Styrene - Matyjaszewski Polymer Group - Carnegie Mellon University

ATRP of Styrene

Preparation of Low Molecular Weight Monofunctional Polystyrene Macroinitiators:

ATRP of Styrene Initiated by Alkyl Dithiocarbamate

Preparation of a Di-functional Polystyrene Macroinitiator

AGET ATRP of Styrene:

General Procedure for ARGET ATRP of Styrene with 50 ppm of Cu(II)

Styrene Polymerization in Aqueous Dispersed Media

Substituted Styrene's

Preparation of Low Molecular Weight Monofunctional Polystyrene Macroinitiators:

MW 1100 and 2000.

0.3146g CuBr (2.2 x 10-3 mole) was added to a round bottom flask.  The flask was sealed with a rubber septum, degassed and back-filled with N2 3 times.  50 ml of deoxygenated styrene (4.4 x 10-2 mole) was added via syringe, followed by 1 ml deoxygenated anisole as an internal standard for GC analysis of convertion of monomer.  0.46 ml of deoxygenated PMDETA (2.2 x 10-3 mole) was added via syringe.  The solution turned light green as the CuBr/PMDETA complex formed. The reaction medium remained heterogeneous.  After the majority of the metal complex had formed, 3 ml of 1-phenylethyl bromide (2.2 x 10-3 mole) was added.  A sample was removed to measure the initial monomer to internal standard ratio and compare that against the final ratio to determine total monomer conversion.  The flask was placed in an oil bath thermostated at the desired temperature and the polymerization was allowed to proceed for a given amount of time.  After the flask was removed from the oil bath, a sample was dissolved in tetrahydrofuran, monomer conversion was determined and molecular weight analysis was performed.

Macroinitiator Purification:

The contents of the flask were dissolved in acetone, slurried with DOWEX MSC macroporous ion-exchange resin for up to one hour, then filtered through alumina.  Both the resin and alumina served to remove the copper catalyst from the polymer.  The acetone was removed by evaporation and the residual polymer was redissolved in diethyl ether, and then precipitated by addition into MeOH. The dissolution/precipitation procedure was repeated 1 or 2 more times, or until the polymer precipitated as a powder instead of a sticky liquid (this is a function of the amount of monomer, solvent, or internal standard remaining).  It was then dried under vacuum and analyzed by 1H NMR to determine degree of polymerization (for lower molecular weights).

Summary of Reaction Conditions: 

Ratio of reagents, media, reaction temperature, time, Mn,th, Mn,exp, and Mw/Mn (PDI) of polymer.

1)  [St]:[1-PEBr]:[CuBr]:[PMDETA] = 10:1:0.1:0.1; bulk, 100°C, 260 min

            Mn,th = 1070; Mn,exp = 1,300; Mw/Mn = 1.23.

2)  [St]:[1-PEBr]:[CuBr]:[PMDETA] = 20:1:0.1:0.1, bulk, 100°C, 240 min

            Mn,th = 1100; Mn,exp = 1,100; Mw/Mn = 1.15.

3)  [St]:[1-PEBr]:[CuBr]:[PMDETA] = 20:1:0.2:0.2; bulk, 100°C, 480 min

            Mn,th = 2030; Mn,exp = 2,800; Mw/Mn = 1.15.

Low MW Polystyrene, MW 4500:

Conditions similar to those detailed above.

[St]:[MBrP]:[CuBr]:[PMDETA] = 78:1:1:1 in anisole at 90 ºC

Conv. 55.5%, Mn,th = 4,500; Mn,exp = 4,420; Mw/Mn = 1.10

Low MW Polystyrene, MW 6400:

CuBr was degassed in a Schlenk flask by three vacuum/nitrogen-inletting cycles. Then the pre-deoxygenated monomer (styrene) and ligand (PMDETA) were injected into the flask. After stirring for 20 minutes at room temperature, to form the catalyst complex, the flask was placed in an oil bath set at a temperature of 80°C. The initiator, methyl 2-bromopropionate was injected into the flask to start the reaction. Samples were removed from the flask by degassed syringes, at timed intervals, to analyze conversion. When the conversion reached around 50%, the reaction was stopped by cooling the reaction down to room temperature and opening the flask to air. The mixture was passed through a neutral aluminum oxide column to remove the oxidized catalyst. The polymer was purified by precipitation into methanol. After drying under vacuum, the macroinitiator was obtained as a white powder.

