Atom Transfer Radical Copolymerization of Styrene and Butyl Acrylate

Chapter 14

Atom Transfer Radical Copolymerization of Styrene and Butyl Acrylate 1

Grégory Chambard and Bert Klumperman

Downloaded by UCSF LIB CKM RSCS MGMT on August 24, 2014 | http://pubs.acs.org Publication Date: August 15, 2000 | doi: 10.1021/bk-2000-0768.ch014

Laboratory of Polymer Chemistry, Eindhoven University of Technology, P.O. Box 513,5600 MB Eindhoven, The Netherlands

Atom transfer radical polymerization of styrene and butyl acrylate has been investigated from a kinetic point of view. Attention is focused on the activation of the dormant species as well as on the termination that plays a role in these reactions. It has been shown that the activation of a styrene dormant species is much slower in methyl methacrylate compared to reported data obtained in styrene. Termination reactions seem to play a minor role in ATRP, although in the early stages of the reaction loss of functionality is observed. The reactivity ratios for the copolymerization of styrene and butyl acrylate are determined as well and are very similar to the ones for conventional free -radical polymerization. From simulations it is clear that the system is self-tuning in the sense that a difference in equilibrium constants between both dormant species does not lead to a change in radical ratios in the reaction mixture.

Introduction Free-radical polymerization is one of the most widely used techniques to produce polymers. It is robust, since it is relatively inert towards impurities such as water, and can be applied with a wide variety of monomers. A great disadvantage, however, is the inability to control polymer properties, such as tacticity, and chain topology (e.g. in the case of block copolymers). Since a decade, techniques have been developed that combine the robustness andflexibilityof free-radical polymerization with the ability to keep control over polymer properties (1,2,3,4). Atom transfer radical polymerization (ATRP) is one of the most promising (3,4). Styrene and derivatives, as well as (meth)acrylates can be (co)polymerized (5,6,7). There are, however, still many features that remain underexposed up to now and need to be looked at in more detail. For instance, the amount of dead chains that are inevitably formed during the course of reaction is an important parameter when preparing e.g. block copolymers. Corresponding author.

© 2000 American Chemical Society

In Controlled/Living Radical Polymerization; Matyjaszewski, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 2000.

198 Furthermore, copolymerization is only rarely reported in literature (8,9,10,11,12) and needs to be closely examined. This paper deals with the above-mentioned 'gaps' in literature. In homopolymerizations of both styrene (S) and butyl acrylate (BA), attention has been focused on kinetics and termination events. Furthermore, the copolymerization of styrene and butyl acrylate has been investigated from a kinetic point of view, taking knowledge on both homopolymerizations into account.


Theory The kinetics of ATRP rely on the reversible activation and deactivation of a dormant species, an alkyl halide. The halide atom is transferred back and forth from the alkyl halide to a transition metal complex, usually a copper complex (scheme 1). k d e a c t

+

R-Br

+

2

Cu

*

R.

+

+

Cu Br

Scheme 1. Activation/deactivation process in ATRP. When a dormant species is activated, it is able to propagate via normal radical kinetics. To describe the monomer concentration, the following expression can be used for most of the reaction time (13,14): +

K [R~BrUCu ] )

-ln(l-X) = | v

a

0

(1)

3*,

In this equation, X stands for overall conversion and K for the equilibrium constant for the reversible activation/deactivation process as shown in scheme 1. For a given monomer, the evolution of ln(l-X) is dependent on the concentrations of dormant species and catalyst, and on the 'system constants' k , k and K . It is very interesting to obtain information about the activation parameter, k , in the kinetic scheme. Fukuda et al introduced a very simple and elegant method to determine this parameter (15). The method is based on the monitoring of the concentration of a macroinitiator species as a function of time with size exclusion chromatography (SEC). A minor amount of radical initiator is added to ensure that the macroinitiator, after it is activated, reacts irreversibly with the radicals derived from this radical initiator. This causes a separation of the original macroinitiator peak and the peak of the product of the trapped macroinitiator radical in the SEC chromatograms. a

p

t

a

act

In f\

+

= k [Cu ]-t acr

(2)


In this equation, S stands for the area of under the macroinitiator peak in the SEC chromatogram at t=0, while S is this area at later times. When plotting the natural logarithm of the ratio of the two areas vs. time, this leads to a value of the activation rate parameter, k . 0

t

act

Experimental


General: Styrene (S, Aldrich) and butyl acrylate (BA, Aldrich) were distilled prior to use. Xylene (Aldrich), CuBr (98%, Aldrich), tosylchloride (TsCl, 99%, Aldrich) were used as received. Di-4,4'-n-heptyl-2,2'-bipyridine (dHbpy) was synthesized according to a known synthesis route (16).

