1 Professur für Makromolekulare Chemie Wahlpraktikum MC für Bachelor Chemie
Free and controlled Radical Polymerization
1. Task In this experiment the basics of free and controlled radical polymerization will be studied. One of the most widely used monomers methyl methacrylate (MMA) (Fig. 1) will be polymerized by both free and radical polymerization. The characteristics of the yielded polymers, such as chemical structure, molar mass and dispersity will be studied and compared.
Figure 1: Chemical formula of MMA.
2. Motivation The free radical polymerization is an important industrial technique to produce various plastic materials on a large scale. A lot of developments in macromolecular chemistry are based on this technique and it should be known by every polymer chemist. [1], [2] The controlled radical polymerization is a further development of this popular technic and normally used to produce defined polymers with adjustable properties. [2]
3. Introduction 3.1 Free radical polymerization (FRP)
The free radical polymerization is a typical chain growth reaction and contains three steps: initiation, propagation and termination, which are described briefly below. [3]
During the initiation, an active center (radical species) is formed, from which a polymer chain is generated. In most cases this is achieved by thermal decomposition 2 Professur für Makromolekulare Chemie Wahlpraktikum MC für Bachelor Chemie
of a suitable initiator. Alternatively, redox systems can be used as initiators. The radicals can be formed by thermal excitation or ultraviolet (UV) radiation, the rate of radical formation depends on various factors such as temperature and solvent. [3]
By repeated addition of monomer to the active chain end and transmission of radicals to the most recently accreted monomer it comes to propagation, where the polymer continuously increases its chain length. For the free radical polymerization, the propagation will be stopped when there is no monomer added or until termination occurs.
Termination of the polymerization can be processed by recombination or disproportionation of the active radical chain end, by which the active chain end is destroyed. Alternatively the chain transfer with solvent, monomer, initiator or the polymer itself can influence the growth of the chain. The FRP can be carried out in bulk monomer, solvent or as a suspension or emulsion polymerization. [1], [3] Due to the high reactivity and low selectivity of the radical chain end and thus related side reactions, the FRP usually results in polymers with broad molar mass distribution (Ð > 2). It is hardly impossible to control the architecture and molecular weight of the polymers, and the maximum yield molar weight is always limited. This procedure is suitable for industrial productions in large scale, but not ideal for a variety of applications, such as biomedicine, where defined polymers with defined properties are required. In this case, the controlled radical polymerization is utilized. [1]
3.2 Controlled radical polymerization (CRP) The controlled radical polymerization is based on the principle that the concentration of the active specie was reduced by the conversion to an inactive or “dormant” specie and thus suppress the chain growth and lead to termination. Depending on the kinds of “dormant” species, the controlled radical polymerization can be classified into three methods: Atom Transfer Radical Polymerization (ATRP), Reversible Addition and Fragmentation Chain Transfer Polymerization (RAFT) and Nitroxy-Mediated Radical Polymerisation (NMP). [4] In this experiment the most widely used ATRP will be utilized. The atom transfer polymerization was discovered by the group of Matyjaszewski in 1995 and has been proved to be a very powerful controlled/“living” free-radical polymerization (LRP) technique for the preparation of polymers. [5] The 3 Professur für Makromolekulare Chemie Wahlpraktikum MC für Bachelor Chemie
mechanism of ATRP is based on the balance between propagating radicals and an inactive "dormant" species, mainly alkyl halides. [4] Periodic reactions of the "dormant"
species (PnX) with a transition metal complex as an activator in its lower oxidation m state (Mt /L; m is oxidation state, L is ligand) result in the active radical species (Pn •) and a deactivating transition metal complex in its higher oxidation state, coordinated with the halide as ligand (X-Mtm+1/L) (Fig. 2).
Figure 2: Basic mechanism of ATRP (reproduced from [5]).
This reversible process rapidly forms an equilibrium that is predominately shifted to the side with very low radical concentrations. The number of polymer chains is determined by the number of initiators. Each growing chain has the same probability to propagate with monomers to form living/dormant polymer chains. As a result, polymers with similar molecular weights and narrow molecular weight distribution can be prepared. ATRP reactions are very robust. They are tolerant aganist many functional groups like allyl, amino, epoxy, hydroxy and vinyl groups present in either the monomer or the initiator.[5] ATRP methods are also advantageous due to the ease of preparation, commercially available and inexpensive catalysts (copper complexes), pyridine based ligands and initiators (alkyl halides). [4], [5]
４. Experiments
4.1 Chemicals
All the chemicals are bought from Sigma Aldrich or Acros Organics and used as received, if there is no additional indication. The MMA will be purified by passing through an aluminum oxide column to remove the inhibitor. A short summary of the used chemicals are listed in Tab. 1.
