Living Radical Polymerization - American Chemical Society

Chapter 19

Downloaded by CORNELL UNIV on August 8, 2012 | http://pubs.acs.org Publication Date: September 7, 2006 | doi: 10.1021/bk-2006-0944.ch019

Synthesis of Inorganic-Polymer Nanocomposites by Atom Transfer Radical Polymerization Using Initiators or Polymerizable Groups Attached to Transition Metals via Bidentate Ligands Guido Kickelbick, Dieter Holzinger, Dieter Rutzinger, and Sorin Ivanovici Institute of Materials Chemistry, Vienna University of Technology, Getreidemarkt 9, A1060 Vienna, Austria

Metal alkoxides were functionalized by bidentate ligands based on β-diketones or acetoacetoxy-derivatives. The resulting complexes either carried initiator or polymerizable groups for ATRP. In addition these compounds were used as precursors for the sol-gel process. Depending on the functional group attached to the metal and the further processing of the derived compounds core-shell nanoparticles or crosslinked materials were obtained. Atom transfer radical polymerization proved that it is a valuable tool for the controlled formation of inorganic-organic hybrid materials.

© 2006 American Chemical Society

In Controlled/Living Radical Polymerization; Matyjaszewski, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 2006.

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270


Introduction Inorganic-organic nanocomposites have currently attracted much scientific interest due to the combination of two moieties in one material and their resulting properties.(l, 2) The properties of the resulting nanocomposites are often distinguished by physical size effects and the large surface to volume ratio of the inorganic building blocks. In many cases free radical polymerizations were used for the preparation of these materials, such as cluster-reinforced polymers or nanoparticle embedded systems.(3, 4) The major advantage of radical polymerization compared to other techniques is its robustness against various reaction conditions (temperature, purity, solvents). One of the drawbacks is the lack of structural control of the synthesized polymers. The development of controlled living polymerization techniques, which allow a good control over composition and morphology of organic polymers, opened new ways for a tailored preparation of inorganic-organic nanocomposites.(5) In particular the functionalization of inorganic surfaces by well-defined organic polymers applying grafting from or grafting to techniques established as a important domain of ATRP.(6-8) Various inorganic building blocks can be used for the incorporation into polymers (Fig. 1). Different types of materials are produced depending on the function (polymerizable or initating groups) and the number of functionalities (linear, crosslinked or star shaped polymers) attached to the inorganic compounds. Probably the most prominent representative of these building blocks are polyhedral oligomeric silsesquioxanes (POSS) or spherosilicates which both have a rigid inorganic silicon-based cube as the major structural element in common, which can be functionalized by organic groups forming for example macroinitiators or monomers for ATRP.(9-12) Depending on their number of polymerizable groups the inorganic building blocks resulted in crosslinked polymers or polymer chains containing pending inorganic molecules. If these systems were used as initiators either end-functionalized or star polymers were obtained. The organic functionalization of the inorganic components occurred either in situ during the preparation of the building blocks, for example in the case of POSS-based compunds, or via a post-functionalization. In most cases the chemical connection between organic group and the inorganic segment was formed using silane coupling agents of the general formula R^SiXn (n = 1-3, X = CI or OR) yieding to strong covalent bonds that connect the functional organic group to the inorganic core. Similar interactions can also be applied for silica or silsesquioxane nanoparticles.(13-15) Contrary, in the case of metal oxides often other interaction mechanisms have to be used, for example coordinative interactions, due to the lack of reactivity of surface M-OH groups, which are necessary for a covalent attachment of silane coupling agents. In previous studies we showed that carboxylate surface-functionalization of metal oxo clusters can



271 be used to attach initiator or polymerizable groups to the surface of these inorganic species.(16-18) In all mentioned studies it was not intended that the inorganic part of the molecule undergoes further reactions but forms a rigid and stable building block. A possibile incorporation of reactive inorganic groups would, however, open the way for the preparation of a second crosslinked network, for example applying the sol-gel process. Compounds containing both an organic group that allows ATRP polymerization and an inorganic group for the formation of a metal oxide newtork are particularly interesting because two totally different polymerization methods can be applied under dissimilar conditions providing an additional parameter to control the structure of the resulting nanocomposites. In the present study we show how /?-diketones and acetoacetoxy-derivatives containing polymerizable or initiator groups can be used to coordinatively bind early transition metal alkoxides that can be used as precursors for sol-gel reactions and thus the formation of metal oxide/polymer nanocomposites.