[St]:[MBrP]:[CuBr]:[PMDETA] = 100:1:1:1 and T = 80°C.

Mn 6,390; Mw/Mn = 1.18

ATRP of Styrene Initiated by Alkyl Dithiocarbamate

(Kwak, Y.; Matyjaszewski, K. Macromolecules 2008, 41, 6627-6635.)

a)  Low MW Polystyrene, MW 1900:

[St]:[EMADC]:[CuBr]:[PMDETA] = 100:1:2:6, bulk at 120 ºC

CuBr (275 mg) was added to a dried Schlenk flask equipped with a stir bar. After sealing with a rubber septum, the flask was degassed and backfilled with N2 five times and then left under N2. Subsequently, St (10.0 g), PMDETA (998 mg), and 2-(N,N-diethyldithiocarbamyl)-isobutyric acid ethyl ester (EMADC) (253 mg) were added to a glass vial and degassed by three freeze-pump-thaw cycles. The solution was then transferred to the Schlenk flask, which was placed in a thermostated oil bath at 120 oC. The polymerization was stopped after a 45 min reaction by opening the flask and exposing the catalyst to air. The mixture was diluted with 20 mL dichloromethane and passed through a neutral alumina column. The solution was concentrated by rotary evaporation and the polymer was precipitated by addition of the purified solution to a large amount of cold methanol. Dissolution and precipitation was repeated until a white powder was obtained. The precipitated polymer was dried in a vacuum oven at 40 oC until a constant weight was reached and analyzed by GPC.

Conv. 20.5%, Mn,th = 2,140; Mn,exp = 1,900; Mw/Mn = 1.06

b) Higher MW Polystyrene, MW 3900 and 9000:

CuBr (41.3 mg, 0.29 mmol) was added to a dried Schlenk flask equipped with a stir bar. After sealing with a rubber septum, the flask was degassed and backfilled with nitrogen (N2) five times and then left under N2. Subsequently, a mixture of St (3.00 g, 28.9 mmol), initiator, and ligand was added to a glass vial and degassed by three freeze-pump-thaw cycles. It was then transferred to the Schlenk flask, which was placed in a thermostated oil bath at the desired temperature. Samples were taken periodically under N2 using an N2-purged syringe, diluted by THF to a known concentration, passed through a column filled with neutral alumina to remove the copper complex, and analyzed by GPC.

St/EMADC/CuBr/PMDETA = 100/1/1/1, bulk, 120 oC, 1h

            Conv. = 38.2%; Mn,th = 3,980; Mn,exp = 3,040; Mw/Mn = 1.08.

St/EMADC/CuBr/PMDETA = 100/1/1/1, bulk, 120 oC, 6h

            Conv. = 90.0%; Mn,th = 9,370; Mn,exp = 8,660; Mw/Mn = 1.08.

Preparation of a Di-functional Polystyrene Macroinitiator:

Ratio [St]:[ DMDBHD]:[CuBr]:[PMDETA] = 300:1:0.5:0.5

Styrene was purified by passing through a basic alumina column and then bubbled with N2 for 30 minutes. CuBr was charged in a flask and after 30 min under nitrogen atmosphere, styrene, PMDETA and 0.5 mL of anisole were added. The solution turned light green as the catalyst complex formation occurred. A sample was removed to measure the initial monomer/internal standard ratio used to determine the conversion as the reaction progressed. Dimethyl 2,6-dibromoheptanedioate (DMDBHD) was then added and the flask was then placed in an oil bath thermostated at 100°C for 11 hours 10 minutes. The flask was removed from the oil bath and the solution was diluted in tetrahydrofuran and purified by passing through a neutral alumina column. The solvent and monomer were then removed under vacuum at 45°C.

Conv. = 58.2%; Mn = 18,145 g/mol; Mw/Mn = 1.08.

