Set-LRP polymerization of acrylates in the presence of acids

Abstract

SET-LRP polymerization of acrylic monomers under acidic conditions is described. The source of the acidity may be the solvent (e.g., an acetic acid-containing solvent) or in the monomer content (e.g., acrylic acid or methacrylic acid, optionally in combination with other monomers such as methyl methacrylate).

Claims

1. A method of making an acrylic polymer or copolymer, comprising performing a single-electron transfer living radical polymerization of a reaction mixture comprising: (a) one or more acrylic monomers, including a monomer of the formula: ##STR00008## wherein R.sub.1 is hydrogen or alkyl and R.sub.2 is carboxyl or carboxylate; (b) a metal single-electron transfer catalyst; (c) a component comprising a solvent and optional sulfide-free ligand, where said component or combination of component and monomer is used to disproportionate the metal single-electron transfer catalyst; and (d) an organohalide initiator; with the proviso that the solvent includes a compound comprising a carboxyl group.

2. The method of claim 1, wherein the metal single-electron transfer catalyst is Cu(I)X where X is Cl, Br, or I, and the component or combination of component and monomer is capable of disproportionating the metal catalyst Cu(I)X into Cu(0) and Cu(II)X.sub.2.

3. The method of claim 2, wherein the component includes a ligand that comprises N-ligand groups.

4. The method of claim 1, wherein the component includes a ligand that comprises N-ligand groups.

5. The method of claim 1, wherein R.sub.2 is carboxyl.

Description

EXAMPLES

Examples 1-7

(1) The copolymerization of methyl methacrylate (MMA) and acetic acid (AA) in protic media was conducted by SET-LRP using TosCl as radical initiator with different concentration of acrylic acid according to the reaction scheme:

(2) ##STR00006##

(3) A solution of the monomer (MMA, 1.00 mL, 9.4 mmol), acetic acid (54 l, 0.94 mmol) solvent (0.50 mL), TosCl (9.0 mg, 0.047 mmol), Cu(0) catalyst (12.5 cm of gauge 20 wire, wrapped around a Teflon-coated stirbar), and ligand (Me.sub.6-TREN, 1.3 l, 0.004 mmol) was prepared and transferred to a 25 mL Schlenk tube. The reaction mixture was thereafter degassed by 6 freeze-pump-thaw cycles and placed in an oil bath thermostated at the desired temperature with stirring. The side arm of the tube was purged with nitrogen before it was opened for samples to be removed at predetermined times, with an airtight syringe. Samples were dissolved in CDCl.sub.3, and the conversion was measured by .sup.1H NMR spectroscopy. The M.sub.n and M.sub.w/M.sub.n, values were determined by GPC with PMMA standards. The polymerization mixture was dissolved in 5 mL of CH.sub.2Cl.sub.2 and passed through a small basic Al.sub.2O.sub.3 chromatographic column to remove any residual nascent Cu(0) catalyst and Cu(II) deactivator. The resulting solution was precipitated twice in 50 mL methanol with stiffing. Methanol was removed by filtration, and the final colorless polymer was dried under vacuum until constant weight was reached.

(4) Cu wire (20 gauge from Fisher), methanol (MeOH), ethanol (EtOH) (Fisher, certified ACS, 99.9%) and acetic acid (98%, Aldrich) were used as received. Tosyl chloride (99%, Aldrich) was recrystallized twice from hexane. Dimethyl sulfoxide (DMSO) (99.9%, Acros) was distilled under reduced pressure prior to use. Methyl methacrylate (MMA) was purchased from Aldrich and passed through basic aluminum oxide in order to remove the inhibitor. Hexamethylated tris(2-aminoethyl)amine (Me.sub.6-TREN) was synthesized as described in Inorg. Chem. 1966, 5, 41-44.

(5) 500 MHz .sup.1H NMR spectra were recorded on a Bruker DRX500 NMR instrument at 20 C. in CDCl.sub.3 with tetramethylsilane (TMS) as internal standard. Gel permeation chromatographic (GPC) analysis of the polymer samples were done on a Perkin-Elmer Series 10 high-performance liquid chromatograph, equipped with an LC-100 column over (40 C.), a Nelson Analytical 900 Series integration data station, a Perkin-Elmer 785A UV-vis detector (254 nm), a Varian star 4090 refractive index (RI) detector, and two AM gel (500 , 5 m and 104 , 5 m) columns. THF (Fisher, HPLC grade) was used as eluent at a flow rate of 1 mL/min. The number-average (M.sub.n) and weight-average (M.sub.w) molecular weights of the PMAMA samples were determined with PMMA standards purchased from American Polymer Standards.

