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An Eco-Friendly Pathway to Thermosensitive Micellar Nanoobjects via photoRAFT PISA:

The Full Guide to Poly(N-Acryloylpyrrolidin)-block-Polystyrene Diblock Copolymers

Felix Lauterbach1, Volker Abetz1,2,*

Supporting Information

Abstract

Macromolecular aggregates, so-called latexes, consisting of polystyrene (PS) resemble a relevant class of synthetic polymers used for a plethora of applications ranging from coatings or lubricants to biomedical applications. Their synthesis is usually tailored to the respective application where emulsifiers, radical initiators, or other additives still play a major role in achieving the desired properties. Herein, we demonstrate an alternative based on the photoiniferter reversible addition–fragmentation chain transfer (RAFT) polymerization-induced self-assembly (PISA) of Poly(N-acryloylpyrrolidin)-block-polystyrene (PAPy-b-PS). This approach yields monodisperse nanospheres with tunable sizes based on an aqueous formulation with only two ingredients. These nanospheres are additionally thermosensitive, meaning that they change their hydrodynamic diameter linearly with the temperature in a broad range between 10 °C and 70 °C. Combined with the eco-friendly synthesis in pure water at 40 °C, the herein presented route constitutes an unprecedented pathway to thermosensitive diblock copolymer aggregates in short reaction times without any additives.

1Institute of Physical Chemistry, Universität Hamburg, Grindelallee 117, 20146 Hamburg

(Germany).2Institute of Polymer Research, Helmholtz-Zentrum Geesthacht, Max-Planck-Str. 1,

21502 Geesthacht (Germany)

Electronic Supplementary Material (ESI) for Soft Matter.This journal is © The Royal Society of Chemistry 2020

Experimental Setup

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1 Experimental Setup

1.1 Millireactor Design

Reactions in continuous flow to produce the PAPy homopolymers were performed in a FLOWLAB™ platform (UNIQSIS LTD.) fitted with a glass chip (LITTLE THINGS FACTORY GMBH, internal volume: 2 mL) containing a static micromixer. The system was maintained at 20 bar of pressure for all experiments by means of a backpressure regulator. The reaction mixture is introduced into the reactor via an isocratic pump (KNAUER WISSENSCHAFTLICHE GERÄTE GMBH) through PFA 1/16” tubing and Tefzel fittings with an additional 10 bar backpressure regulator to minimize backflow and to maintain the reactor under pressure. The pump can deliver the reaction solution at flowrates between 0.01 and 10 mL∙min−1. The reactor was mounted on a heating plate and the temperature controlled externally with a thermal imager (BOSCH®). The glass chip of the reactor was illuminated with UV light via a UV-LED (OMNICURE® AC450, spectral emission 350 – 450 nm, maximum at 384 nm) at a distance of 5 cm giving a light intensity of 30 mW∙cm−2 at the chip surface.

Figure S1: Millireactor setup as used for all PAPy homopolymerizations. . Stock solution flask connected to the pump via PFA tubing. 2. Knauer 20P isocratic pump. 3. 2 mL glass chip mounted on a heating plate for temperature regulation. The glass chip is additionally depicted on the right side to show the static mixing unit. The UV lamp is the same as for the batch reactions (see Figure S1).

Experimental Setup

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1.2 Batch Reaction Design

All batch reactions were performed in rubber-sealed screw-capped vials (LABSOLUTE®, 12 mL) placed in a water bath at the desired temperature and typically stirred at 600 rpm. The distance between the sample and the light source was always set to 5 cm and the UV light source (OMNICURE® AC450, spectral emission 350 – 450 nm, maximum at 384 nm) had a power output at the sample surface of 30 mW∙cm−2.

Figure S2: Pictures of the UV-LED used for all polymerizations with self-made beam-widening to increase the illuminated area to 10 ∙ 10 cm. The attachment itself is 15 cm long. The distance between the sample and the front end of the beam-widening was always set to 5 cm.

