New Concepts for Controlled Radical Polymerization: The DPE System

Chapter 42 New Concepts for Controlled Radical Polymerization: The DPE System 1

1,

2

2

P . C. Wieland , O. Nuyken *, Y. Heischkel ,B.Raether , Downloaded by UNIV LAVAL on July 11, 2016 | http://pubs.acs.org Publication Date: June 26, 2003 | doi: 10.1021/bk-2003-0854.ch042

and C. Strissel

1

1Lehrstuhl für Makromolekulare Stoffe, Technical University of M ü n c h e n , D-85747 Garching, Germany B A S F AG, Ludwigshafen, Germany 2

The DPE-System is a new, versatile method for controlled radical polymerization which consists of a conventional radical polymerization system where a small amount of 1,1diphenylethylene (DPE) is added. It allows the facile synthesis of block copolymers with numerous industrial important monomers. The mechanism is believed to involve the combination of two stable diphenylmethylradicals to a thermolabile quinoid unit as a key step. The exact mechanism is not clear in all details yet, but some results shown in this article give good evidence on the suggested mechanism.

Introduction The synthesis of new block copolymer structures is of great interest in modern polymer chemistry. Next to ionic polymerization techniques, which require highly pure monomers and solvents, living radical polymerization has been established since several years for the synthesis of all kind of polymer architectures. Atom transfer radical polymerization (ATRP) (7-5), nitroxide mediated polymerization (NMP) (4-6) or the reversible addition fragmentation transfer (RAFT) (7-P) process are well known techniques in this field. Additionally, degenerative transfer with alkyl iodides is a potential method for controlled radical polymerization (10). Nevertheless, no broad industrial

© 2003 American Chemical Society

Matyjaszewski; Advances in Controlled/Living Radical Polymerization ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

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Downloaded by UNIV LAVAL on July 11, 2016 | http://pubs.acs.org Publication Date: June 26, 2003 | doi: 10.1021/bk-2003-0854.ch042

These are the use of toxic metal catalysts, an unsatisfactory universality or the formation of thiols during the polymerization. Due to this problem, there are extensive investigations for new systems which are free of these limitations. Recently, we reported a new additive for controlled radical polymerization , which is especially suitable for an easy synthesis of block copolymers and free of the above mentioned disadvantages (11,12). Addition of 1,1-diphenylethylene (DPE) to a conventional radical polymerization system changes the polymerization characteristics into a controlled like system. Furthermore, the polymers which were synthesized in the presence of DPE are able to initiate the polymerization of a second monomer at elevated temperatures which leads to block copolymers.

The DPE-System In general, the DPE-system consists of a conventional system for free radical polymerization where a small amount of 1,1-diphenylethylene (DPE) is added. The amount of DPE is typically in the range of 0.3 mol-% in respect to the monomer and in a 1:1 mole-ratio to the initiator. Suitable initiators are azo isobutyronitrile (AIBN) or benzoyl peroxide (BPO). The polymers which were synthesized in the presence of DPE can be reinitiated at elevated temperatures from 70 to 110 °C. If the re-initiation is carried out in the presence of a second monomer block copolymers are obtained in good yields. There is almost no limitation in the choice of monomers. Styrenes, acrylates, methacrylates N-vinylmonomers etc. can be polymerized and combined to block copolymers. The polymerization can be carried out in water-based systems like emulsion or suspension polymerization systems (13), in solution and in bulk.

Homopolymerization The DPE system shows characteristics which are different from a free radical polymerization. Figure 1 shows the time-conversion plot for the bulk polymerization of styrene at 80 °C with and without DPE. The DPE-free polymerization was performed with A I B N as initiator in the same amount as in the polymerization in presence of DPE. A retarding effect on the conversion is observed by adding a small amount of DPE in comparison to the free radical polymerization. Nevertheless, the conversions are rather good in the given time compared to other systems e.g. like the 2,2,6,6-tetramethyl-piperidin-l-oxyl (TEMPO) controlled polymerization (4). Figure 2 shows the molecular weight and the polydispersity index (PDI) versus conversion of the bulk polymerization of styrene with and without DPE. The DPE controlled polymerization shows a slightly increasing molecular weight with conversion after a short period of uncontrolled polymerization. During this period polymers with a relatively high molecular weight are formed. The DPE free polymerization shows the expected behavior. The increase of the molecular weight at higher conversion is due to the increasing viscosity of the system and the higher probability for radical combination. Matyjaszewski; Advances in Controlled/Living Radical Polymerization ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

621


60

Ί

time/min

Figure 1: Time-conversion plot of the radical bulk polymerization ofstyrene in the presence of DPE and without DPE at 80 °C; conc.(AIBN) = 0.3 mol-%, conc.(DPE) = 0.3 mol-%.