Results and Discussions β-Diketones (Scheme 1) are well-known for their coordination chemistry of transition metals. These ligands are also often used to limit reactivity of metal alkoxides in the sol-gel process by blocking free coordination sites. In addition, usually the coordination of these ligands survives the conditions of the sol-gel process and thus they allow an organic modification of metal oxides prepared by this method. Preparation of bidentate ligand coordinated metal alkoxides can occur, for example, by simple ligand exchange reactions.(19, 20) If a stable linkage between the sol-gel precursor and the organic polymer should be formed either polymerizable or initiating groups have to be attached to the diketones. For such functionalization reactions the central position between the two carbonyl groups is prefered because it can be easily deprotonated and reacted with nucleophiles. Applying this approach we introduced various organic groups that can act as initiators.(19) A selection of prepared ligands is shown in Scheme 2. The modification of acetylacetone with polymerizable groups at the same position was much more difficult due to polymerization induced during the reaction with polymerizable group containing synthons. Only small yields of the desired products were obtained and therefore we applied the acetylacetoxy-based systems as described below for a functionalization with polymerizable groups. Coordination via alkoxide exchange was proven for Ti(0 Pr) Zr(O Bu) , Ta(OEt) , Y(OPr) , and VO(OEt) . The resulting bidentate ligand functionalized precursors were used in a microemulsion-based sol-gel approach for the preparation of surface-fiinctionalized nanoparticles. FT-IR analysis of the particles revealed a surface-functionalization with the organic groups. The materials were used as multifunctional initiators in ATRP reactions for the 1

n

4

5

3

3


4


Figure L Well-defined molecular inorganic building block for the preparation of nanocomposites.


273 Scheme 1 Ο

Ο

/^-Diketones R"= OR: Acetoacetoxy-Derivatives


Scheme 2

grafting of polystyrene and poly(methyl methacrylate) from the amorphous metal oxide core (19). The diameter of the polymer shell was easily controlled by the kinetics of ATRP. The polymers were cleaved from the surface of the particles and it was shown that a good control over molecular weight and molecular weight distribution can be achieved. An alternative type of ligand that can be used to connect metal alkoxides to (meth)acrylates are acetoacetoxy derivatives (Scheme 1). In these compounds the functional group is introduced via the ester functionality. A prominent type of these compounds is 2-(methacryloyloxy)ethyl acetoacetate (HAAEMA). These ligands can be used for the preparation of metal containing polymers or to crosslink an organic polymer via a metal or metal oxide species.(21-23) We studied this type of /?-keto ester and its modification with various Ti alkoxides (Ti(O Pr) and Ti(OEt) ). HAAEMA was coordinated to the metal alkoxide via the above mentioned alkoxide exchange reaction. The resulting product is a mixture of monosubstituted dimers in equilibrium with disubstituted monomers. This was proven via NMR and single crystal X-ray structure analysis. A separation of the two different complexes is not possible due to fast exchange reactions. We investigated this mixture in homo- and copolymerizations with MMA. Polymerizations were performed under commonly applied ATRP reaction conditions using CuBr/PMDETA as catalyst, 2-ethylbromoisobutyrate as initiator in solvents such as dichlorobenzene or toluene or in bulk. The temperature of the reactions was 85°C and the reaction time was varied between 4 and 8 hours. The metal containing polymers were not soluble in common solvents, most likely because crosslinking through the metal alkoxides occurred (Scheme 3). Due to fast gelation during polymerization even in presence of a i

