continuous atom transfer radical polymerization in a tubular reactor

Full Paper

Continuous Atom Transfer RadicalPolymerization in a Tubular Reactor

Matthias Muller, Michael F. Cunningham,* Robin A. Hutchinson*

The use of a tubular reactor for conducting living radical polymerizations by atom transferradical polymerization (ATRP) was investigated. Solution polymerization experiments wereperformed with styrene and butyl acrylate to elucidate the influence of a continuous reactionprocess on conversion, molecular weight, andpolydispersity compared to batch polymeri-zation experiments. The continuous polymeri-zations were well controlled. Initial conversionwas found to be slightly higher in the tubularreactor than in a batch polymerization run atsimilar conditions, while number average mo-lecular weight and polydispersity are compar-able between the continuous and batchprocesses. Residence time distribution studiesshowed the reactor exhibits nearly plug flowbehavior.

Introduction

Living/controlled radical polymerization (L/CRP) offers the

potential for major advances in the manufacture of poly-

meric materials through its control of polymer micro-

structure - narrowing of molecular weight distributions

(MWD), controlled composition distribution along the

chain, and targeted placement of functional groups and

branch points. Most L/CRP studies have been done using

bulk or solution polymerization in batch reactors. There

are only a few references on living radical polymerization

in a continuous reactor. Homogeneous bulk atom transfer

radical polymerization (ATRP) of methyl methacrylate in a

continuous packed bed of silica supported catalyst has

been demonstrated successfully by Zhu and co-workers.[1,2]

M. Muller, M. F. Cunningham, R. A. HutchinsonDepartment of Chemical Engineering, Dupuis Hall, Queen’sUniversity, 19 Division St., Kingston, ON, Canada K7L 3N6Fax: (þ1) 613 533 6637;E-mail: [email protected],[email protected]

Macromol. React. Eng. 2008, 2, 31–36

� 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Recently, Noda et al.[3] reported the preparation of MMA

homo- and block co-polymers via ATRP using amicroreactor

consisting of a poly(tetrafluoroethylene) (PTFE) tube of�10

mL volume. Schork and Smulder[4] have investigated

theoretical aspects of using a continuous stirred tank

reactor for ATRP, NMP, and RAFT polymerizations. They

predicted a polydispersity index (PDI) of two compared to

the theoretical ideal polydispersity of one for a batch

reactor. Enright et al.[5] investigated nitroxide mediated L/

CRP in miniemulsion using a tubular reactor. RAFT

polymerization in miniemulsion has been studied

by Smulders et al.[6] using a train of CSTRs and Russum

et al.[7–9] using a tubular reactor.

There are several potential benefits of using a contin-

uous tubular reactor for ATRP polymerizations. Synthesiz-

ing polymers with controlled microstructure (e.g., block

copolymers) can be simplified by using feed streams

situated along the reactor. The conversion at which the

additional monomer is added can be easily adjusted by

varying the flow rate. A tubular reactor permits better

control of the temperature profile, especially during

the heating period, and temperature changes during

DOI: 10.1002/mren.200700029 31

M. Muller, M. F. Cunningham, R. A. Hutchinson

32

polymerization, which in the end leads to better controlled

polymer properties. Furthermore, operating a tubular

reactor under pressure, which is needed close to or above

the boiling point of monomer or solvent, is much easier

than running a stirred tank reactor under the same

pressure, for both laboratory and industrial scale. This

paper describes initial results using a continuous tubular

reactor to produce homopolymers of butyl acrylate and

styrene via ATRP.

Experimental Part

The reactor setup is based on a similar setup used by Enright et al.[5]

to investigate nitroxide mediated polymerization inminiemulsion.

The tubular reactor setup consists of stainless steel tubing (150 m

length, 3.2 mm outer diameter, 2.1 mm inner diameter, reactor

volume �0.52 L) immersed in an oil bath. The flow was controlled

by aMasterflex peristaltic pump situated at the outlet of the reactor

tube. Between the pump and oil bath, a heat exchanger is used

to cool the reaction mixture down. The actual mass flow was

measured by a balance (Mettler Toledo PG 5002s) at the outlet. A

scheme of the reactor is shown in Figure 1. Two storage tanks were

used in order to be able to refill one storage tank while the other is

used for feeding the reactor. Therefore, operation time is not limited

by the volume of the storage tanks.