4 Professur für Makromolekulare Chemie Wahlpraktikum MC für Bachelor Chemie
Table 1: Used Chemicals
Chemicals Abbreviation Properties
Azo-bis-isobutyronitril AIBN M = 164.21 g/mol; solid
Methyl methacrylate MMA M = 100.12 g/mol; liquid;
ρ = 0.944 g/cm3
Ethyl-2-bromoisobutyrat EBIB M = 195.05 g/mol;
ρ = 1.329 g/cm3
Copper(I) Bromide CuBr M = 143.45 g/mol; solid
Copper(II) Bromide CuBr2 M = 223.35 g/mol; solid
2,2‘-Bipyridin bpy M = 156.19 g/mol; solid
4.2 Free radical polymerization of MMA
In a test tube with stirring bar 16.4 mg AIBN (0.1 mmol) are weighed and dissolved in 2.19 ml MMA (0.02 mol). The test tube will be sealed with a septum and degassed by nitrogen for 20 min. Afterwards the test tube will be put into an oil bath and the reaction will be performed under 60 ℃ for 45 min. After the reaction ends and cooled down to room temperature, the yielded polymers are precipitated in an excess amount of methanol. The methanol is then removed by decantation and the products are dried under vacuum overnight. If the polymerization solution is too viscous, it can be diluted by adding 1 - 2 ml toluene. Afterwards ~10 mg yield polymers will be
dissolved in 0.7 ml CDCl3 and analyzed by Nuclear Magnetic Resonance (NMR).
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4.3 ATRP of MMA
In a test tube 1.10 ml MMA (0.01 mol, 10 eq) are weighed and dissolved in 2 ml solution of 5 mM EBIB (0.01 mmol, 1 eq) in dimethyl sulphoxide (DMSO). The test tube is sealed by a septum and degassed with nitrogen for 30 min. In another test tube with stirring bar, 7.5 mg (0.05 mmol, 0.05 eq) CuBr, 2.23 mg (0.01 mmol, 0.01
eq) CuBr2 and 34.4 mg (0.22 mmol, 0.22 eq) bpy are weighed and sealed with a septum and carefully flushed with nitrogen. Afterwards the purged solution is transferred carefully into the test tube with catalyst and ligand. The reaction is performed under room temperature with stirring for 2 h. After reaction the catalyst is removed by passing the solution through an aluminum oxide column. The resulting polymer is perticipatied in excess amount methanol (~20 ml) and dried overnight . Afterwards ~10 mg product are dissolved in 0.7 mL DMSO-d6 and analyzed by NMR
Questions:
1. What are the basic steps of free radical polymerization? 2. What is the difference between free radical and controlled/living radical polymerization? Name the advantages and disadvantages of both polymerizations. 3. Name some methods of controlled radical polymerization. 4. Simply describe the mechanisms of ATRP. 5. Give some applications of the PMMA. Why is this polymer so popular? 6. What are polymer brushes? How can they be synthesized (reminder: two routes)? 7. How to characterize the molar mass and distribution of the yielded polymers?
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Literature:
[1] Elias, H. G.; Makromoleküle, Bd.1. Chemische Struktur und Synthesen, 1999, sechste vollständige und erweiterte Auflage, WILEY-VCH.
[2] Elias, H. G.; Makromoleküle, Bd.3. Industrielle Polymere und Synthesen, 2001, sechste vollständige und erweiterte Auflage, WILEY-VCH.
[3] Braun, D.; Chedron, H.; Ritter, H.; Praktikum der Makromolekularen Stoffe – Grundlagen, Synthesen, Modifizierung, Charakterisierung, 1999, WILEY-VCH.
[4] Matyjaszewski, K.; Xia, J.; Chemical Reviews, 2001, 101, 2921 – 2990.
[5] Wang, J.; Matyjaszewski, K., J. Am. Chem. SOC., 1995, 117, 5614-5615.