4

4



274 solvent we decided to copolymerize the preformed substituted Ti complexes with M M A in 1:1, 1:5 and 1:10 ratios. As model systems we prepared non-metal containing copolymers of HAAEMA and M M A in the same ratios for reasons of comparison. In Figure 2 SEC-plots of the unmodified poly(HAAEMAcoMMA) in the various ratios are presented. A monomodal molecular weight distribution was observed and it could be shown that both of the monomers were successfully incorporated in the backbone by NMR. However, the molecular weight distribution increased with increasing content of M M A (Table 1). One potential reason for this observation is the different reactivity of the two monomers. Polymerization of metal-containing monomers resulted in metal-alkoxide crosslinked polymers. This can be based on two facts, on the one hand already disubstituted monomers were used on the other hand ligand exchange reactions can increase the crosslinking density. Therefore the final polymers could not be analyzed by SEC. Instead C MAS NMR and FT-IR were used to investigate the resulting polymers. A representative FT-IR spectrum of polytTKO'Pr^AAEMAcoMMA] is shown in Figure 3. The band at 1724 cm' corresponds to the C=0 stretching of the ester groups of the methacrylate moieties. The intensity is high corresponding to a higher content of M M A in the copolymer chains. The most important observation is that the linkage between the β-keto ester group and the Ti atom is maintained after the polymerization. This is proven by the presence of the specific bands of the chelating ligand at 1619.81 cm' (C=C) and 1529.38 cm" (C-O). The C-0 bands of the alkoxide moieties are present at 1145.8 cm' while the C H groups give a signal at 750.35 cm" . 13

1

1

1

1

2

1

w

' 1 .

200-.-

Poly(HAAEMAcoMMA) 1:1 Poly(HAAEMAcoMMA) 1:5 Poly(HAAEMAcoMMA) 1:10

Poly(HAAEMA)

150^ 100500-ξ

15 Time (minutes)

Figure 2. SEC-plot ofpoly(HAAEMAcoMMA) copolymers in 1:1, 1:5 and 1:10 monomer ratios.



275

Β


276

Table 1. SEC data for ATRP [CuBr/PMDETA, 2-ethylbromoisobutyrate initiator, in toluene at 85°C]


Polymer

M Ratio between monomers

n

Poly(HAAEMA) Poly(HAAEMAcoMMA) Poly(HAAEMAcoMMA) Poly(HAAEMAcoMMA)

1:1 1:5 1:10

37040 28120 27920 43250

PDI

1.23 1.39 1.82 1.89

DPtheor DPobsSEC

200 100 200 400

175 90 195 360

GO alkoxide ligands C=0 (methacrylate)

1500

1000

Figure 3. FT-IR spectrum of poly[Ti(0'Pr)3AAEMAC0MMA] (1:5 ratio metal complex/MMA)


277


13

C MAS NMR analysis revealed that in the cases of the metal-containing monomers the polymerization is not complete even after very long reaction times. This observation is supported by the detection of the bands corresponding to residual unreacted monomers at 1498 cm and at 968 cm (HC—CH) in the FT-IR spectrum. Most likely residual monomer is still present because ligand exchange reactions lead to a descreased mobility of the polymerizable groups that hinder further chain growth. This was not observed in polymerizations of pure HAAEMA homo- and copolymers where a complete conversion of the double bonds was obtained. With regard to the formation of inorganic-organic nanocomposites the most important feature is that the linkage between the inorganic part and the organic polymer is still maintained after polymerization. The resulting hybrid polymers allowed for the preparation of such materials via hydrolysis and condensation of the metal alkoxides in sol-gel reactions.

Conclusions Metal alkoxides that can be used as precursors in the sol-gel process can easily be modified with functional bidentate ligands such as β-diketones or acetylacetoxy-derivatives. Depending on the functionalization the resulting metal complexes are precursors that contain either initiating or polymerizable groups for ATRP. β-Diketone derivatives were successfully used in the preparation of surface-functionalized amorphous metal oxide nanoparticles which were used as multifunctional initiators for ATRP. These systems allowed a grafting from functionalization from the surface and a good control over the polymer layer diameter. 2-(Methacryloyloxy)ethyl acetoacetate was homopolymerized and copolymerized with M M A by ATRP. This monomer was also used to attach methacrylate groups to titanium alkoxides by alkoxide exchange reactions resulting in a mixture of mono and disubstituted titanium alkoxide complexes. These systems were successfully applied in ATRP reactions leading to crosslinked nanocomposites in which the titanium atoms were coordinatively connected to the organic polymer.