Batch reactions for comparison to the continuous reactor were

done utilizing a 1-L glass autoclave built into a Mettler-Toledo

LabMax system.

Materials

Styrene (99%, Aldrich), butyl acrylate (99%, Aldrich), toluene (99%,

Aldrich), benzonitrile (99%, Aldrich), acetonitrile (99%, Aldrich),

Cu(I)Br (98%, Aldrich), N,N,N0,N00,N0- pentamethyldiethylenetri-

amine (PMDETA, 99%, Aldrich), and methyl 2-bromopropionate

(MBrP, 98%, Aldrich) were used as received.

Figure 1. Continuous tubular reactor apparatus for ATRP polymerizat



Analyses

The molecular weight and PDI were measured by gel permeation

chromatography (GPC). The samples were first passed through a

column packed with basic alumina to remove the residual copper.

The GPC instrument was equipped with a Waters 2960 module

containing four Styragel columns with pore sizes of 100, 500, 103,

and 104 A, coupled with a Waters 410 differential refractive index

(RI) detector (480 nm) and a Wyatt Technology DAWN EOS

photometer multi-angle light scattering (LS) detector (690 nm,

30 mW Ga-As laser). THF was used as the eluent and the flow rate

was set to 1mL �min�1. The LSdetectorwas calibratedwith toluene,

normalizedwith 30000 g �mol�1 narrow polystyrene standard, and

the datawere processed using Astra (Version 4.90.08) software. The

system was calibrated with 11 narrow polystyrene standards.

Conversion was analyzed gravimetrically by drying small samples

at room temperature for 2 d and by gas chromatography using

a Varian CP 3800 system fitted with a 50 m� 0.25 mm inner

diameter fused-silica CP-Sil wall coated open tubular capillary

column (WCOT). Sample injectionwas controlled by a Varianmodel

CP-8410 injector. Analytes were detected using a flame ionization

detector. The GC was controlled by Varian Star software.

Polymerization Procedure

Monomer, solvent, CuBr, and PMDETA were premixed for at least

30 min in an Erlenmeyer flask while purging the solution with

nitrogen. If there was no insoluble copper at the bottom after the

initialmixing period, the initiator was added and the solutionwas

poured into the two storage tanks of the tubular reactor setup. In

the case where insoluble copper was observed, the mixing period

was extended by 30 min, followed by 20 min to let the insoluble

parts settle to prevent transfer to the storage tanks. In most cases

therewas no insoluble catalyst. After purgingwith nitrogen for an

additional 10 min the storage tanks were closed and the nitrogen

valve was regulated to provide a pressure of 2 bar.

To begin the continuous reactions, the reactor tube was quickly

ion.

filled from storage tank #2 which took 5–6

min. The pump head of the peristaltic pump

was closed and the pump started with the

desired flow rate. Samples were taken at

intervals at the outlet of the reactor, which

results in samples with successively longer

reaction times and therefore higher conver-

sion until the full residence time of the tubular

reactor is reached. In principle it is possible to

take samples in the middle of the reactor

using T-junctions, but thiswould interrupt the

flow pattern and therefore was not done in

this study.

Residence Time Distributions (RTDs)

The RTD of the tubular reactor was deter-

mined using water. After applying a pulse of

NaCl solution, the conductivity was measured

using a conductivity meter placed between

DOI: 10.1002/mren.200700029

Continuous Atom Transfer Radical Polymerization in a Tubular Reactor

the heat exchanger and the peristaltic pump. Control experiments

without the reactor were done to determine the time the salt

solution needs to reach the reactor from the injection point and to

reach the conductivity meter after leaving the tubular reactor.

Figure 3. Cumulative distribution function measured by applyinga pulse of NaCl solution in water. The vertical line is the theo-retical expectation for an ideal tubular reactor. The temperatureof the oil bath was 21 8C.

Results

The rate of polymerization for styrene and butyl acrylate

using ATRP is significantly lower than the rate of the

corresponding normal free radical polymerization. There-

fore, longer times are needed to achieve the same level

of conversion. Using a tubular reactor setup there are

basically two different design options within the limita-

tion of a reasonable flow rate range: a short tubewith large

inner diameter or a long tube with small inner diameter.

For this setup the latter was chosen in order to provide a

reasonable average velocity along the tube. Higher velocity

should help to prevent fouling at the walls, which is

especially important at high conversion and molecular

weight.