Examples 1-3

(6) FIGS. 1A-B, 2A-B, and 3A-B depict the results for SET-LRP of 1.00 ml MMA in 0.5 ml methanol/water (95/5) at 50 C. with 1% acetic acid (FIGS. 1A-B), 2.5% acetic acid (FIGS. 2A-B), and 25% acetic acid (FIGS. 3A-B). The ratio of [MMA].sub.0/[TosCL].sub.0/Me.sub.6-TREN].sub.0 was 200/1/0.1. Polymerization solutions were prepared and AcOH was added in 2, 5, 20 and 50 equivalents with respect to TosCl (1, 2.5, 10 and 25% monomer concentration). In all of theses cases, very high and even complete conversions were obtained within a few hours reaction time. The polymerization followed straight first order kinetic, and the apparent rate constant of polymerization k.sub.p.sup.app somewhat increased with [AcOH].sub.0 and varied from 0.008 to 0.012 min.sup.1. It is unclear whether this is a trend since it was found for the SET-LRP of MMA under identical conditions but in absence of acetic acid that k.sub.p.sup.app=0.010 min.sup.1. (see Fleischmann, S. & Percec., V., J. Polym. Sci. Part A: Polym. Chem., 48, 2010, 2236-2242).

(7) Generally, the molecular weight evolution followed the common principles of SET-LRP. The chain growth proceeded linearly with conversion and there was a good agreement between calculated and experimental molecular weight. I.sub.eff in the presence of AcOH seems to be significantly increased and values of 70-80% were found. Another fact of the SET-LRP process is that the polydispersity index decreased with conversion and eventually fairly narrow molecular weights distributions are achieved with M.sub.w/M.sub.n1.3. In some examples, the two last data points showed an increase of the M.sub.w/M.sub.n values. It cannot be ruled out that this is due to the high viscosity of the reaction mixture at this stage. Nonetheless, the experiment with 25% acetic acid especially demonstrates that the SET-LRP process seems to be quite tolerant towards acidic media. FIGS. 1-3 show that the reaction proceeded without induction period and reached a very high conversion (>95%) within 240 min, retaining first order kinetic.

Examples 4-6

(8) There was also no spectroscopic evidence that AcOH was basically quenched by an in situ esterification with methanol. That is in clear agreement with experiments conducted in other solvent systems. FIGS. 4A-B, 5A-B, and 6A-B depict the results for SET-LRP of 1.00 ml MMA in 0.5 ml different solvents at 50 C. with 10% acetic acid. The solvents were methanol/water (95/5) (FIGS. 4A-B), ethanol/water (95/5) (FIGS. 5A-B), and DMSO (FIGS. 6A-B. The ratio of [MMA].sub.0/[TosCL].sub.0/Me.sub.6-TREN].sub.0 was 200/1/0.1. FIGS. 4-6 show that the SET-LRP of MMA at 50 C. proceeded with 10% acetic acid content in methanol, ethanol, and DMSO. The reaction in DMSO obeyed first order kinetic (k.sub.p.sup.app-=0.019 min.sup.1) and complete conversions was reached within 180 min. The molecular weight increased monotonically with conversion and M.sub.w/M.sub.n stayed rather low at 1.3. The comparison in MeOH (k.sub.p.sup.app=0.012 min.sup.1) and EtOH (k.sub.p.sup.app=0.008 min.sup.1) exhibits a difference in rate, a fact that has also been observed in earlier work for the SET-LRP of methyl acrylate in neutral media. In any case, in both situations the control over the molecular weight evolution was high.

Example 7

(9) This Example addressed the question of whether SET-LRP can be conducted in acetic acid as solvent. The main points addressed were whether AcOH irreversibly disrupts the Cu(II)/Me.sub.6-TREN complex and whether it facilitates the disproportionation of the Cu(I) species. In FIG. 7, the kinetic plot of the SET-LRP of 1.0 ml of MMA in 0.5 ml AcOH at 50 C. is displayed. The ratio of [MMA].sub.0/[TosCL].sub.0/Me.sub.6-TREN].sub.0 was 200/1/0.1. Two aspects became immediately visible. First, the reaction can be driven to completion. This is an important observation because excessive termination would result in an interruption of the polymerization process. Second, the monomer consumption follows first order kinetic so that the overall radical concentration must stay fairly constant.

