reducing copper concentration in polymers prepared via atom transfer radical polymerization

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180

Reducing Copper Concentration in PolymersPrepared via Atom Transfer RadicalPolymerization

Laura Mueller, Krzysztof Matyjaszewski*

Three different initiation systems for atom transfer radical polymerization (ATRP): normalATRP, activators regenerated by electron transfer (ARGET) ATRP, and initiators for continuousactivator regeneration (ICAR) ATRP were used to synthesize polystyrene with a number-average molecular weight �20 000 g �mol�1. Each polymerization mixture was divided andsubjected to one or more purification methods includ-ing passing through a column filled with neutralalumina, stirring with an ion exchange resin (ATRPPure), or precipitation into hexanes or methanol.Inductively coupled mass spectrometry was used toanalyze the concentration of copper in each sample.For the polystyrene prepared by normal ATRP, purifi-cation by a neutral alumina column was the mosteffective at removing copper (down to �2 parts permillion by mass). For the ARGET and ICAR ATRP, puri-fication by a neutral alumina column and stirring with10wt.-% ATRP Pure gave comparable results (each �1part per million by mass).

Introduction

Since its discovery more than a decade ago, copper-

mediated atom transfer radical polymerization (ATRP)

has become one of the most widely known synthetic tools

in polymer chemistry.[1–3] Similar to other controlled

radical polymerization techniques, ATRP allows the synth-

esis of polymers with controllable molecular weights,

narrow molecular weight distributions, and a wide range of

functionalities, compositions and topologies.[4–10]

L. Mueller, K. MatyjaszewskiDepartment of Chemistry, Center for MacromolecularEngineering, Carnegie Mellon University, 4400 Fifth Avenue,Pittsburgh, Pennsylvania 15213, USAFax: þ412 268 6897; E-mail: [email protected]

Macromol. React. Eng. 2010, 4, 180–185

� 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

The general mechanism of ATRP involves a reversible

redox reaction between an alkyl halide and a low oxidation

state metal complex (activator) to generate an active radical

species and the corresponding high oxidation state metal

complex. The active species continues propagation with

monomer to form polymer until it is deactivated to form a

dormant alkyl halide species by the high oxidation state

metal complex (deactivator). Control in ATRP is established

from the dynamic equilibrium of the redox reaction that

maintains a majority of the polymer chains in the dormant

state, where they are unable to self-terminate. ATRP is

typically conducted with a copper catalyst, although

successful polymerization has been reported with other

metals as well.[11–14]

One disadvantage to normal ATRP is the relatively large

concentration of catalyst which is required.[15] The copper

DOI: 10.1002/mren.200900067

Reducing Copper Concentration in Polymers Prepared via Atom . . .

complexes which are typically used are considered mildly

toxic and are highly colored. Sufficient concentration of

copper complex is necessary to compensate for the

unavoidable radical termination reactions which irrever-

sibly consume the activator. For example, if less than

5 mol-% of copper catalyst is used relative to the alkyl halide

initiator, then it will all be converted to deactivator after

�5% of polymer chains terminate. To overcome this

problem, two new ATRP initiation systems were developed

which allow for the concentration of copper to be reduced

down to parts per million (ppm) levels.[16–19] With these

techniques, known as activators regenerated by electron

transfer (ARGET) ATRP and ICAR ATRP, polymerization can

be conducted with low levels of copper catalyst by the

continuous generation of activator with either an appro-

priate reducing agent or free radical initiator (for ARGET or

ICAR ATRP, respectively). It is also important to choose an

appropriate ligand for ARGET and ICAR ATRP in order to

form an active catalyst complex which is stable under high

dilution.[20–23]

ARGET and ICAR ATRP are industrially relevant as they

can be conducted in the presence of limited amounts of air

and the final products obtained from these techniques are

essentially colorless.[24] However, for certain applications

(e.g., electronic and biomedical) it may be important to

further reduce the catalyst concentration. Unfortunately,

control over molecular weight distribution during poly-

merization is lost if the concentration of catalyst is too

low.[25] It is therefore necessary for some applications to

reduce the residual catalyst concentration after polymer-

ization.

There are several methods for the removal of copper from

a polymerization solution.[26] These include passing the

solution through a column filled with an absorbent such as

silica or alumina,[27] stirring with an ion exchange resin,[28]

precipitation of the polymer into a nonsolvent,[29,30] or the

use of a heterogeneous catalyst that is easily isolated after

polymerization.[31–41] In this report, the effectiveness of

several purification methods was investigated for a normal,

ARGET, and ICAR ATRP.