Figure 2: Plot of molecular weight and PDI versus conversion for the radical bulk polymerization ofstyrene in the presence of DPE and without DPE at 80 °C; conc.(AIBN) = 0.3 mol-%, conc.(DPE) = 0.3 mol-%.



622 The PDI of the polymers is clearly lower if the polymerization is carried out in the presence of DPE than for the DPE free system. That means that the number of termination reactions is decreased by the added DPE which is important evidence for control during the polymerization process. Additionally, we investigated the temperature dependence of the polymerization of methyl methacrylate in the presence of 0.3 mol-% DPE with 0.3 mol-% A I B N as initiator. The polymerization was performed in toluene to avoid viscosity effects. Figure 3 shows the time conversion plot in a temperature range of 70 to 90 °C. The conversion increases clearly with increasing temperature but is always in a good range from 40 to 75 %. Figure 4 shows the plot of the molecular weight and the PDI with the conversion for the different temperatures. The molecular weight shows no increase at 70 °C and 80 °C. In contrast, there is an increase of the molecular weight versus the conversion at 90 °C. This effect can be ascribed to a more frequent exchange between a dormant and an active species due to higher temperature. Apparently, lower temperatures are not high enough for an effective exchange between the two species. The PDI in the range between 1.5 and 2.0 is much smaller than in common free radical polymerization at all temperatures for a monomer with such a strong transfer tendency as methyl methacrylate. This indicates again the decrease of termination and transfer reactions due to the addition of DPE.

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go °C 80 °C 70 °C

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—ι—·—ι—>—ι—•—ι—»—ι—ι—ι—«—ι—·—ι—«—ι—»—I 50 100 150 200 250 300 350 400 450 500

time / min Figure 3: Time-conversion plot of the radical polymenzation of methyl methacrylate in the presence of DPE at different temperatures; cone.(AIBN) =0.3 mol-%, cone.(DPE) = 0.3 mol-%.


623 30000 -

-95 53 58 65

624 Obviously, the block copolymer yield Y changes with the monomer type of Pi from 50 % for styrene to >95 % for methyl methacrylate. The reason for this result is not clear yet but the investigation of the mechanism of the polymerization will open ways for improvements especially of the block yield. Next to common monomers like styrene or (meth)acrylates it is also possible to synthesize block copolymers with functional monomers like 4chloromethyl styrene. This result is interesting for the synthesis of new macroinitiators for numerous polymerization techniques like cationic polymerization or ATRP (14-17).


Mechanism The emphasis of our investigations is to clear the mechanism of this polymerization type which is important for further improvements of this system. Figure 5 shows how we suggest that the polymerization and the reinitiation occurs. The first step is the formation of an active chain end ? { by common radical initiation. P,' reacts with a DPE molecule under formation of a stabilized diphenylmethyl radical P \ This radical can be seen as a dormant species which can avoid transfer reactions. If the formation of P ' is reversible all needs for an equilibrium between active and dormant species are given. This equilibrium is required for all controlled polymerizations. According to our view, the final dormant species is built by combination of two P * radicals to a quinone like structure as have been shown for other hindered tetraphenyl ethanes (18,19). Such units are thermolabile, as has been shown by different authors (20-22). Furthermore, it is also possible to get combination between P ' and P, under formation of a triphenylmethane unit which should also be thermolabile. If the polymers formed by combination of P * and P " are heated in the presence of a second monomer, they split into two P ' radicals. The P * radicals split into P,* and free DPE. P," can initiate the polymerization of the second monomer whereby block copolymers are formed. 2

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2

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Proof of the mechanism The proof of this mechanism is under intensive investigation by the synthesis of model compounds and the investigation of their initiator properties. Furthermore, spectroscopic investigations can help to clear the exact mechanism. Figure 6 shows a part of the H - N M R spectrum of low molecular weight poly(methyl methacrylate) with M = 4450 g/mol which has been synthesized in the presence of DPE. Next to the signals for the aromatic protons are four broad signals at 6.70 ppm, 6.25 ppm, 5.90 ppm and 5.65 ppm detectable. These signals are characteristic for quinoid protons as we suggest them for the combination of two P * radicals in Figure 5. !

