1.5.1 Block copolymer synthesis
Block copolymer structures are accessible through a variety of “living” and/ or controlled polymerization techniques, including anionic and cationic polymerization, controlled radical polymerization (ATRP, RAFT, NMP) and ring opening (methatesis) polymerization (ROP, ROMP).90,91 All these methods have in common that the ability of the polymer chains to grow is preserved at any time, which in turn opens the principle opportunity to grow a second block from the first one. By using multifunctional initiating sites (grafting-from) or macromonomers (grafting-through), one has further access to brush- or star-like architectures. Additional techniques like post polymerization modification and coupling reactions finally give access to a great variety of block copolymer structures and sequences. In the following, a short introduction to ATRP, anionic polymerization with special focus on epoxide polymerization, and common coupling reactions will be given.
1.5.1.1 Atom transfer radical polymerization (ATRP)
The principle of all controlled radical polymerization procedures is a significant decrease of the concentration of active, propagating radicals compared to free radical polymerization, which in consequence suppresses typical radical involving side reactions like recombination or disproportionation.91-93 In ATRP, this is achieved by keeping the vast majority of
23 propagating chains in a dormant state. (Re-)activation occurs via homolytic cleavage of a functional group from the chain end (mostly a halogen atom), with the help of complexed Cu(I) salts or other transition metal complexes. The general reaction mechanism is shown in Figure 1.8.
Figure 1.8. Reaction mechanism of the atom transfer radical polymerization (ATRP).
The main advantage of ATRP is its insensitivity to a huge number of functional groups, which renders ATRP a very versatile polymerization method. Furthermore, ATRP initiators are easily accessible or even commercially available. Some monomers, however, interfere with the catalyst system, for instance by complexing Cu(I) (e.g. 2-vinylpyridine). In such cases, one has to use strong ligands competing with the complexing monomer.93 Acidic monomers on the other hand, tend to oxidize Cu(I) and/ or protonate the ligand. Such monomers can only be polymerized in a protected state. Other problems occurring in ATRP are inadequate blocking efficiencies and sequence limitations. Propagating acrylate radicals for instance are not able to add methacrylate monomers.94
1.5.1.2 Anionic polymerization
The propagating chain ends in anionic polymerization carry a negative charge, which excludes termination reactions, due to electrostatic repulsion. Consequently, the chain ends stay active at any time. Since system immanent termination reactions cannot take place at all, achievable polydispersities are usually very low and follow a POISON distribution.95 Another advantage is the ease of chain end functionalization, since carb- or oxyanions readily react with a number of compounds such as acids or alkyl halides. A negative aspect is the necessity of a strict purification of monomers and solvents, since carbanions are very sensitive to air and moisture. The choice of polymerizable monomers is limited with respect to radical polymerization techniques and the block sequences are determined by the nucleophilicity of
Introduction
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the active chain ends of the involved monomers. Due to the high basicity of the active chain ends, transfer reactions to solvent molecules or to the monomer might further take place.
The polymerization of epoxide monomers constitutes a special field within anionic polymerization. Monomer addition occurs here simultaneously with a ring opening (Fig.
1.9A). This ring opening is the driving force for the addition, since it causes a release of ring strain. The “living” chain ends are constituted by oxyanions, which are generally more stable than carbanions, but can still be considered as strong bases which are able to react with a variety of acidic compounds. Especially protic solvents should be excluded, since residual water or alcohol molecules can serve as additional initiating sites, due to a proton exchange equilibrium between these molecules and the propagating oxyanion chain ends.96
Figure 1.9. A) Reaction mechanism of the anionic ring opening polymerization of epoxide derivatives; B) Chemical structure of the phosphazene base t-BuP4.
The type of counterion of the propagating oxyanion chain end has a strong impact on the polymerization behavior. Depending on the size and polarizability of the distinct cation used, different degrees of association of the active chain ends occur, which influences the propagation rate and stereoregularity of the resulting polymer.97,98 Li+ is the least favorable counterion since it causes the formation of unreactive associates. Yet, its utilization is a key step towards new block sequences involving certain vinyl and epoxide monomers at the same time.99 The discovery of the phosphazene base t-BuP4 (Fig. 1.9B) as a very effective complexing agent for Li+ finally solved that problem, rendering Li+ a suitable counterion for epoxide polymerization.99,100 The structure of t-BuP4,with the inner amino groups, and the outer non-polar methyl groups, fulfils the criterium for a cryptand-like behavior. The inner free volume of the molecule is suitable for complexing the rather compact Li+-ion. It is noted, that this coordination step consumes a rather long time period as expressed by an induction period at the beginning of the polymerization.99
25 A last crucial point is the occurrence of transfer reactions in the anionic polymerization of ethylene oxide derivatives of the general structure depicted in Figure 1.10, which limits the maximum achievable molar masses.101 So far, no suitable anionic initiating system based on alkoxides is known leading to high molar mass polyglycidol derivatives.
Figure 1.10. Mechanisms of transfer reactions occurring during the anionic ring opening polymerization of epoxide derivatives using alkoxide initiators.
1.5.1.3 Coupling methods
As pointed out, common polymerization techniques for the synthesis of block copolymers come along with several restrictions, limiting the possibilities with respect to block architectures and sequences. To overcome such limitations, it might be suitable to first synthesize a variety of polymer precursors and couple them subsequently, using suitable coupling agents. Figure 1.11 shows linking reactions which are commonly used for such purposes.90,102 Especially highlighted is the copper(I)-catalyzed 1,3-dipolar Huisgen cycloaddition, better known as “click” reaction. This reaction is known to take place under ambient conditions, proceeds fast and is quantitative.103,104 Its success is well documented by a huge amount of publications dealing not only with the synthesis of various block copolymer architectures but also with the functionalization of chain ends, particles and surfaces.105-107 It is further emphasized that “click” chemistry is easily combinable with ATRP. Common ATRP initiating sites, which are mostly preserved after finishing the polymerization, can be easily transformed into “click”-components by a simple nucleophilic substitution of the halogen end group by an azide moiety.108,109 Another combined polymerization/ coupling method worth to be mentioned is the recently developed RAFT hetero-Diels-Alder addition (RAFT-HDA), also known as “clack” reaction. Here, a RAFT chain transfer agent moiety, situated at the chain end of a previously synthesized polymer is directly reacted with a diene, attached to a second polymer chain (Fig. 1.11).
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Figure 1.11. Selection of coupling methods applied in block copolymer synthesis.