Experimental Setup

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1.3 UV-Emission and CTA extinction

Figure S3: Top: Main equilibrium of the photoiniferter RAFT mechanism. Bottom: UV-Vis absorption spectrum of TTC2 (blue) and emission spectrum of the UV-LED (grey) used for all experiments. The inset shows an enlarged area of the absorption and emission spectra and the spectral overlap is higlighted in light-grey. The UV-LED excites both the spin-allowed π → π* and the spin-forbidden n → π* electronic transition of the thiocarbonyl moiety.1,2

Exemplary extinction of TTC2 at relevant wavelengths (both other CTAs absorb in the same order of magnitude due to similar environments of the thiocarbonyl moiety):

ε (308 nm) = 18800 L∙mol−1∙cm−1 (π → π* electronic transition)ε (384 nm) = 26.6 L∙mol−1∙cm−1 (emission maximum UV-LED)ε (451 nm) = 58.6 L∙mol−1∙cm−1 (n → π* electronic transition)

General NMR Analysis

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2 General NMR Analysis

Figure S4: 1H-NMR spectrum of N-Acryloylpyrrolidine (APy) in CDCl3. All protons are assigned to the respective signals.


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Figure S5: 1H-NMR spectrum of 4-Cyano-4-(butylsulfanylthiocarbonyl)sulfanylpentanoic acid (TTC1) in CDCl3 after purification by silica gel column chromatograhpy. All protons are assigned to the respective signals.


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Figure S6: Complete spectrum and enlarged area of a 1H-NMR spectrum in D2O of a typical reaction mixture for APy polymerization. N,N-Dimethylformamide (DMF) is added as an internal reference to calculate the monomer conversion. Note that the signals of the vinylic protons appear at a slightly different chemical shift than in CDCl3.

The monomer conversion p of APy after normalization to the DMF signal integral is calculated as follows (IA/B is the respective signal integral either before or after the polymerization):

(1)𝑝 (𝐴𝑃𝑦) = 1 ‒

𝐼𝐴,𝑒𝑛𝑑 + 𝐼𝐴',𝑒𝑛𝑑+ 𝐼𝐵,𝑒𝑛𝑑

𝐼𝐴,𝑖𝑛𝑖 + 𝐼𝐴',𝑖𝑛𝑖+ 𝐼𝐵,𝑖𝑛𝑖


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Figure S7: 1H-NMR spectrum (in tetrahydrofuran-d8) of a sample during polymerization of styrene to calculate the monomer conversion. All relevant protons of styrene are assigned to the respective signals: The monomer signals are marked in blue and the polymer signals are marked in green.

The monomer conversion of styrene after normalization to the protons E and E’ is calculated as follows:

(2)𝑝(𝑠𝑡𝑦𝑟𝑒𝑛𝑒) =

𝐼𝑃𝑜𝑙𝑦𝑚𝑒𝑟𝐼𝑃𝑜𝑙𝑦𝑚𝑒𝑟 + 𝐼𝑀𝑜𝑛𝑜𝑚𝑒𝑟

=𝐼𝐴 + 𝐵 + 𝐶 + 𝐷 + 𝐴/𝐶 + 𝐵 ‒ 6 ∙ 𝐼𝐸𝐼𝐴 + 𝐵 + 𝐶 + 𝐷 + 𝐴/𝐶 + 𝐵 ‒ 1 ∙ 𝐼𝐸

The theoretical molecular weights of all polymers are calculated from the following equation:

PAPy Homopolymers

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(3)𝑀𝑛, 𝑡ℎ𝑒𝑜𝑟. =

[𝑀𝑜𝑛𝑜𝑚𝑒𝑟][𝐶𝑇𝐴]

𝑝𝑀𝑀𝑜𝑛𝑜𝑚𝑒𝑟 + 𝑀𝐶𝑇𝐴

3 PAPy Homopolymers

3.1 Homopolymerization Kinetics of N-Acryloylpyrrolidine

Figure S8: Evolution of the monomer conversion with time and the respective pseudo first-order kinetics plot (left) of the APy homopolymerization with TTC1. The corresponding SEC traces are and dispersities are also given (right). Experimental conditions: [APy]:[TTC1] = 158:1, c = 15 % (w/w), water:1,4-dioxane = 40:60 (v/v), I = 30 mW∙cm−2, T = 70 °C.