Examples 8-11

(10) The copolymerization of MMA and methacrylic acid (MAA) in protic media was conducted by SET-LRP using TosCl as radical initiator with different concentration of methacrylic acid according to the reaction scheme:

(11) ##STR00007##

(12) A solution of the MMA (1.00 mL, 9.4 mmol), MAA (80 l, 0.94 mmol) solvent (0.50 mL), TosCl (9.4 mg, 0.047 mmol), Cu(0) catalyst (12.5 cm of gauge 20 wire, wrapped around a Teflon-coated stirbar), ligand (Me.sub.6-TREN, 1.3 l, 0.004 mmol) and approximately 20 mg dimethylsulfone (as internal standard for quantification of the NMR experiments) was prepared and transferred to a 25 mL Schlenk tube. The reaction mixture was thereafter degassed by 6 freeze-pump-thaw cycles and placed in an oil bath thermostated at the desired temperature with stiffing. The side arm of the tube was purged with nitrogen before it was opened for samples to be removed at predetermined times, with an airtight syringe. Samples were dissolved in CDCl.sub.3, and the conversion was measured by .sup.1H NMR spectroscopy. The M.sub.n and M.sub.w/M.sub.n values were determined by GPC with PMMA standards. The polymerization mixture was dissolved in 2 mL of CH.sub.2Cl.sub.2 and passed through a small basic Al.sub.2O.sub.3 chromatographic column to remove any residual nascent Cu(0) catalyst and Cu(II) deactivator. The resulting solution was precipitated twice in 50 mL methanol with stiffing. Methanol was removed by filtration, and the final colorless polymer was dried under vacuum until constant weight was reached.

(13) Cu wire (20 gauge from Fisher) and methanol (MeOH), (Fisher, certified ACS, 99.9%) were used as received. Tosyl chloride (99%, Aldrich) was recrystallized twice from hexane. Methyl methacrylate (MMA) and methacrylic acid were purchased from Aldrich and passed through basic aluminum oxide in order to remove the inhibitor. Hexamethylated tris(2-aminoethyl)amine (Me.sub.6-TREN) was synthesized as described by M. Ciampolini in Examples 1-7.

(14) 500 MHz .sup.1H NMR spectra were recorded on a Bruker DRX500 NMR instrument at 20 C. in CDCl.sub.3 with tetramethylsilane (TMS) as internal standard. Gel permeation chromatographic (GPC) analysis of the polymer samples were done on a Perkin-Elmer Series 10 high-performance liquid chromatograph, equipped with an LC-100 column over (40 C.), a Nelson Analytical 900 Series integration data station, a Perkin-Elmer 785A UV-vis detector (254 nm), a Varian star 4090 refractive index (RI) detector, and two AM gel (500 , 5 m and 104 , 5 m) columns. THF (Fisher, HPLC grade) was used as eluent at a flow rate of 1 mL/min. The number-average (M.sub.n) and weight-average (M.sub.w) molecular weights of the PMMA samples were determined with PMMA standards purchased from American Polymer Standards.

(15) In FIGS. 8A and 8B, the SET-LRP of 1.00 ml MMA with 2.5% MAA in 0.50 ml methanol/water (95:5) at 50 C. is displayed. The ratio of [MMA].sub.0/[MAA].sub.0/TosCL].sub.0/Me.sub.6-TREN].sub.0 was 200/5/1/0.1. At this very low acidic monomer content the polymerization obeyed first order kinetic (k.sub.p.sup.app=0.008 min.sup.1) and very high conversion was reached after 360 min. The apparent rate constant of propagation k.sub.p.sup.app was therefore in the same ranges reported in the literature for the SET-LRP in absence of any acidity (k.sub.p.sup.app=0.010 min.sup.1) or in presence of 2.5% AcOH (k.sub.p.sup.app=0.009 min.sup.1). The molecular weight evolved linearly with conversion with an initiator efficiency of I.sub.eff=67%. The low value for M.sub.w/M.sub.n1.3 suggests the living nature of the process.