Experimental Part

Chemicals

Styrene (St; Aldrich, 99%) was passed through a column filled with

basic alumina (Sorbent Technologies, Act. I, 50–200mm) to remove

inhibitor. Copper(I) bromide (CuBr; Acros, 99%) was purified using a

modified literature procedure.[42] Ethyl 2-bromoisobutyrate (EBriB;

Acros, 98%), copper(II) bromide (CuBr2; Acros, 99%), N,N,N,N,N-

pentamethyldiethylenetriamine (PMDETA; Aldrich, 99%), tris(2-

pyridylmethyl)amine (TPMA; ATRP Solutions, Inc., 99%), N,N-

dimethylformamide (DMF; Aldrich, 99.9%), 2,20-azoisobutyronitrile

(AIBN; Aldrich, 98%), tin(II) 2-ethylhexanoate (Sn(EH)2; Aldrich,

95%), anisole (Aldrich, 99%), diphenyl ether (Aldrich, 99%)



tetrahydrofuran (THF; Aldrich, 99%), hexanes (Fisher, 99%),

methanol (Fisher, 99.9%), ATRP Pure resin (ATRP Solutions, Inc.,

60mm, catalog number R-0101), and neutral alumina (Sorbent

Technologies, Act. I, 50–200mm) were used as received.

Synthesis of Polystyrene using Normal ATRP

St (50 mL, 440 mmol), EBriB (320mL, 2.2 mmol), PMDETA (456mL,

2.2 mmol), and DMF (2.5 mL) were placed in a 100-mL Schlenk

flask. Oxygen was removed by three freeze–pump–thaw cycles,

and CuBr (313 mg, 2.2 mmol) was added under nitrogen flow.

Polymerization was conducted at 90 8C for 20 h. The reaction

was stopped by removing from the heat and opening the flask

to air.

Purification of Polystyrene Prepared using Normal

ATRP

The polymerization solution was diluted with THF (97.5 mL) to

make the total volume of the solution equal to 150 mL. The solution

was divided into three equal parts, 50 mL each. The first 50-mL

portion was divided into two equal parts, 25 mL each. One of the

25-mL portions was dried without purification. The other 25-mL

portion was precipitated into 600 mL of hexanes (precipitation

technique: the polymer solution was poured into a 20-mL plastic

syringe fitted with a needle [20 gauge, 1.5 inch], which was placed

above a beaker full of hexanes, stirred by magnetic stir bar, such

that the polymer solution dropped from the syringe at a rate of

about one drop per second). After precipitation, the hexanes were

decanted away from the polymer, and the polymer was then dried.

The second 50-mL portion was passed by gravity through a column

of neutral alumina (30 g of neutral alumina was used in a 2.54 cm

diameter glass column with a piece of cotton at the bottom). After

the entire solution passed through the column, additional THF

(50 mL) was passed through the column to remove the remaining

residue. The total solution obtained from the column was divided

into two equal portions. One portion was dried without further

purification. The other portion was concentrated until it was a total

volume of 25 mL, and then it was precipitated into hexanes and

dried as described before. The third 50-mL portion was diluted with

THF (100 mL) for a total volume of 150 mL. This was then stirred

with 10% by weight of ATRP Pure resin versus polymer (1.2 g of the

resin was used, it was washed with methanol and dried thoroughly

before use). This solution was stirred for 12 h (enough time for the

solution to appear colorless), and then the liquid was decanted

away from the resin. The resin was washed five times with 1-mL

portions of THF, followed by decanting after each washing. These

washings were combined with the polymer solution, and the

resulting solution was divided into two equal parts. One half was

dried without further purification, and the other was concentrated

to 25 mL, precipitated into hexanes, and dried. A total of six samples

were prepared. Each was crushed to a powder after an initial drying,

followed by further drying under vacuum. In each sample, less than

1% of monomer and solvent remained (determined by 1H NMR

spectroscopy).

www.mre-journal.de 181

L. Mueller, K. Matyjaszewski

182

Synthesis of Polystyrene using ARGET ATRP

St (50 mL, 440 mmol), EBriB (320mL, 2.2 mmol), TPMA (38.0 mg,

0.13 mmol), and CuBr2 (4.87 mg, 0.022 mmol) were placed in a

100-mL Schlenk flask. Oxygen was removed by three freeze–pump–

thaw cycles, and Sn(EH)2 (70.7mL, 0.22 mmol) in DMF (2.5 mL) was

added under nitrogen flow. Polymerization was conducted at 90 8Cfor 14 h. The reaction was stopped by removing from the heat and

opening the flask to air.