Acknowledgments We gratefully acknowledge the financial support by the Fonds zur Fôrderung der wissenschaftlichen Forschung Austria.


278 References 1. 2. 3.


4. 5. 6. 7.

8.

9. 10. 11.

12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23.

Kickelbick, G. Progr. Polym. Sci. 2002, 28, 83-114. Sanchez, C.; Julian, B.; Belleville, P.; Popall, M . J. Mater. Chem. 2005, 15, 3559-3592. Caseri, W., Nanocomposites of polymers and inorganic particles. In Encyclopedia ofNanoscience and Nanotechnology, Nalwa, H.S., Ed. American Scientific Publishers: Stevenson Ranch, CA, USA, 2004; Vol. 6, pp 235-247. Schubert, U. Chem. Mater. 2001, 13, 3487-3494. Pyun, J.; Matyjaszewski, K. Chem. Mater. 2001, 13, 3436-3448. Kruk, M.; Dufour, B.; Celer, E.B.; Kowalewski, T.; Jaroniec, M.; Matyjaszewski, K. J. Phys. Chem. Β 2005, 109, 9216-9225. Matyjaszewski, K.; Miller, Ρ.J.; Shukla, N.; Immaraporn, B.; Gelman, Α.; Luokala, B.B.; Siclovan, T.M.; Kickelbick, G.; Vallant, T.; Hoffmann, H.; Pakula, T. Macromolecules 1999, 32, 8716-8724. Saleh, N.; Phenrat, T.; Sirk, K.; Dufour, B.; Ok, J.; Sarbu, T.; Matyjaszewski, K.; Tilton, R.D.; Lowry, G.V. Nano Lett. 2005, 5, 24892494. Costa, R.O.R.; Vasconcelos, W.L.; Tamaki, R.; Laine, R.M. Macromolecules 2001, 34, 5398-5407. Holzinger, D.; Kickelbick, G. J. Polym. Sci., Part A: Polym. Chem. 2002, 40, 3858-3872. Ohno, K.; Sugiyama, S.; Koh, K.; Tsujii, Y.; Fukuda, T.; Yamahiro, M.; Oikawa, H.; Yamamoto, Y.; Ootake, N.; Watanabe, K. Macromolecules 2004,37,8517-8522. Pyun, J.; Xia, J.; Matyjaszewski, K. ACS Symp. Ser. 2003, 838(Synthesis and Properties ofSilicones and Silicone-Modified Materials), 273-284. Liu, P.; Tian, J.; Liu, W.; Xue, Q. Polym. Int. 2004, 53, 127-130. Pyun, J.; Matyjaszewski, K.; Kowalewski, T.; Savin, D.; Patterson, G.; Kickelbick, G.; Hüsing, N. J. Am. Chem. Soc. 2001, 123, 9445-9446. Savin, D.A.; Pyun, J.; Patterson, G.D.; Kowalewski, T.; Matyjaszewski, K. J. Polym. Sci., Part B: Polym. Phys. 2002, 40, 2667-2676. Kickelbick, G.; Holzinger, D.; Brick, C.; Trimmel, G.; Moons, E. Chem. Mater. 2002, 14, 4382-4389. Kickelbick, G.; Schubert, U. Chem. Ber. 1997, 130, 473-477. Kickelbick, G.; Schubert, U. Monatsh. Chem. 2001, 132, 13-30. Holzinger, D.; Kickelbick, G. Chem. Mater. 2003, 15, 4944-4948. Zhang, J.; Luo, S.; Gui, L. J. Mater. Sci. 1997, 32, 1469-1472. Gbureck, U.; Probst, J.; Thull, R. J. Sol-Gel Sci. Techn. 2003, 27, 157-162. Grosse-Sommer, Α.; In, M.; Prud'homme, R.K. J. Polym. Sci., Part A: Polym. Chem. 1996, 34, 1447-1453. Miele-Pajot, N.; Hubert-Pfalzgraf, L.G.; Papiernik, R.; Vaissermann, J.; Collier, R. J. Mater. Chem. 1999, 9, 3027-3033.


Living Radical Polymerization - American Chemical Society

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