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+ M

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2

Ph Ph

.Ph Ρ* + H C=C Ph 2

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Block copolymer

Figure 5: Suggested mechanism of the radical polymerization with small amounts of DPE.

The signals at 5.45 ppm and 6.20 ppm can be attributed to olefinic endgroups in poly(methyl methacrylate) as the result of disproportionation reactions. Signals in the same range of the spectrum can be detected i f the combination product of A I B N and DPE is investigated by ' H - N M R spectroscopy. The reaction product between these two compounds after 14 h reaction time at 80 °C is clearly the quinoid structure as shown in Figure 7. This is a clear indication for the presence of such groups in the re-initiatable polymers.



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Figure 6. Part of the H-NMR spectrum ofpoly(methyl methacrylate) (M = 4450 g/mol) which has been polymerized in the presence ofDPE. n

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Figure 7. Combination product of the reaction between AIBN and DPE.



627 Next to spectroscopic investigations we studied the thermal behavior of the DPE-containing polymers. We heated DPE-containing poly(methyl methacrylate) in the presence of radical scavengers to 85 °C for 3 h and observed i f a decrease of the molecular weight occurs. We have chosen the two radical scavengers 2,2,6,6-tetramethyl-piperidin-l-oxyl (TEMPO) and galvinoxyl. Both are excellent radical scavengers for alkyl radicals which are widely used in the investigation of radical reactions. The results are shown in Figure 8. It is clearly indicated that the polymer chains break at the chosen temperatures and the resulting radicals are capped by the radical scavengers resulting in a decrease of the molecular weight of ca. 20 %. That supports our view of the incorporation of DPE in the polymer cain under formation of a thermolabile unit. We repeated this experiment under the same conditions with poly(methyl methacrylate) which has been synthesized by conventional free radical polymerization with A I B N as initiator. The results are shown in Figure 9. For this polymer no decrease of the molecular weight occurs under the chosen conditions. That means that DPE is clearly responsible for the cleavage of the polymer chains at elevated temperature.

Conclusions It has been shown that the DPE-system is a new, versatile method for controlled radical polymerization which is especially useful for a facile synthesis of numerous block copolymers under mild conditions. The polymerization control is not as good as in the established systems like ATRP, N M P or RAFT, but the versatility of the DPE system makes it interesting especially for the synthesis of block copolymers even on a industrial scale. The exact mechanism is not clear in all details yet, but some results give a good indication on what occurs during the polymerization. We suggest that the mechanism is probably based on the reversible addition of DPE on the growing chain ends under formation of a stabilized diphenylmethyl radical. This radical has the function of a dormant species. Two diphenylmethyl radicals can combine under formation of a thermolabile quinoid unit. Evidences of such quinoid units have been found in the Ή-ΝΙνΠΙ spectra of low molecular weight DPE-containing poly(methyl methacrylate). Furthermore, we observed a decrease of the molecular weight of poly(methyl methacrylate) which has been synthesized in the presence of DPE i f these polymers are heated in the presence of radical scavengers. This indicates clearly that the groups which are responsible for the reinitiation are incorporated in the polymer chain.


a] before heating (M =23500 g/mol) after heating (M„=1850Q g/mol)


n

Galvinoxyl b) before heating (M « 26100 g/mol) after heating (M„ = 20900 g/mol) n

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1

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elution time/mm

\ure 8. Decrease of the molecular weight of DPE-containing polyfmethyl methacrylate) in presence of radical scavengers at 85 °C.


629 before heating - after heating

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Figure 9. Behavior of the molecular weight ofpoly(methyl methacrylate) after heating the polymer to 85 °Cfor 3 h.


630


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New Concepts for Controlled Radical Polymerization: The DPE System

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