Chapter 9
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RAFT for the Control of Monomer Sequence Distribution – Single Unit Monomer Insertion (SUMI) into Dithiobenzoate RAFT Agents Graeme Moad,* Carlos Guerrero-Sanchez, Joris J. Haven, Daniel J. Keddie, Almar Postma, Ezio Rizzardo, and San H. Thang CSIRO Materials Science and Engineering, Bag 10, Clayton, Victoria 3168, Australia *E-mail:
[email protected] In this paper we explore RAFT (reversible addition-fragmentation chain transfer) single unit monomer insertion (SUMI) into dithiobenzoates. Styrene and N-isopropylacrylamide (NIPAm) were successfully inserted into 2-cyanopropan-2-yl dithiobenzoate. Attempted SUMI of methyl methacylate (MMA) provided an oligomeric insertion product due to the low transfer constant of the dithiobenzoate in MMA polymerization. A very low yield with maleic anhydride (MAH) reflects the low reactivity of MAH towards 2-cyanopropan-2-yl radicals. We also examined insertion of MAH, styrene and NIPAm into the styrene SUMI product. Insertion of MAH was rapid and efficient. SUMI with styrene and NIPAm was slower, which is attributed both to the low monomer concentrations used and the poor leaving group ability of the propagating species. The reaction with NIPAm is additionally complicated by initiator-derived by-products.
Precisely controlled compositions, well-defined architectures and narrow molecular weight distributions are now an expected (though not always achieved) outcome when applying techniques for reversible-deactivation radical polymerization (RDRP) (1), such as reversible addition-fragmentation chain transfer (RAFT) polymerization (2–9). However, precise control over
© 2014 American Chemical Society In Sequence-Controlled Polymers: Synthesis, Self-Assembly, and Properties; Lutz, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.
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sequence distribution at the monomer level, as is achieved by nature in protein or polynucleotide biosynthesis, remains an elusive goal in the field of radical polymerization (10–12). Zard and coworkers (13) first performed selective single unit monomer insertion (SUMI) in the late 80s for an N-alkylmaleimide or a vinyl sulfone into a xanthate. They (14–19) have subsequently provided many examples of this chemistry mainly in the context of organic synthesis. The noted the reaction was not effective with more-activated monomers (MAMs). In 2004, Chen and coworkers (20) applied SUMI in their synthesis of light harvesting polymers when they prepared new dithiobenzoate macro-RAFT agents by selectively inserting a single unit of a styrene derivatives. SUMI has since been applied to a wider range of examples involving styrene or vinylthiophene derivatives [all (MAMs) (21)] and either trithiocarbonate or dithiobenzoates RAFT agents (22–25). SUMI into macro-RAFT agents has also been developed as a method of chain-end functionalization with monomers such as maleic anhydride (MAH) (26–29), N-alkylmaleimide derivatives (30, 31) or β-pinene (31). Success in these experiments was attributed to the monomers not readily undergoing homopolymerization. This meant that the monomer could be used in excess with respect to the macro-RAFT agent (e.g. macro-RAFT agent:monomer>1:20 (30)) with little risk of multiple insertion (oligomerization). McLeary, Klumperman and colleagues (29, 32–37) observed that complete conversion of the initial RAFT agent to a species incorporating a single monomer unit is common to many well-behaved RAFT polymerizations (including those of styrene (32, 35), methyl acrylate (MA) (34, 37), N-vinylpyrrolidone (36) and vinyl acetate (VAc) (36)). They termed the behavior selective initialization. However, no similar selectivity was seen for subsequent monomer insertions. We have made similar observations for styrene polymerization and found that the phenomenon was strongly dependent on the specific RAFT agent and the polymerization conditions (38). Specifically, with 4.3 M styrene and 0.5 M RAFT agent, selective initialization is observed with 2-cyanopropan-2-yl and cumyl dithiobenzoates but not with benzyl dithiobenzoate or 2-cyanopropan-2-yl dodecyl trithiocarbonate. Selective initialization may be observed with 2-cyanopropan-2-yl dodecyl trithiocarbonate only when higher RAFT agent to styrene ratios are used (25). Quiclet-Sire et al. (19) found that two sequential SUMI for a xanthate could be achieved where the first monomer is the electron poor less activated monomer (LAM), specifically N-vinylphthalimide, and the second monomer is an electronrich LAM (a functional propene). In a recent paper (25), we explored the scope and limitations for performing (two) successive SUMI for MAMs (styrene or N-isopropylacrylamide (NIPAm)) into a trithiocarbonate RAFT agent. We also made predictions with respect to the scope of the process and pointed out some of the limitations. Recently, a number of reports of the synthesis of multiblock copolymers (meth)acrylates or acrylamides by sequential RAFT (39–42) (or atom transfer radical polymerization (ATRP (43))) steps have appeared. In that a single unit can be considered as a block with length unity, many of the factors important to the success of these experiments are also important in forming polymers through multiple SUMI steps. However, an additional criteria for successful SUMI is a 134 In Sequence-Controlled Polymers: Synthesis, Self-Assembly, and Properties; Lutz, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.
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very high transfer constant for the RAFT agent for the monomer being inserted (25). Vandenbergh et al. (44) performed four consecutive SUMI of acrylate monomers into a trithiocarbonate RAFT agent. Excess (10-fold) monomer was used in the experiments and the degree of oligomerization was controlled by limiting the monomer conversion through short (10 min) reaction times. Automated recycle size exclusion chromatography (SEC) was developed to provide a pure SUMI product after each step. A goal in designing our experiments has been to obtain achieve sufficient selectivity to render complex purification procedures unnecessary. In principle, higher selectivities for single unit insertion of MAMs might be expected with the use dithiobenzoates because of their higher transfer constants in RAFT polymerization (45). In this paper we explore single unit insertion of styrene, NIPAm, methyl methacylate (MMA) and MAH into 2-cyanopropan-2-yl dithiobenzoate (1). We then examine insertion of styrene, NIPAm and MAH into the so-formed styrene single unit insertion product.
Results and Discussion RAFT Single Unit Monomer Insertion into 2-Cyanopropan-2-yl Dithiobenzoate (1) The initial RAFT agent used in the present work was 2-cyanopropan-2-yl dithiobenzoate (1). The use of azobis(isobutyronitrile) (AIBN) as initiator then ensured there would be no initiator-derived insertion products in the first SUMI step. SUMI experiments with styrene, NIPAm, MMA and MAH were carried out as in situ NMR experiments as described in our previous study (25). For these experiments used solutions were prepared in a nominal 5:5:1 [Monomer]:[RAFT agent (1)]:[AIBN] ratio with the exact ratios being determined by 1H NMR (see Table 1 in Experimental). Difficulties experienced in maintaining the tuning of the nuclear magnetic resonance (NMR) spectrometer (with MMA) and inconvenient peak overlaps precluded a detailed kinetic analysis. SUMI of styrene (Figure 1) or NIPAm (Figure 2) into 1 provided 2 or 3, respectively (Scheme 1). The reaction with styrene showed a higher degree of specificity than the similar reaction previously carried out with dodecyl 2-cyanopropan-2-yl trithiocarbonate (25) to the extent that “dimers” or higher oligomers from multiple monomer insertion were not detected at all. The products from cage reaction of the initiator-derived radicals, TMSM, IBN and KB (Scheme 2), were observed in the anticipated amounts. For the reactions with styrene, NIPAM and MAH the rate determining step, that limiting the rate of disappearance of monomer and the initial RAFT agent, is the rate of cyanoisopropyl adding to monomer. This increases in the series MAH