AGET ATRP of Styrene:

[St]:[EBiB]:[CuCl2]:[dNbpy]:[Sn(EH)2] = 216:1:1:0.45 (i.e. less than one equivalent of reducing agent in order to leave some Cu(II) present at all times.)

Styrene (5.0 ml, 44 mmol), CuCl2 (29.3 mg, 21.8´10-2 mmol) and dNbpy (178 mg, 43.6´10-2 mmol) were placed in a 25 mL Schlenk flask and bubbled with nitrogen for 15 min. Sn(EH)2 (32 ml, 9.8´10-2 mmol), and a purged solution of EBiB (29.7 ml, 20.3´10-2 mmol) in toluene were added, and the sealed flask was placed in thermostated oil bath at 110 °C. The polymerization was stopped after 7 hours by opening the flask and exposing the catalyst to air.

Conv. = 83%; Mn = 14,000; Mw/Mn= 1.37.

General Procedure for ARGET ATRP of Styrene with 50 ppm of Cu(II)

(Jakubowski, W.;  Min, K.; Matyjaszewski, K. Macromolecules 2006, 39, 39-45.)

[St]:[EBiB]:[CuCl2]:[Me6TREN]:[Sn(EH)2]:[Me6TREN] = 216:1:0.001:0.01:0.01 in anisole. 

Degassed styrene (5.0 ml, 44 mmol) and anisole (1.5 ml) were transferred via degassed syringes to a dry, thoroughly purged by flushing with nitrogen, Schlenk flask. Next, a solution of CuCl2 (0.29 mg, 0.22´10-2 mmol)/Me6TREN (0.57 ml, 0.22´10-2 mmol) complex in degassed anisole (0.5 ml) was added via a syringe. The mixture was stirred for 10 minutes and then a purged solution of Sn(EH)2 (7.0 ml, 2.2´10-2 mmol) and Me6TREN (5.7 ml, 2.2´10-2 mmol) in anisole (0.5 ml) was added. Finally the initiator EBiB (32.1 ml, 21.9´10-2 mmol) was added. An initial sample was taken and the sealed flask was placed in thermostated oil bath at 110 °C.  Samples were taken at timed intervals and analyzed by gas chromatography and gel permeation chromatography to follow the kinetic of the polymerization which was stopped after 7.6 hr by opening the flask and exposing the catalyst to air.

Conv. = 59%; Mn = 12,700; Mw/Mn = 1.11.

When the polymerization was run with lower amounts of ligand the polymerization was stopped after 20 hr by opening the flask and exposing the catalyst to air, conversion was higher.

[St]:[EBiB]:[CuCl2]:[Me6TREN]:[Sn(EH)2]:[Me6TREN] = 216:1:0.001:0.01:0.03 in anisole.  Conv. = 76%; Mn = 15,900; Mw/Mn = 1.28.

Styrene Polymerization in Aqueous Dispersed Media

Early work in the Matyjaszewski group established that ATRP emulsion/miniemulsion systems using non-ionic surfactants (i.e. Brij 98) and bipyridine ligands with long alkyl substituents (i.e. dNbpy) was feasible.(1,2)   However, a large amount of surfactant (13.5 wt% based on monomer) was required to obtain stable latexes with relatively low solids content (~ 13 %).   

Several improvements had to be made to make this procedure environmentally acceptable.  These included: starting with a stable catalyst complex, reducing the concentration of catalyst, removing/recycling the catalyst, bringing the ratio of surfactant to monomer closer to industrially acceptable ratios, increasing % solids, and finally developing a "true" emulsion polymerization procedure.(3)

1)  Gaynor, S. G.;  Qiu, J.; Matyjaszewski, K. Macromolecules 1998, 31, 5951-5954.

2)  Gaynor, S. G.;  Qiu, J.;  Shipp, D.; Matyjaszewski, K. Polym. Mater. Sci. Eng. 1999, 80, 536-537.

3) Min, K.;  Gao, H.; Matyjaszewski, K. Journal of the American Chemical Society 2006, 128, 10521-10526.

The following examples describe the critical steps.

