6 0 10

100 Size (nm)

1000

Hydrodynamic radius (nm)

(Figure 5B), which can be explained by the fact that both blocks of P161 have become fully hydrophobic.

720

B

640 120 90 60 30 0 20

30

40 50 60 70  Temperature ( C)

80

90

20

Figure 6. A) DLS size measurement of reactive copolymers modified with benzylamine and N,Ndiethylethylenediamine (P161) 4.2 mg/mL in 0.1M HCl (black square connected line) and 0.1 M NaOH (red dot connected line) at 25 ºC. B) Temperature-responsive behavior of P161 in 0.1 M NaOH. This result clearly demonstrates that the synthesis of dual responsive diblock copolymers was for the first time successful employing a one-pot sequential post-polymerization modification route. Further preliminary self-assembly studies showed that the block copolymers can yield polymeric micelles in water. The formation of micelles not only proved that the successful sequential aminolysis based on the reactivity difference between PFPA and MSA can be fully controlled but also indicated that this synthetic approach can be also used to prepare temperature and pH dual responsive block copolymers. Additionally, diblock copolymers prepared via sequential aminolysis with isopropylamine or cyclopropylamine and N,N-diethylethylenediamine yielded also multi stimuli-responsive block copoylmers. However, their responsive behavior was very complex due to three competing stimuli (2 times temperature and 1 time pH) and will be subject of a forthcoming study.

Conclusion In conclusion, we explored a new facile method for introducing functional moieties into block copolymers by sequential aminolysis of PFPA ester and MSA ester. For the first time, the reactivity difference of two esters toward amines was demonstrated and documented by on-line IR. RAFT polymerization of block copolymer poly(MSA)-b-poly(PFPA) was reported and the yielded block copolymers were subsequently modified step-by-step with different aliphatic amines, affording polymers with the bespoke functionality. The selective reactivity of these two esters (PFPA and MSA) towards aliphatic amines, such as isopropylamine, cyclopropylamine and amino compounds containing tertiary amine moieties, provided a novel synthetic approach in making temperature and pH responsive polymers. 21

In addition, the self-assembly behavior of the obtained block copolymers was investigated and a dual responsiveness was documented by DLS and TEM measurements.

Acknowledgements Qilu Zhang (UGhent) is gratefully acknowledged for fruitful discussions on polymerizations. L.H. gratefully acknowledges the China Scholarship Council (CSC, Grant 201206240006) for partial support of this work.

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Highlight     

RAFT polymerization of poly(methyl salicylate acrylate)-block-poly(pentafluorophenyl acrylate) [PMSA-b-PPFPA]; Reactivity difference of two activated esters MSA and PFPA; Sequential post-polymerization modification of PMSA-b-PPFPA with amines; Diblock copolymers obtained from one-pot sequential aminolysis reaction; A novel synthetic approach in making temperature and pH dual responsive polymers.

25