Table 1. Results of the residence time distribution experimentsusing water as solvent. The conductivity was measured after

Residence Time Distribution Studies

A typical result of the residence time distribution is shown

in Figure 2. Three replicate experiments are shown using

the same flow rate. Figure 3 shows the cumulative distri-

bution function for one of the detected signals given in

Figure 2. For comparison, the theoretical graphs expected

for laminar and plug flow are also shown. The experi-

mental RTD is close to plug flow. This is most probably

caused by the fact that the tubular reactor is coiled to fit

in the oil bath. Due to the coiled tubing secondary flow

patterns are introduced leading to better mixing and

therefore a narrow residence time distribution. The

Figure 2. Results of three replicate residence time distributionexperiments at a flow rate of 3.1 g �min�1. The temperature of theoil bath was 21 8C.



Reynolds number of water under these conditions is

smaller than ten due to the very low flow rate. Therefore,

axial mixing can play an important role as well. When

Russum et al.[8] investigated RAFT miniemulsion poly-

merization in a tubular reactor under low flow rates, they

also found residence time distributions close to plug flow

and attributed this to a high axial mixing.

The results of all residence time distribution experiments

are given in Table 1. The full width at half maximum of the

signals (like the ones shown in Figure 2) becomes smaller

with increasing flow rate and increasing temperature. At

applying a pulse of NaCl solution. For each temperature and flowrate, three experiments were done giving the mean residencetime and full width at half maximum of the NaCl pulse.

Temperature Mass

flow

Mean

residence

time

Full width

at half

maximum

-C g �minS1 s s

21 2.13 16 068 751

21 2.11 15 859 754

21 2.07 15 621 763

21 3.11 10 708 600

21 3.11 10 698 600

21 3.14 10 639 591

80 3.17 10 262 360

80 3.17 10 240 354

80 3.19 10 192 350

www.mre-journal.de 33


Table 2. Formulations used in each tubular and batch polymerization experiment of the Figure 4 and 5. The masses and moles of monomer,initiator, catalyst, and ligand are given.

BA batch BA tubular STY batch STY tubular

m n m n m n m n

g mol g mol g mol g mol

Monomer 362.57 2.83 900.83 7.03 302.58 2.91 1 142.92 10.97

CuBr 1.53 10.7� 10S3 3.79 26.4� 10S3 4.16 29.0� 10S3 15.73 109.7� 10S3

PMDETA 6.06 35.0� 10S3 15.21 87.8� 10S3 10.03 57.9� 10S3 37.96 219.0� 10S3

MBrP 11.94 71.5� 10S3 29.65 177.5� 10S3 9.67 57.9� 10S3 36.53 218.7� 10S3

Temperature [-C] 80 80 110 110

Solvent [g] 155.44 (acetonitrile) 386.85 (acetonitrile) 129.13 (toluene) 488.99 (toluene)

34

80 8C, the full width at halfmaximum is 355 s and themean

residence time is 10 240 s, only slightly different (�3.5%)

than the expected value. Since the polymerizations are

carried out at 80 8C and higher, one can assume that

the effect of the residence time distribution on the PDI will

be small.

Polymerizations

Solution homopolymerizations of butyl acrylate and

styrene were investigated using both the continuous

tubular reactor and the batch reactor. First experiments

were tried with a copper to initiator ratio of 1:1. Since

the copper bromide was not fully soluble under these

conditions the amount of copper was decreased until

the solution became homogeneous, while the amount of

ligand was held constant. The formulations are described

in Table 2. The amount of initiator was chosen to reach a

molecular weight of about 5 000 g �mol�1 at full conver-

sion. This value was chosen as it is typical of that used in

automotive coatings. The temperatures of 80 8C for butyl

acrylate and 110 8C for styrene were chosen to be the same

used by Fu working on heterogeneous ATRP.[10,11]

Table 3. Summary of end results of experiments using the for-mulations given in Table 2. Listed are the conversion, molecularweight, and polydispersity of the last sample taken.

BA

batch

BA

tubular

STY

batch

STY

tubular

Conversion [%] 82 71 75 50

Mn 4797 4 160 4 426 2807

PDI 1.09 1.09 1.13 1.14

Homopolymerization of Styrene

The results of the experiments with styrene using the

formulation in Table 2 are given in Table 3 and Figure 4.