Figure S9: Evolution of the monomer conversion with time and the respective pseudo first-order kinetics plot (left) of the APy homopolymerization with TTC2. The corresponding SEC traces and dispersities are also given (right). Experimental conditions: [APy]:[TTC1] = 156:1, c = 15 % (w/w), water:1,4-dioxane = 40:60 (v/v), I = 30 mW∙cm−2, T = 70 °C.

PAPy Homopolymers

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3.2 PAPy Homopolymers Used for Emulsions

Figure S10: SEC traces of all PAPy homopolymers that were used in this study. The three different trithiocarbonate CTAs that were used are assigned to the respective trace by color. Additionally, theoretical, number averaged molecular weights are given besides the dispersity.

Table S1: Synthesized PAPy homopolymers used for subsequent emulsion polymerization with styrene. The number averaged molecular weight as well as dispersity (as measured by SEC in DMAc with PMMA calibration, see method section in the main paper) are given besides the calculated theoretical number averaged molecular weight. The reaction time and monomer conversion reached at the end of the polymerization are depicted as well.

Code Time / minConversion

/ %Mn,SEC /

kDaMn,theor. /

kDa Đ

PAPy13.8-TTC1

40 69.3 8.7 14.1 1.36

PAPy3.1-TTC2 20 50.0 1.1 3.5 1.39PAPy8.2-TTC2 20 70.8 4.5 8.6 1.33

PAPy23.6-TTC2

20 58.6 13.4 23.9 1.51

PAPy9.7-TTC3 20 65.5 7.6 10.0 1.29

It is worth mentioning that the dispersities of the homopolymer appear quite high at first sight. However, our objective was to synthesize the homopolymer in a minimum amount of time. Therefore, we performed the photoRAFT polymerization in a continuous flow reactor since it reduces the reaction time for photoinduced polymerizations significantly.3 The short reaction time at elevated temperatures reduces the risk of losing chain end livingness which is crucial for the diblock copolymerization. Albeit the advantages, such flow reactors sometimes increase the dispersity due to a broadening of residence times. Thus, the admittedly higher dispersities in the table above are attributed to flow effects rather than termination reactions. That the chain end livingness is still given to almost full extent, is proven in Chapter 4.

PAPy Homopolymers

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3.3 PAPy Aggregation in Aqueous Solution

Figure S11: Optical cloud points of all PAPy homopolymers estimated by visual turbidimetry. The cloud points were taken from the onset of the beginning turbidity during heating of the polymer solution. All depicted cloud points are the average of three heating–cooling cycles.

In the dynamic light scattering measurements, the intensity correlation function (g2(τ)) was fitted with the so-called cumulant method using the following general formula:

(4)𝑔2(𝜏) = 𝐵 + 𝛽( 𝑁∑

𝑖 = 1

𝑓𝑖exp ( ‒ Γ̅𝑖𝜏)(1 + 𝜇2,𝑖2! 𝜏2 ‒ 𝜇3,𝑖3! 𝜏3 + …))2In Equation 4, B is the baseline, β is the stretching factor (meaning the maximum of the correlation function), N is the number of size distributions and therefore decays in the curve, and fi represents the relative fraction of the respective size distribution, while Γi, μ2, and μ3 are the respective cumulants. The hydrodynamic radius RH of each species is calculated from the first order cumulant via the Stokes-Einstein equation

(5)𝑅𝐻,𝑖 =

𝑘𝑇𝑞2

6𝜋𝜂Γ̅𝑖

with k being the Boltzmann constant, T the temperature, q the scattering vector, and η the viscosity. The dispersity of the respective size distribution is given by Equation 6:

(6)𝑃𝐷𝐼 =

𝜇2,𝑖

Γ̅𝑖2

PAPy Homopolymers

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Figure S12: Left: Temperature-dependent evolution of the hydrodynamic radii from the PAPy-TTC1 homopolymer in aqueous solution (c = 1 % (w/w)). The red dots depict the radii during heating of the polymer solution while the blue dots represent the respective radii during the cooling step. In all homopolymer samples the most suitable fit indicated two distinct aggregate species; both are shown in the graphs with different marker sizes, while the area of the marker represents the relative intensity weighted fraction of the particular species. The optical cloud point observed via turbidimetry is indicated at the transition from white to grey background. Right: Size distributions of hydrodynamic radii for selected temperatures during the temperature-dependent DLS analysis of the PAPy-TTC1 homopolymer.