Typical polymerization procedure: Monomer is put into a 100 mL round-bottomed flask, together with xylene and dHbpy and purged for 1 hour with argon. The typical total reaction volume was always kept at around 20 mL. After this, an appropriate amount of CuBr was added, so that Cu:dHbpy was 1:2, and the solution was degassed for another half hour. Then, the mixture was heated up to reaction temperature (typically 110°C), after which the reaction was started by addition of previously degassed ethyl 2bromoisobutyrate (EiB, for the homopolymerizations) or a solution of tosyl chloride (TsCl) in xylene (in the case of copolymerizations). Samples were taken at different time intervals and monomer concentrations were determined by gas chromatography using a HP 5890 equipped with A T Wax column (Alltech, length 30m, film thickness 1.0m) with autosampler. In the case of copolymerizations, partial monomer conversions were calculated as well as the fraction of S (f ) as a function of total conversion (X). Since these data are all calculated from the monomer concentrations measured, they all contain an error and therefore the X,f -data were analyzed using the nonlinear least squares method that takes errors in all variables into account (17). Molecular weights were determined by SEC at 40°C using tetrahydrofuran as solvent and polystyrene standards from Polymer Laboratories. Poly(butyl acrylate) molecular weights were corrected via the universal calibration principle using Mark-Houwink parameters (18), while the molecular weights of the copolymers were calculated relative to polystyrene standards. s

s

Results and Discussion

Homopolymerizations ofstyrene and butyl acrylate: In figure 1 and 2, the kinetic plots are depicted for the homopolymerization of butyl acrylate and styrene, respectively.



200

5000

20000

Time (s)

Figure 1. Kinetic plot for the polymerization of Β A in xylene. [BA] M, [Cu]=0.0168 M [EiB]=0.0222M at 110°C. Dotted line is the be through the experimental data according to equation 1. t

120000

Time, [s]

Figure 2. Kinetic plot for the polymerization of S in xylene. [SJ-4 [Cu]=0.0153 M, [EiB]=0.0434 M at 110°C. Dotted line is the through the experimental data according to equation 1.

273

When the -ln(l-X) data is fitted against t , we get a very good estimation of the set of constants (A) in equation 1. For Β A, a value of 1.67-10 s' is obtained, while for S this value is 1.53-10" s~ . When we take the difference in k into account (about 1600 Lnnoll^s" for S and 76000 Lunol'^s" ) together with the concentrations of copper complex and initiator and we assume that k is 5 · 1 0 L mol' s" in both cases, we obtain a value of the equilibrium constant K of 5.85· 10" for S and 1.26·10" for BA. For S, this value is an order of magnitude smaller than reported in literature (16). The difference can be found in the fact that we used another initiator and that the literature value was obtained for a bulk system. The first factor in particular is known to have -3

2/3

4

2/3

1

p

1

8

e

Ie

1

t

10

a

π



201 a significant influence on the course of polymerization (19,20). Furthermore, in their analysis Matyjaszewski et al ignored the persistent radical effect (14) and calculated the activation/deactivation equilibrium constant assuming a constant deactivator concentration. The value for BA is almost 50 times lower than that of S, which is very large compared to the results of Arehart (12). It must be noted, however, that their value for the equilibrium constant of Β A was obtained at 100°C. One of the drawbacks of the ATRP system is its oxygen sensitivity. Unlike conventional radical polymerization, where a small amount of oxygen in the reaction mixture is eliminated due to the constant production of radicals, a small amount of oxygen affects the whole course of reaction (21). Oxygen can react with the C u complex in a simple redox reaction to yield a C u complex that is no longer able to activate the dormant species. Moreover, oxygen can also trap the radicals originating from the dormant species, although the concentration of the latter is much lower than the Cu concentration. In both cases, the [Cu ] will decrease and, as a result, the reaction rate will drop. This is illustrated by figure 3, where a small amount of oxygen is introduced by a syringe during the course of reaction. +