Purification of Polystyrene Prepared using

ARGET ATRP

The polymerization solution was subjected to the purification

techniques exactly as described for the polystyrene prepared using

normal ATRP, to again make a total of six samples. Each was

crushed to a powder after an initial drying, followed by further

drying under vacuum. In each sample, less than 1% of monomer

and solvent remained (determined by 1H NMR spectroscopy).

Synthesis of Polystyrene using ICAR ATRP

St (50 mL, 440 mmol), EBriB (320mL, 2.2 mmol), TPMA (19.0 mg,

0.065 mmol), and CuBr2 (4.87 mg, 0.022 mmol) were placed in a

100-mL Schlenk flask. Oxygen was removed by three freeze–pump–

thaw cycles, and AIBN (35.8 mg, 0.22 mmol) in DMF (2.5 mL) was

added under nitrogen flow. Polymerization was conducted at 70 8C.

After 24 h, it was necessary to add additional AIBN (35.8 mg,

0.22 mmol) in 0.25 mL anisole in order to reach the desired

conversion. After 52 h, the reaction was stopped by removing from

the heat and opening the flask to air.

Purification of Polystyrene Prepared using ICAR ATRP

The polymerization solution was diluted with THF (147.5 mL) to

make the total volume of the solution equal to 200 mL. This was

then divided into five portions (two portions with 50 mL each and

four portions with 25 mL each). One of the 25-mL portions was

precipitated into 600 mL of hexanes and the other into 600 mL of

methanol (using the same precipitation procedure as before). After

precipitation, each polymer was isolated and dried. One of the

50-mL portions was passed by gravity through a column of neutral

alumina (30 g of neutral alumina was used in a 2.54 cm diameter

glass column with a piece of cotton at the bottom). After the entire

solution passed through the column, additional THF (50 mL) was

passed through the column to remove the remaining residue. This

solution was concentrated back to a total of 50 mL, and then it

was split into two equal parts. One part was precipitated to hexanes

and the other to methanol and both polymers were dried. The

second 50-mL portion was diluted with an additional 50 mL of THF

and was stirred for 12 h with 10% by weight of ATRP Pure resin

versus polymer (0.93 g of the resin was used, it was washed with

methanol and dried thoroughly before use). After stirring, the

polymer solution was decanted from the resin. The resin was

washed five times with 1-mL portions of THF, followed by

decanting after each washing. These washings were combined



with the polymer solution, and the entire solution was concen-

trated down to a volume of 37.5 mL. This mixture was split into two

equal parts. One part was precipitated into hexanes and then dried.

The other part was precipitated into hexanes, dried to remove

hexanes, redissolved into 20 mL of THF, precipitated a second time

into hexanes, and then dried. Six samples were prepared. Each was

crushed to a powder after an initial drying, followed by further

drying under vacuum. In each sample, less than 1% of monomer

and solvent remained (determined by 1H NMR spectroscopy).

Characterization

Monomer conversions were determined from the concentration of

the unreacted monomer in the samples periodically removed from

the reactions using a gas chromatograph (Shimadzu GC-14A),

equipped with a capillary column (DBWax, 30 m�0.54 mm�0.5mm, J&W Scientific). DMF was used as internal standard for

calculation of monomer conversions. The polymer samples were

measured by gel permeation chromatography (GPC) (Polymer

Standards Services (PSS) columns (guard, 105, 103, and 102 A), with

THF eluent at 35 8C, flow rate¼1.00 mL �min�1, and differential

refractive index (RI) detector (Waters, 2410)). Diphenyl ether was

used as the internal standard to correct the fluctuation of the THF

flow rate. The apparent molecular weights and polydispersity were

determined with a calibration based on linear polystyrene

standards using WinGPC 6.0 software from PSS. The concentrations

of copper and tin in each sample were determined using an

inductively coupled plasma mass spectrometer by Analytical

Consulting Services (ACS) Labs in Houston, TX.

Results and Discussion

Synthesis and Purification of Polystyrene usingNormal, ARGET, and ICAR ATRP

Polystyrene was synthesized using three different ATRP

initiation techniques: normal ATRP, ARGET ATRP, and ICAR

ATRP, Table 1. For each reaction, the targeted degree of

polymerization was 200 and each was stopped when

conversion reached �80%. The normal ATRP reached 80%

conversion in 20 h to yield polystyrene with number-

average molecular weight (Mn)¼ 22 500 g �mol�1 and

polydispersity index (PDI)¼ 1.11. The ARGET ATRP reached

84% conversion after 14 h to yield polystyrene with

Mn ¼ 20 700 g �mol�1 and PDI¼ 1.08. The ICAR ATRP

reached 82% conversion after 52 h to yield polystyrene

with Mn ¼ 15 000 g �mol�1 and PDI¼ 1.12 (the longer

reaction time was due to the polymerization being run at

70 8C, as opposed to 90 8C for normal and ARGET ATRP, and

the lower molecular weight is due to new chains initiated

by AIBN). After each reaction was completed, the poly-

merization solution was divided into several parts and each

part was subjected to a different purification method. The

purification methods used include passing through a

column of neutral alumina, stirring with an ion exchange

DOI: 10.1002/mren.200900067


Table 1. Experimental conditions and properties of polystyrene prepared using various ATRP initiation techniques.a)