PAPy Homopolymers

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Figure S13: Temperature-dependent evolution of the hydrodynamic radii from all PAPy-TTC2 homopolymers in aqueous solution (c = 1 % (w/w)) obtained by DLS. The red dots depict the radii during heating of the polymer solution while the blue dots represent the respective radii during the cooling step. In all homopolymer samples the most suitable fit indicated two distinct aggregate species; both are shown in the graphs with different marker sizes, while the area of the marker represents the relative intensity weighted fraction of the particular species. The optical cloud point observed via turbidimetry is indicated at the transition from white to grey background.

PAPy Homopolymers

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Figure S14: Size distributions of hydrodynamic radii for selected temperatures during the temperature-dependent DLS analysis of all PAPy-TTC2 homopolymers.

PAPy Homopolymers

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Figure S15: Top Left: Temperature-dependent evolution of the hydrodynamic radii from the PAPy-TTC3 homopolymer in aqueous solution (c = 1 % (w/w)). The red dots depict the radii during heating of the polymer solution while the blue dots represent the respective radii during the cooling step. In all homopolymer samples the most suitable fit indicated two distinct aggregate species; both are shown in the graphs with different marker sizes, while the area of the marker represents the relative intensity weighted fraction of the particular species. The optical cloud point observed via turbidimetry is indicated at the transition from white to grey background. Top Right: Size distributions of hydrodynamic radii for selected temperatures during the temperature-dependent DLS analysis of the PAPy-TTC3 homopolymer.

Figure S16: Temperature-dependent evolution of the hydrodynamic radii from the PAPy9.7-TTC3 homopolymer in aqueous solution (c = 1 % (w/w)) during three heating–cooling cycles. The red dots depict the radii during heating of the polymer solution while the blue dots represent the respective radii during the cooling step. In this sample the most suitable fit indicated two distinct aggregate species; both are shown in the graphs with different marker sizes, while the area of the marker represents the relative intensity weighted fraction of the particular species. The optical cloud point observed via turbidimetry is indicated at the transition from white to grey background.

PAPy-b-PS Diblock Copolymer Emulsion Polymerizations

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4 PAPy-b-PS Diblock Copolymer Emulsion Polymerizations

4.1 PAPy-TTC1 Emulsion Polymerization Kinetics

Figure S17: Top Left: Pseudo first-order kinetics plot of the styrene emulsion polymerizations with the PAPy13.8-TTC1 macroCTA at a total solids concentration of 10 % (w/w), 15 % (w/w), and 20 % (w/w). Remaining: SEC traces for the samples taken from this kinetic studies. The first SEC trace is the macroCTA itself, the remaining traces represent the diblock copolymer samples. The dispersities and overall theoretical molecular weights are given as well.


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Table S2: Synthesized PAPy-b-PS diblock copolymers with TTC1 endgroup. The number averaged molecular weight as well as dispersity (as measured by SEC in DMAc with PS calibration, see method section in the main paper) are given besides the calculated theoretical number averaged molecular weight. The reaction time and monomer conversion are depicted as well.