Circles and squares indicate results from batch and tubular

experiments, respectively. The batch experiments show a

continuous increase in conversion, as expected. In case of

the tubular runs, the conversion stays constant after the

operating time exceeds approximately onemean residence

time of the reactor, since the reaction time is now limited

by the residence time and not the operating time any

more. The value of themean residence determined at 80 8Cin water is indicated by the vertical line in the upper



part of the graph. There is a slight difference in con-

version between the tubular and batch experiment. The

conversion-time profiles shown in the left part of the

figures show a higher conversion for the tubular experi-

ment, even if the time to fill the tubular reactor is taken

into account. On the other hand, the evolution of molar

masswith respect to the conversion is the same for the two

reactor types. This indicates that there is no loss of growing

chains in the batch reactor compared to the tubular

reactor. The differences between the theoretical and

experimental molecular weights are small, indicating

high initiation efficiency and a nearly constant number of

living chains during polymerization. The evolution of

ln(M0/M) with time depicted by the open symbols in the

left part is quite linear. According to the equation ln(M0/

M)¼ kpRt (where M is the monomer concentration, M0

the monomer concentration at time zero, kp the overall

propagation rate coefficient, R the radical concentration,

and t is the time) this is an evidence of no noticeable loss of

active radicals during polymerization.

The PDI except for the first samples taken from the

tubular reactor is less than 1.15, which is indicative of

excellent control of the polymerization.

A possible reason of the difference in conversion might

be the mode of operation. Russum et al.[9] also found an

increase in conversion using a tubular reactor compared to

the batch control experiment in their investigation of

DOI: 10.1002/mren.200700029

Continuous Atom Transfer Radical Polymerization in a Tubular Reactor

Figure 4. Results of styrene homopolymerization. Tubular and batch experimental results aredepicted by squares and circles, respectively. Open symbols correspond to the right axis andclosed symbols to the left. The vertical line in the upper left part indicates the mean residencetime of the high temperature experiment in Table 1.

RAFT polymerization in miniemulsion. In case of the

batch experiments, the initiator is added after the reaction

mixture is heated up, to be as close as possible to the

tubular reactor, in which the reactionmixture is heated up

nearly instantaneously entering the reactor. Adding the

initiator in the batch experiments usually takes between

40 and 50 s and additional time to mix. Therefore, the

initiation period may be a little longer in the case of the

batch experiments.

After styrene polymerizations in the tubular reactor

were completed, at the beginning of the cleaning proce-

dure, it was observed that small particles were washed out

of the reactor. On one occasion the reactor became blocked

during the experiment. This is most probably due to

precipitation of the Cu(II) catalyst complex. The solubility

of the Cu(II) catalyst is lower than that of Cu(I).[10,11] During

polymerization there may be gradual accumulation of

Cu(II) due to a small but finite termination rate. While this

is not a serious problem in a stirred tank batch reactor, it

can cause serious operability issues with a tubular reactor.

There are several possible approaches to solve this

problem. The easiest would be to choose a different ligand

such as 4,4-di-(5-nonyl)-2,2-bipyridine (dNbpy) which

leads to a homogeneous reaction mixture.[12] However,

this ligand is not commercially available in large volumes

and is therefore too expensive for industrial applications.

Alternative approaches to make the reaction mixture

homogeneous are to reduce the amount of copper in the

solution or switch to another solvent which is a better

solvent for the catalyst complex. The former choice would

lead to a lower conversion and likely negatively impact



control of the polymerization, and

therefore we investigated the use

of a different solvent. Due to its

low boiling point of 82 8C, acet-onitrile was deemed unsuitable,

and benzonitrile (bp¼ 189 8C)was chosen instead.

Initial experiments with ben-

zonitrile showed no precipitation

in a batch experiment but when

used in the tubular reactor did

lead to a blocked tube after �2 h

of operation. This time the block-

age occurred in or following

the heat exchanger, when the

reaction mixture was cooling

down. Opening a three-way valve

attached between the reactor

and the heat exchanger for test-

ing purposes revealed that the

hot reaction mixture was homo-

geneous. Further tests are cur-

rently being done to optimize the

cooling unit to prevent blockage as a result of catalyst

precipitation during the cooling step.

Homopolymerization of Butyl Acrylate

The results of the experiments with butyl acrylate are

shown in Table 3 and Figure 5. As with styrene there is a

slight difference in conversion between the batch and the

tubular experimental results, which can be explained

following the same arguments given for styrene.