Miniemulsion Polymerization

Typical recipes for miniemulsion polymerizations employing different procedures for catalyst activation are listed below.

a) Reverse ATRP in Miniemulsion:

Monomer : Surfactant : Solid content : Conversion,  Mn,exp  Mn,th  Mw/Mn

Styrene : Brij 98 : 20% : 78.4%,  31,200,  32,600, 1.49

[M]0 : [tNtpy] : [CuBr2] : [VA-044] = 400 : 1 : 1 : 1;

[Surfactant] = 5 mM (0.58 wt % based on water); * [Brij 98] = 7.5 mM;

[Hexadecane] = 3.6 wt % based on monomer;

20 % solid content; reaction temperature = 70 oC.

b) Typical Recipe for SN&RI ATRP in a Miniemulsion Systema

Monomer

BA

5.0 g

200 equiv.

Alkyl halide

MBrP

0.0326 g

1 equiv.

Ligand

tNtpy

0.0239 g

0.2 equiv.

Catalyst

CuBr2

0.0087 g

0.2 equiv.

Costabilizer

Hexadecane

0.18 g


Surfactant

Brij 98

0.115 g


Deionized water

H2O

19.88 g


Initiator

AIBN

0.004 g

0.125 equiv.

a [Brij 98] : [hexadecane] = 2.3 : 3.6 wt% based on monomer;

solid content = 20 % (based on 100% conversion); 80 oC.

Detailed Procedure: 

The ATRP deactivator (CuBr2 and ligand), monomer, and costablizer (hexadecane) were charged to a round-bottom flask, and heated with magnetic stirring at 60 oC for 10 to 20 minutes to form a homogeneous solution.  The surfactant solution was added after cooling the solution down to room temperature, and the mixture was ultrasonicated (Heat Systems Ultrasonics W-385 sonicator; output control set at 8 and duty cycle at 70% for 1 minute) in an ice bath to prevent a significant temperature rise resulting from sonification. The resulting miniemulsion exhibited good shelf life stability at room temperature, as evidence by a lack of visible creaming or phase separation over 3 days of aging. After homogenization, the miniemulsion was then transferred to a 25 ml Schlenk flask, where pure argon was bubbled through the miniemulsion for 30 minutes before it was immersed in an oil bath thermostated at 80 oC. The speed of the magnetic stirrer was set at 700 rpm. When a water-soluble azo initiator, (e.g. VA-044), was used the polymerization was initiated by the injection of a pre-deoxygenated aqueous solution of the initiator into the miniemulsion. When a water-insoluble azo initiator (e.g. AIBN) was employed it was pre-dissolved in the oil phase at room temperature before sonification. Time zero for the polymerization was marked as the moment when the Schlenk flask was immersed in the oil bath. Samples were withdrawn periodically via a pre-degassed syringe to monitor the monomer conversion and molecular weight.

Miniemulsion Polymerization of Styrene with a Macroinitiator PBA-(b-PS)3

(Gao, H.;  Min, K.; Matyjaszewski, K. Macromolecular Chemistry and Physics 2006, 207, 1709-1717.)

This example used a trifunctional PBA-Br3 macroinitiator, prepared via bulk ATRP, in a miniemulsion system where the initiation or activation of the catalyst complex was conducted by means of SR&NI.

[St]/[PBA-Br3]/[CuIIBr2-tNtpy]/[VA-044] = 300/1/0.6/0.375

(the reactant ratio would be 100/1/0.2/0.125 for each arm); at 80 oC.

Miniemulsion system: [Brij 98]/[Hexadecane] = 2.3/3.6 wt % based on monomer;

20 % solids, based on 100% conversion.

Conversion was 93% therefore solids = 18%, Mw/Mn =1.37.

The final polymer had ~12% homopolymer present in the product.

(Indeed this observation was the motivation for the development of AGET ATRP)

c) AGET Miniemulsion Polymerization of Styrene from a Three-arm PBA Star Macroinitiator PBA-(b-PS)3

Polymerization conditions for the preparation of a pure star block copolymer.

[Styrene]0:[(PMA-Br)3]0:[CuBr2/BPMODA]0:[Ascorbic Acid]0 = 400: 1: 0.6: 0.24; 80 oC.  

Miniemulsion conditions: [Brij 98] = 0.58 wt % with respect to water (2.3 wt % with respect to the oil phase); [Hexadecane] = 3.6 wt % with respect to monomer.