/ %Mn,SEC /

kDaMn,theor. /

kDa Đ

PAPy110-b-PS9524.0-TTC1

120 16.8 21.0 24.0 1.69


150 42.5 43.2 39.2 1.47


180 59.4 49.2 49.2 1.13


210 74.1 57.8 57.9 1.14


240 84.9 72.8 64.3 1.20


270 90.1 73.2 67.4 1.27


300 92.7 73.8 68.9 1.29


90 13.3 19.6 21.7 1.56


120 33.7 34.3 33.3 1.61


150 48.6 47.1 41.7 1.11


180 58.5 54.8 47.4 1.12


210 68.8 64.1 53.2 1.12


240 77.9 71.2 58.4 1.13


270 85.9 84.6 62.9 1.21


300 90.8 85.8 65.7 1.25


450 77.0 106.3 113.7 1.24


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Figure S18: Left: Pseudo first-order kinetics plot of the styrene emulsion polymerization with the PAPy3.1-TTC2 macroCTA at a total solids concentration of 20 % (w/w). Right: SEC traces for the samples taken from this kinetic study. The first SEC trace is the macroCTA itself, the remaining traces represent the diblock copolymer samples. The dispersities and overall theoretical molecular weights are given as well.

Figure S19: Top Left: Pseudo first-order kinetics plot of the styrene emulsion polymerizations with the PAPy8.2-TTC2 macroCTA at a total solids concentration of 10 % (w/w) and 15 % (w/w). Remaining: SEC traces for the samples taken from this kinetic studies. The first SEC trace is the macroCTA itself, the remaining traces represent the diblock copolymer samples. The dispersities and overall theoretical molecular weights are given as well.


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Figure S20: Top Left: Pseudo first-order kinetics plot of the styrene emulsion polymerizations with the PAPy23.6-TTC2 macroCTA at a total solids concentration of 10 % (w/w) and 15 % (w/w). Remaining: SEC traces for the samples taken from this kinetic studies. The first SEC trace is the macroCTA itself, the remaining traces represent the diblock copolymer samples. The dispersities and overall theoretical molecular weights are given as well.



/ %Mn,SEC /

kDaMn,theor. /

kDa Đ


420 12.6 7.0 7.7 1.16


540 42.7 25.9 17.8 1.16


330 19.6 12.3 15.6 2.13


360 31.1 17.0 19.7 1.83


390 42.4 22.8 23.7 1.20


420 51.4 29.0 26.9 1.17

PAPy66-b- 450 59.7 34.3 29.8 1.17


20


/ %Mn,SEC /

kDaMn,theor. /

kDa Đ

PS20429.8-TTC2PAPy66-b-

PS22131.6-TTC2480 64.8 37.7 31.6 1.16


360 27.5 11.7 18.0 1.74


420 42.5 19.6 23.2 1.58


480 59.2 26.8 28.9 1.17


540 67.8 30.1 31.8 1.16


600 74.7 34.1 34.2 1.15


150 24.7 35.4 47.9 1.75


180 38.7 54.3 61.4 1.57


210 50.2 69.6 72.6 1.50


240 60.4 83.8 82.5 1.20


240 30.6 36.6 53.4 1.73


300 50.9 65.3 72.9 1.50


360 67.2 73.9 88.6 1.22


420 79.6 96.5 100.5 1.21


480 87.1 106.5 107.7 1.28


540 90.0 107.2 110.5 1.32


600 91.5 117.4 111.8 1.37


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Figure S21: Top Left: Pseudo first-order kinetics plot of the styrene emulsion polymerizations with the PAPy9.7-TTC3 macroCTA at a total solids concentration of 15 % (w/w), 20 % (w/w), and 25 % (w/w). Remaining: SEC traces for the samples taken from this kinetic studies. The first SEC trace is the macroCTA itself, the remaining traces represent the diblock copolymer samples. The dispersities and overall theoretical molecular weights are given as well.



/ %Mn,SEC /

kDaMn,theor. /

kDa Đ


120 21.9 20.0 18.9 1.13


190 38.2 26.7 25.4 1.20


240 47.1 29.4 29.0 1.26


300 56.7 30.7 32.9 1.33


360 64.4 34.4 36.0 1.36


22


/ %Mn,SEC /

kDaMn,theor. /

kDa Đ


480 84.0 40.6 43.9 1.45


130 23.4 19.9 19.4 1.14


200 39.5 26.2 25.8 1.16


260 49.4 34.9 29.7 1.21


320 56.9 38.3 32.7 1.27


380 62.7 39.4 35.0 1.31


440 66.7 40.7 36.6 1.31


170 42.8 29.1 27.2 1.13


240 61.9 37.4 34.8 1.20


300 74.0 42.3 39.7 1.23


370 88.4 51.0 45.5 1.35


480 93.0 52.3 47.3 1.37


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4.4 PAPy-b-PS-TTC1 Aggregates in Aqueous Emulsion