One can observe a slight curvature in the ln(M0/M)

versus time profile for the batch experiment. This indicates

a decrease in the active radical concentration during

polymerization. There are two possible reasons, either a

loss of radicals by termination or side reactions. Radicals

may be lost by reactions with the ligand PMDETA as pro-

posed by Sharma et al.[13] On the other hand, the deviation

between the theoretical molar mass and the experimental

values is small, which indicates that there is no significant

loss of radicals during polymerization and that the

initiation efficiency is close to unity. Another possibility

is the backbiting reaction which leads to a mid-chain

radical which is more stable and therefore propagates

slower than the normal radical. At higher conversion the

backbiting reaction becomes more favored.[14] The impli-

cations of these side reactions on ATRP will be studied in

future work.

The agreement between the results for evolution of

molar mass and polydispersity with conversion of the

batch and tubular experiment is remarkable. Except for

www.mre-journal.de 35


Figure 5. Results of butyl acrylate homopolymerization. Tubular and batch experimental resultsare depicted by squares and circles, respectively. Open symbols correspond to the right axis andclosed symbols to the left. The vertical line in the upper left part indicates the mean residencetime of the high temperature experiment in Table 1.

36

the first two data points, the differences between the

two experimental setups lie within the analytical uncer-

tainties.

In contrast to the styrene experiments, the tubular

reactor with butyl acrylate was run for 6 h without any

problem. Cleaning of the reactor showed no evidence of

solid particles washed out with the cleaning solvents

water and acetone. Therefore, longer operation should be

feasible. Higher solubility of the catalyst complex (espe-

cially the Cu(II) species) in BA compared to styrene explains

the absence of fouling, and underscores the importance of

the catalyst solubility in the polymerization to the overall

operability of a tubular reactor.

Conclusion

Living radical polymerization of styrene and butyl acrylate

using ATRP was demonstrated in a continuous tubular

reactor, resulting in polymer with low PDI. The reaction

kinetics was similar for batch and continuous reactors. The

conversion in the continuous reactor is slightly higher

than in the batch control experiments. The molecular

weight evolution with conversion and the polydispersity

is very close to the ones obtained during batch control

experiments.

Using a tubular instead of a batch reactor for ATRP

polymerization imposes an additional constraint on the



polymerization system. The for-

mulation has to be homogeneous

throughout the full conversion

and temperature range used in

the reactor setup. Even small

amounts of precipitated catalyst

particles can lead to problems

with long-term operation, which

is not a concern for batch reactors.

This is an initial work done to

determine the feasibility of using

a tubular reactor. Further details

will be published in a future paper.

Acknowledgements: Financial sup-port from the ‘‘German AcademicExchange Service’’ (DAAD) and fromthe Natural Sciences and EngineeringResearch Council of Canada is grate-fully acknowledged.

Received: July 27, 2007; Revised:October 5, 2007; Accepted: October

18, 2007; DOI: 10.1002/mren.200700029

Keywords: ATRP; butyl acrylate; continuous; kinetics; styrene;tubular reactor

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[2] Y. Shen, S. Zhu, F. Zeng, R. H. Pelton, Macromolecules 2000,33, 5427.

[3] T. Noda, A. J. Grice, M. E. Levere, D. M. Haddleton, Eur.Polym. J. 2007, 43, 2321.

[4] F. J. Schork, W. Smulders, J. Appl. Polym. Sci. 2004, 92, 539.[5] T. E. Enright, M. C. Cunningham, B. Keoshkerian, Macromol.

Rapid Commun. 2005, 26, 221.[6] W. W. Smulders, C. W. Jones, F. J. Schork, Macromolecules

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2005, 44, 2484.[10] Y. Fu, Ph. D. Thesis, Queen’s University, Kingston, ON 2007.[11] Y. Fu, A. Mirzaei, M. F. Cunningham, R. A. Hutchinson,

Macromol. React. Eng. 2007, 1, 425.[12] K. Matyjaszewski, T. E. Patten, J. Xia, J. Am. Chem. Soc. 1997,

119, 674.[13] R. Sharma, A. Goyal, J. M. Caruthers, Y.-Y. Won, Macromol-

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DOI: 10.1002/mren.200700029

continuous atom transfer radical polymerization in a tubular reactor

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