The copolymerization was faster in miniemulsion than in bulk, which indicated a gradual diffusion of a fraction of the CuII complex out of the monomer droplets into the aqueous phase.  Therefore at high conversion star-star coupling reactions, within each droplet, were difficult to avoid especially for styrene polymerization.  The contribution of coupling reaction increases with conversion but could be reduced by stopping the polymerization at a limited conversion ~50%.

Substituted Styrene's

An introduction to the polymerization of substituted styrene's can be found in a paper authored by Jain Qiu.(Qiu, J.; Matyjaszewski, K. Macromolecules 1997, 30, 5643-5648.) A series of substituted styrenic monomers were studied: 4-CN, 4-CF3, 3-CF3, 4-Br, 4-Cl, 4-F, 4-H, 3-Me, 4-Me, 4-CMe3, and 4-OMe. 

The ATRP of the substituted styrenes were conducted in diphenyl ether at 110 °C. [M]0 = 4.37 M and [M]0:[1-PEBr]0: [CuBr]0:[bipy]0  = 100:1:1:3.  Monomers with electron-withdrawing (EW) substituents resulted in better control over the polymerization and faster polymerization than the polymerization of monomers with electron-donating (ED) substituents. The apparent polymerization rate constants followed the Hammett equation with ρ = 1.5. The differences between the polymerization rates for the different monomers can be attributed to differences in both the propagation rate constant, kp, and the equilibrium constant, Keq, for atom transfer.  Monomers with EW substituents have larger kp and Keq values than those with ED substituents; therefore, EW substituents increase the monomer reactivity and decrease the stability of dormant species, while ED substituents have the opposite effect.

The general procedure for the polymerizations was similar to that described above for the polymerization of styrene.  CuBr and ligand (bpy or dNbpy were employed since the examples were conducted early in the development of ATRP) were added to a round bottom flask followed by the degassed solvent and monomer then the addition of the initiator. The flask was then immersed in an oil bath thermostated at 110 °C.  At timed intervals, the same amount of sample was withdrawn from the flask and dissolved in THF for further analysis. The polydispersity for polymers with EW substituents had PDI's below 1.2 while polymerization of monomers with ED substituents resulted in formation of polymers with broader PDI's, but still  >1.5, except for 4-OMe styrene which did not yield high MW polymer.

Sodium Styrene Sulfonate (NaSS)

a) Standard Conditions: 

The initial example of polymerization of NaSS was conducted with H2O as solvent using a CuI-based catalyst complex and conducting the reaction at 30oC.  After 15 min conversion was 30% and Mn was 6,620 with Mw/Mn = 1.81, and after 150 min conversion was 32% and Mn was 6,800 with Mw/Mn = 2.06.  Therefore in this case of NaSS polymerization, catalyst disproportionation presumably took place; the solutions turned blue several minutes after initiation, although precipitation of Cu0 particles was difficult to observe. Conversions reached 20-30 % in less than 20 minutes and after this point the reactions did not proceed further.

However, once the role of a "pseudo ligand" was understood addition of pyridine (PyH ) to the reaction improved control.

Conducting the reaction in H2O-PyH (1:1), CuI-based catalyst, at 30oC after 15 min conversion was 38% and Mn was 8,100 with Mw/Mn = 1.20 and after 150 min conversion was 70% and Mn was 10,800 with Mw/Mn = 1.26.

 b) Improved Polymerization Conditions:

The monomer (5 mmol) was dissolved in 3 ml of solvent (D2O or mixtures of D2O and Pyridine) and the solution was degassed by five freeze-pump-thaw cycles. The reaction flask was filled with nitrogen and the catalyst (mixture of copper (I) and (II) bromides (0.05 mmol of total copper) and 0.0156 g (0.1 mmol) of 2,2'-bipyridine (bpy)) was added to the still frozen solution. The flask was closed and evacuated and back-filled with nitrogen three times. A homogeneous brown solution was obtained after the flask was immersed in a water bath thermostatted at 25oC or 30oC then the initiator, 2-bromopropionate for the ATRP of NaSS, was added.