(16) In the next step, the level of MMA was increased and a batch of 1.00 ml MMA containing 10% MAA in 0.50 ml methanol/water (95:5) was polymerized via SET-LRP at 50 C. The ratio of [MMA].sub.0/[MAA].sub.0/TosCL].sub.0/Me.sub.6-TREN].sub.0 was 200/20/1/0.1. The kinetic plot of such an experiment can be seen in FIG. 9A. The reaction was slower compared to the previous case, but still followed first order kinetic with k.sub.p.sup.app=0.004 min.sup.1. The incorporation of the two monomers into the growing chain occurred simultaneously. As can be seen from FIG. 9D, the decrease of both MMA and MAA occurred almost synchronically. The molecular weight monotonically with the monomer consumption with I.sub.eff=50% was significantly reduced (FIGS. 9B and 9C). The M.sub.w/M.sub.n values became smaller as the reaction proceeded, however, and increased slightly toward higher conversions. It is not clear whether broadening of the molecular weight distribution has a real physical cause or originates from an artifact by introducing oxygen while sampling. In contrast to that, an experiment where the acidity was further increased by the addition of acetic acid (equimolar to MAA) revealed some interesting trends. The monomer consumption followed the same first order kinetic with k.sub.p.sup.app=0.006 min.sup.1. Also, the molecular weight increases linearly with conversion with I.sub.eff=58% being just slightly different form the previous case. The M.sub.w/M.sub.n values, in contrast, equilibrated at 1.3 and dramatic increase was observed at the end of the reaction. Clearly, the extra acidity via AcOH addition did not further disturb the SET-LRP of MMA but rather enhances the polymerization.

(17) FIGS. 10A-B depict the polymerization of 1.00 ml MMA with 25% MAA in 0.50 ml methanol/water (95/5) at 50 C. ([MMA].sub.0/[MAA].sub.0/TosCL].sub.0/Me.sub.6-TREN].sub.0=200/20/1/0.1), and FIGS. 11A-B depict the polymerization of 1.00 ml MMA with 10% MAA and 10% acetic acid in 0.50 ml methanol/water (95/5) at 50 C. ([MMA].sub.0/[MAA].sub.0/TosCL].sub.0/Me.sub.6-TREN].sub.0=200/20/1/0.1). The polymerization with 25% MAA resulted in the loss of the livingness as shown in FIGS. 10A and 10B. The reaction followed first order kinetic up to about 50% conversion, and the rate of propagation (k.sub.p.sup.app=0.0035 min.sup.1) in this linear regime was much slower than for the previous experiments. Above this conversion, the rate notably decreased. The molecular weights, however, increased linearly with conversion, so that a certain control over the polymerization was maintained. However, in Example 3, SET-LRP of MMA in the presence of 25% acetic acid under identical conditions the reaction went to completion within 4 hours. Herein, the reaction followed first order kinetic and the molecular weights increased linearly with conversion. For this reason, it is unlikely that the acidity of the reactions mixture yields a disruption of the activator/deactivator complex and is therefore not accountable for the loss of livingness in the process.

(18) An important issue that has to be addressed in this context is the reactivity and nature of the growing macroradical species. It is not just the difference in reactivity of the MMA versus the MAA radical that needs consideration. Buback et al (see Macromolecules 2009, 42, 7753-61) have demonstrated that the rate of polymerization of methacrylic acid in free radical polymerization strongly depends on the degree of ionization. More precisely, k.sub.p for nonionized MAA was an order of magnitude higher than for fully ionized MAA. The addition of extra acetic acid as shown in FIGS. 11A-B might very well contribute to the polymerization process in such a way that it keeps the growing MAA radical protonated. The degree of ionization of polymeric MAA differs remarkably from the monomer since the pk.sub.a of poly(methacrylic acid) is significantly lower than that for its monomer.

(19) On the other hand, in living radical polymerization where the control is based on balanced rates of activation and deactivation of dormant/active species, significant differences in radical reactivity are decisive. Activation/deactivation in SET-LRP is achieved by heterolytic bond cleavage of a carbon halogen double bond and the reversible atom transfer of a halogenide to a carbon centered radical, respectively. The halogen, in turn, plays a pivotal role in balancing the dormant/active equilibrium and, ultimately, the control of the polymerization process.

(20) While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description.