Molar ratios Cu Time Conv Mn;GPC PDI

St EBriB CuBr CuBr2 Ligandb) R.A.c) ppmd) h % g �mol�1

Normal ATRP 200 1 1 – 1 – 3 050 20 80 22 500 1.11

ARGET ATRP 200 1 – 0.01 0.06 0.1 30.5 14 84 20 700 1.08

ICAR ATRP 200 1 – 0.01 0.03 0.1 30.5 52 82 15 000 1.12

a)[St]0¼ 8.3 M; 90 8C for normal and ARGET ATRP, 70 8C for ICAR ATRP; in DMF (5 vol.-%); b)PMDETA for normal ATRP and TPMA for ARGET

and ICAR ATRP; c)R.A. (reducing agent) was Sn(EH)2 for ARGET ATRP and AIBN for ICAR ATRP; d)ppm by mass of copper metal in the initial

polymerization solution.

resin (ATRP Pure), and precipitation into either hexanes or

methanol. A detailed description of the purification

methods is described in the Experimental Part.

Table 2. Copper concentration (in ppm by mass) remaining afterpurification in the polymers prepared by normal, ARGET, and ICARATRP.

Normal

ATRP

ARGET ATRP ICAR

ATRP

Purification

techniquea)[Cu] [Cu] [Sn] [Cu]

ppm ppm ppm ppm

NR/NP 3 718 35.9 107 –

NR/HP 2 636 34.6 53.5 33.4

NR/MP – – – 14.9

AC/NP 2.19 1.44 5.90 –

AC/HP 1.37 1.07 5.80 0.66

AC/MP – – – 0.50

10R/NP 1 451 1.48 7.30 –

10R/HP 1 405 1.18 8.55 1.43

10R/2HP – – – 1.05

a)Where NR¼no removal of copper directly from the polymeri-

zation solution, NP¼no precipitation, HP¼precipitation into

hexanes, MP¼precipitation in methanol, AC¼passing of polymer

solution through a neutral alumina column, 10R¼ stirring poly-

mer solution with 10 wt.-% ATRP Pure resin, and 2HP¼ 2 precipi-

tations into hexanes.

Analysis of Copper Concentration after PolymerPurification

Each sample prepared was thoroughly dried and analyzed

by inductively coupled plasma mass spectrometry (ICP-MS)

to determine the copper concentration (and tin concentra-

tion for the ARGET ATRP samples). ICP-MS is able to detect

metal concentrations down to 0.05 parts per million (ppm)

by mass. The results are summarized in Table 2. The samples

are labeled with the following abbreviations: NR¼no

removal of copper directly from the polymerization

solution, NP¼no precipitation, HP¼precipitation into

hexanes, MP¼precipitation in methanol, AC¼passing of

polymer solution through neutral alumina column,

10R¼ stirring polymer solution with 10 wt.-% ATRP Pure

resin, and 2HP¼ 2 precipitations into hexanes.

For the polystyrene prepared via normal ATRP, the

concentration of copper without purification (NR/NP) was

3 718 ppm. This is close to the predicted value of 3 800 ppm

(determined by the total mass of copper metal initially

added divided by the total mass of dried polymer). With no

direct copper removal, but with precipitation into hexanes

(NR/HP), the copper concentration decreased to a value of

2 636 ppm. The polymer solution which was passed

through a column filled with neutral alumina (AC/NP)

had a much lower copper concentration of 2.19 ppm. After

the polymerization was complete, the copper catalyst

became insoluble and precipitated out of solution. There-

fore, it was very easily filtered by the alumina when the

solution passed through the column. However, the polymer

was still colored, most likely due to a side reaction occurring

with the PMDETA ligand.[43] The concentration of copper in

the polymer that was filtered through alumina column was

reduced to 1.37 ppm after precipitation into hexanes (AC/

HP). The polymer which was stirred with 10 wt.-% of ATRP



Pure had a copper concentration of 1 451 ppm before

precipitation (10R/NP) and 1 405 ppm after precipitation

into hexanes (10R/HP). This concentration of resin was not

high enough to sufficiently remove copper. It is also difficult

to compare the effectiveness of the resin versus the alumina

column, due to the fact that the copper catalyst became

insoluble after polymerization and was simply filtered by

the column. Also, 300 wt.-% of alumina was used compared

to 10 wt.-% of ATRP Pure. A smaller amount of alumina may

have been enough to effectively remove the copper, but as it

is inexpensive, large excess was used to ensure that it was

efficient. A photograph of each sample obtained after


L. Mueller, K. Matyjaszewski

184

purifying the polystyrene prepared via normal ATRP is

shown in Figure 1a.