2+

+

+

Time, [s]

Figure 3. Kinetic plots for two polymerizations of BA in xylene. Λ Wi oxygen introduced during the reaction ([BA]=3.78 M, [Cu]=0.0187 [EiB]=0.0186 Μ), Ο without oxygen ([BA]=3.78 M, [Cu]=0.0168 M [EiB]=0.0222 M) at 110°C It can clearly be seen that in the beginning both reactions proceed at the same rate. From the fifth data point on, however, the rate of the reaction where oxygen is introduced is drastically decreased. Even a minor amount of oxygen in the system will hamper any kinetic investigation on ATRP polymerizations. When the data from the fifth point on is fitted with equation 1, taking only into account the decreased [Cu ], this leads to an estimation of the set of constants equaling 1.94·10" s" , while the reaction without oxygen yielded a constant of 1.69*10 s' . This means that the concentration of Cu has decreased with a factor of about 650. This illustrates that a small amount of oxygen, e.g. a contaminated +

4

3

23

m

+


202 syringe, will slow down the reaction rate to a very large extent. As well as in conventional free-radical polymerization, radicals are involved in ATRP and therefore also termination takes place. The amount of termination is dependent on the equilibrium constant in scheme 1. When K is large, this means that more radicals are present in the steady state situation and as a consequence, more termination will take place. We have monitored the amount of termination that takes place in an ATRP of S by looking at the spectra in *H NMR. Attention is focused on the signals of the protons residing in the initiator fragment and the proton at the end of the dormant species chain, see figure 4. Downloaded by UCSF LIB CKM RSCS MGMT on August 24, 2014 | http://pubs.acs.org Publication Date: August 15, 2000 | doi: 10.1021/bk-2000-0768.ch014

a

1

1J ι 10

I

1 9

1 8

1 7

1 6

1

5 δ (ppm)

1 4

3

ϋ

1 2

1 1

1 0

l

Figure 4. H NMR spectrum of a PS dormant species. The peak at ppm originates from proton at the end of the polymer chain, whi peak at S=3.6 ppm comes from the two protons in the initiator fr 1.0

-3

0.2

0.3

0.4

0.5

Fractional conversion

Figure 5. Fraction of chains functionalised with Br as a functi conversion for a polymerization of S at 110°C in xylene. [SJ=4. [Cu]=0.0314M, [EiB]=0J61 M.


The ratio of the initiator and the end-group peaks in the *H NMR spectrum are a measure for the fraction of chains that can still be activated since they still contain a bromine atom at the end. In figure 5, the fraction of chains that are still functionalised with Br is plotted for samples taken from a S ATRP at different conversions. In the beginning of the reaction, we see that the fraction of chains with a bromine end group has already decreased from 1 to about 0.85. This is logical, since in the beginning of the reaction there is not enough C u present yet and the bimolecular termination of the radicals is still competing with the deactivation reaction. When the persistent radical concentration has been built up, the fraction of terminated chains stays at a relatively constant level. However, the fraction of living chains is lower than reported by Matyjaszewski et al (22), who reported a steady-state concentration of C u of 4-6% relative to the initial Cu concentration. The origin of this phenomenon is currently investigated in our laboratories. It has to be noted, however, that care has to be taken when interpreting the NMR data, since the error in the peak areas can be up to 5%. Both polystyrene (PS, M =1585 g-mol" ) and poly(butyl acrylate) (PBA, M =2200 g-mol" ) were prepared and the PS was used to determine the activation rate parameter, k , with the use of the method described earlier employing equation 2. In figure 6, the SEC chromatograms are shown for the PS dormant species. PS was therefore dissolved in MMA, together with a known amount of copper complex and a small amount of cumyl hydroperoxide (CHP) to enhance the SEC resolution. Note that although the reaction was carried out at 110°C, the reaction mixture did not boil due to the high polymer concentration.