4.4.1 TEM Analysis

Figure S22: TEM images and size distributions of PAPy110-b-PS52768.9-TTC1 (top), PAPy110-b-PS52768.9-TTC1 (middle), and PAPy110-b-PS52768.9-TTC1 (bottom). Scale bars are 500 nm (top and middle) and 2 µm (bottom, respectively). The mean radius of every size distribution is indicated on the particular axis.


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Figure S23: TEM image and size distribution of PAPy25-b-PS13717.7-TTC2. The scale bar is 2 µm. The mean radius of the distribution is indicated on the axis.

Figure S24: TEM images and size distributions of PAPy66-b-PS22131.6-TTC2 (top) and PAPy66-b-PS30740.6-TTC2 (bottom). All scale bars are 500 nm. The mean radius of every size distribution is indicated on the particular axis.


25

Figure S25: TEM images and size distributions of PAPy188-b-PS61387.8-TTC2 (top) and PAPy188-b-PS865114-TTC2 (bottom). All scale bars are 500 nm. The mean radius of every size distribution is indicated on the particular axis.

Figure S26: TEM image and size distribution of PAPy77-b-PS32543.9-TTC3. The scale bar is 500 nm. The mean radius of the distribution is indicated on the axis.


26

Figure S27: TEM image and size distribution of PAPy77-b-PS25636.6-TTC3. The scale bar is 500 nm. The mean radius of the distribution is indicated on the axis.

Figure S28: TEM images and size distributions of PAPy77-b-PS28539.7-TTC3 (top), PAPy77-b-PS34045.5-TTC3 (middle), and PAPy77-b-PS35847.3-TTC1 (bottom). Scale bars are 1 µm (top), 500 nm (middle), and 2 µm (bottom, respectively). The mean radius of every size distribution is indicated on the particular axis.


27

4.4.2 DLS Analysis

Figure S29: Evolution of the hydrodynamic radii of PAPy110-b-PS49665.7-TTC1 at 0.02 % (w/w) solids concentration with temperature calculated from DLS measurements. The red dots represent the radii during heating of the polymer dispersion and the blue dots represent the respective radii during the cooling step. The most probable fit solution yielded one distribution of particle sizes. The PTT of the used homopolymer is also depicted on the temperature axis.

Figure S30: Evolution of the hydrodynamic radii of PAPy110-b-PS957114-TTC1 at 0.02 % (w/w) solids concentration with temperature calculated from DLS measurements. The red dots represent the radii during heating of the polymer dispersion and the blue dots represent the respective radii during the cooling step. The most probable fit solution yielded one distribution of particle sizes. The PTT of the used homopolymer is also depicted on the temperature axis.


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29




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4.4.3 DLS-TEM Comparison

Figure S35: Evolution of the PDI of all polymers made from the PAPy9.7-TTC3 macroCTA at 15 % (w/w), 20 % (w/w), and 25 % (w/w) solids concentration compared to the degree of polymerization (DP) of PS. The PDI measured by TEM are depicted as circles and the PDI calculated from DLS measurements are depicted as squares.

References

31

References

1. Coyle, J. D. The photochemistry of thiocarbonyl compounds. Tetrahedron 41, 5393–5425 (1985).

2. Lu, L., Zhang, H., Yang, N. & Cai, Y. Toward Rapid and Well-Controlled Ambient Temperature RAFT Polymerization under UV-Vis Radiation : Effect of Radiation Wave Range. Macromolecules 39, 3770–3776 (2006).

3. Lauterbach, F., Rubens, M., Abetz, V. & Junkers, T. Ultrafast PhotoRAFT Block Copolymerization of Isoprene and Styrene Facilitated through Continuous-Flow Operation. Angew. Chemie - Int. Ed. 57, 14260–14264 (2018).

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