For the polystyrene prepared via ARGET ATRP, with no

copper removal or precipitation, the concentration of

copper after no type of purification was 35.9 ppm (NR/

NP), which matches well with the predicted value of

36 ppm. After precipitation into hexanes, this value

decreased slightly to 34.6 ppm (NR/HP). The copper con-

centration after passing through a neutral alumina column

and stirring with 10 wt.-% ATRP Pure resin were similar at

1.44 ppm (AC/NP) and 1.48 ppm (10R/NP) before precipita-

tion and 1.07 (AC/HP) and 1.18 (10R/HP) after precipitation

into hexanes, respectively. Tin concentration was also

measured for these samples, as its presence could also be

undesired for certain applications. The tin concentration

decreased from 107 ppm without purification to 53.5 ppm

after precipitation into hexanes. The concentration of tin

was also significantly decreased after both types of

purification. A photograph of each sample obtained after

Figure 1. Photographs of polystyrene samples prepared usingnormal ATRP (a), ARGET ATRP (b), and ICAR ATRP (c) after variouspurification techniques, where NR¼no removal of copperdirectly from the polymerization solution, NP¼no precipitation,HP¼ precipitation into hexanes, AC¼passing of polymersolution through a neutral alumina column, 10R¼ stirring poly-mer solution with 10wt.-% ATRP Pure resin, MP¼precipitation inmethanol, and 2HP¼ 2 precipitations into hexanes.



purifying the polystyrene prepared via ARGET ATRP is

shown in Figure 1b.

For the polystyrene prepared via ICAR ATRP, the

predicted concentration of copper in the polymer after no

type of purification was 37 ppm. The experimental

procedure for the purification of this polymer was slightly

different than that for the normal and ARGET ATRP in order

to make a comparison between precipitation into hexanes

and into methanol. After precipitation into hexanes, the

copper concentration was 33.4 ppm (NR/HP) and after

precipitation into methanol, the copper concentration was

14.9 ppm (NR/MP). This was expected, as methanol is a

better solvent for the copper catalyst and is therefore better

at removing it from the polymer. The concentration of

copper after passing through neutral alumina column was

0.66 ppm after precipitation into hexanes (AC/HP) and

0.50 ppm after precipitation into methanol (AC/MP). The

concentration of copper after stirring with 10 wt.-% ATRP

Pure resin was 1.43 ppm after precipitation into hexanes

(10/HP). This sample was also subjected to a second

precipitation into hexanes, after which the copper con-

centration was 1.05 ppm (10R/2HP). A photograph of each

sample obtained after purifying the polystyrene prepared

via ICAR ATRP is shown in Figure 1c.

Conclusion

The copper-removal methods of passing through a neutral

alumina column, stirring with an ion exchange resin, and

precipitation into hexanes or methanol were tested on

polystyrene prepared via normal, ARGET, and ICAR ATRP.

For polystyrene prepared using normal ATRP, it was found

that passing through a neutral alumina column was the

most effective method at removing copper. This was mostly

due to the copper catalyst becoming heterogeneous at the

end of the polymerization, which was simply filtered by the

column. For the polystyrene prepared using ARGET and

ICAR ATRP, both the neutral alumina column and ion

exchange resin were comparably effective at removing

copper (and tin in the case of ARGET ATRP). For all cases,

precipitation into hexanes provided a slight reduction in

the copper concentration while precipitation into methanol

was more effective at reducing copper concentration,

compared to no precipitation.

Acknowledgements: The authors acknowledge Dr. WojciechJakubowski for helpful discussions, ACS Labs for ICP-MS analysis,NSF CHE 07-15494, and Graduate Research Fellowship (for L.M.).

Received: October 8, 2009; Revised: December 6, 2009; Publishedonline: March 9, 2010; DOI: 10.1002/mren.200900067

Keywords: atom transfer radical polymerization (ATRP); poly-styrene; purification techniques

DOI: 10.1002/mren.200900067


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reducing copper concentration in polymers prepared via atom transfer radical polymerization

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