2+

2+

+

1

n

1

n

act

1750 -,

1

1500 H

I , , , , , , , , , , , , , , , 24

26

28

30

32

34

36

V,[mm]

Figure 6. SEC chromatograms of PS in order to determine k . [PS]=0.0089 M, [Cu]=0.0053 M [CHP]=0A088 M in MMA at 110°C. act

t

From the SEC chromatograms and the concentration of the copper complex, the activation rate parameter for S could be detennined. Note that the chromatograms in figure 6 are obtained with a DRI-detector and that they have been corrected for differences in dn/dc, normalized and subsequently scaled with conversion. The decrease of the macroinitiator peak at V =33 min was monitored as a function of time. It is clear that e


204 the radicals originating from this macroinitiator are trapped by the radicals originating from CHP initiation and yield polymer that is well separated from the macroinitiator in SEC. For PS, k has a value of 0.03 (± 0.003) L-mol" ^" . This is much lower than the 0.45 L-mol" ^" that has been found by Fukuda et al (15)., who determined this parameter in styrene. There is evidence that solvent can have a significant influence on the activation process, which could explain the discrepancy between our results and those reported in literature. We are currently verifying this by using a different and independent method (23). The PS and PB A have also been used to make block copolymers. For chain extension of PS with BA the molecular weight as a function of conversion is plotted in figure 7, and a comparison of the calculated copolymer composition with the experimentally obtained is plotted in figure 8. act


1

1

1

1

12000

looooH

0.1

0.2

0.3

0.4

0.5

Fractional conversion of Β A

Figure 7. M as a function of conversion for a chain extension ofP BA in xylene at 110°C. Solid line: theoretical prediction; dotted li through data. [BA]=3.44M, [Cu]=0.032 M, [PS]=0.042 M. n

0.2

0.4

0.6

0.8

Fractional conversion of Β A

Figure 8. Fraction of S in the block copolymers as a function conversion. Ο Expected from conversion data, + determined by N spectroscopy. Conditions as infigure7.


205 From figure 7, it is clear that the molecular weight grows linearly with conversion. It is important to mention that these molecular weights are relative to polystyrene standards and that differences in detector responses have not been taken into account. This explains why the molecular weights of the block copolymers are systematically higher than theoretically expected. Figure 8 shows that the experimentally determined fraction of styrene in the block copolymer is in good agreement with the expected fraction of styrene in the block copolymer, calculated from conversion data.


Copolymerization of styrene and butyl acrylate: The copolymerization kinetics for an ATRP system are completely analogous to the free-radical copolymerization kinetics (24). For the determination of the reactivity ratios the differential copolymer composition equation can be derived: 2

nf +ff

(3)

2

nf

^^ffi+hfi

By fitting of the copolymer composition at low conversion data as a function of initial monomer feed composition, the reactivity ratios can be derived from this equation. However, when performing ATRP experiments, we are not able to determine copolymer composition at low conversion, since the polymer chains are growing throughout the reaction. It is therefore necessary to make use of the integrated form of the copolymer equation (25): l-f;

1-/

X = l-

fo

(4)

f-s

.l-/ej

Where δ is a function of the reactivity ratios as well:

δ=.

,

(5)

The integrated copolymerization equation can also be used to estimate reactivity ratios (26,27) and has the advantage over the differential copolymerization equation that it can be applied up to high conversion as well. With gas chromatography or on-line techniques as Raman spectroscopy, the monomer feed ratio is determined at various conversions. Fitting equation 4 to these data with nonlinear least squares parameter estimation yields estimations of the reactivity ratios, η and r . 2



206

1

0.0

0.5

1.0

1.5

2.0

2.5

'

3.0

1

3.5

'

1

4.0

'

!

4.5

1

Γ

5.0

5.5

Figure 9. 95% Joint confidence intervals for four ATRP reactions andBAat 1I0°C —f =0.807, fs^0.691, •-•f =0.397, —f =0.2 s

s

s

Four copolymerizations have been carried out at different initial monomer feed ratios. The initial monomer feed ratios were chosen where a large composition drift was expected, i.e. at f =0.255, 0.397, 0.691 and 0.807. As can be seen from figure 9, the shape of the joint confidence intervals is greatly dependent on the initial monomer feed composition. At high f , the r is well-determined, but the error in r A is large. When applying a low fraction of styrene in the monomer feed, the situation is exactly the opposite. To obtain a reliable joint confidence interval for both reactivity ratios, the sum of the squared residuals spaces of experiments at different initial monomer feed ratios must be combined. The combined sum of squared residuals space can be visualized, leading to the following estimation of reactivity ratios: s

s

B

s

0.25-r

0.20-

0.15

Atom Transfer Radical Copolymerization of Styrene and Butyl Acrylate

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