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Chapter 16
Catalytic Chain Transfer Polymerization and Reversible Deactivation Radical Polymerization of Vinyl Acetate Mediated by Cobalt(II) Phenoxy-imine Complexes Yi-Hao Chen, Hung-Hsun Lu, Jia-Qi Li, and Chi-How Peng* Department of Chemistry and Frontier Research Center on Fundamental and Applied Sciences of Matters, National Tsing Hua University, Hsinchu, Taiwan 30013 *E-mail:
[email protected].
Organo-cobalt complexes have demonstrated unique properties in the mediation of radical polymerizations such as catalytic chain transfer polymerization (CCTP) of methacrylates and reversible deactivation radical polymerization (RDRP) of unconjugated monomers particularly vinyl acetate (VAc). Two pathways of reversible termination (RT) and degenerative transfer (DT) have been rationalized as the major control mechanisms in cobalt mediated RDRP. In this chapter, cobalt(II) phenoxy-imine complexes mediated radical polymerization of vinyl acetate is reported. The increasing of electron donating property of phenoxy-imine ligand could switch the polymerization mechanism from RDRP to CCTP, demonstrating the potential of this system to combine CCTP and RDRP techniques.
© 2018 American Chemical Society Matyjaszewski et al.; Reversible Deactivation Radical Polymerization: Mechanisms and Synthetic Methodologies ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
Introduction Chain transfer reaction was described as a side reaction in the chain growth polymerization process (1–6). The active species generated by initiators (usually carbon radicals, anions, or cations) not only can propagate with monomers but also react with other components in the polymerization such as solvent, monomer, or impurity via hydrogen abstraction to transfer the active centers from propagating chains to small molecules and thus causes the decreasing of average molecular weight of polymeric products. The schematic comparison of free radical polymerization and chain transfer polymerization was shown in Figure 1a. In 1975, Smironov and Marchenko et al. reported the application of cobalt porphyrin complex (Figure 2a) for the acceleration of chain transfer reaction in methyl methacrylate polymerization. The average molecular weight of poly(methyl methacrylate) decreased linearly with the equivalent of cobalt complex. The correlation of cobalt concentration and degree of polymerization was schematically shown in Figure 1b (7). The cobaloxime (Figure 2b) was then found as a highly efficient chain transfer agent for methyl methacrylate radical polymerization (8–11) and was commercialized as an industrial technique to produce polymeric products with low molecular weight for the convenience of following processing (1). Recently Haddleton et al. expanded the application of cobaloxime to the synthesis of sequential-controlled multiblock copolymers (12). Poli et al. also contributed to this field by developing new cobalt complexes for catalytic chain transfer polymerization (CCTP) of vinyl acetate (VAc) (13). More details of CCTP could be found in the review article published by Heuts et al. (1)
Figure 1. (a) Illustration of the difference between free radical polymerization and polymerization with chain transfer reaction; (b) Correlation of cobalt concentration and degree of polymerization in catalytic chain transfer polymerization. 336 Matyjaszewski et al.; Reversible Deactivation Radical Polymerization: Mechanisms and Synthetic Methodologies ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
Living polymerization was first reported by Szwarc (14) and defined as a polymerization process with no chain breaking reactions (chain transfer or chain termination), in which the molecular weight of polymeric products increases linearly with conversion, the molecular weight distribution is narrow, and the block copolymers can be obtained by sequential addition of second monomers. However, in radical polymerization, the radicals termination is inevitable so that the term of “living radical polymerization” has been revised to “controlled radical polymerization” or “controlled/living radical polymerization”, and then “reversible deactivation radical polymerization (15, 16)”. Therefore, in this chapter, “reversible deactivation radical polymerization (RDRP)” will be used to refer this technique. Cobalt complexes mediated RDRP was first reported by Wayland et al. in 1994 using cobalt(II) tetramesitylporphyrin (CoII(TMP), Figure 3a) to control the radical polymerization of methyl acrylate (17). At the same time, Harwood et al. used alkylcobaloximes (CoII(dmgH)2, Figure 3b) to mediate the light initiated radical polymerization of methyl acrylate (18). Another milestone of cobalt complexes mediated C/LRP was achieved by Jérôme et al. using cobalt(II) bis-acetylacetonate (CoII(acac)2, Figure 3c) to efficiently control the vinyl acetate radical polymerization (6). Afterward, The β-diketonato and β-ketiminato analogues of CoII(acac)2 (Figure 3d,e) (19), 1,3-Bis(2-pyridylimino) isoindolatocobalt(II) complexes (CoII(acac)(bpi), Figure 3f) (20), cobalt(II) [N,N’-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediamine] (CoII(Salen*), Figure 3g) (21), and cobalt(II) bipyridine bisphenolate (CoII(bpybph), Figure 3h) (22) have been used to control the radical polymerization of acrylates, vinyl acetate, or other vinyl monomers.
Figure 2. Catalytic transfer agent: (a) cobalt tetramethoxy hematoporphyrin-IX and (b) cobaloxime. 337 Matyjaszewski et al.; Reversible Deactivation Radical Polymerization: Mechanisms and Synthetic Methodologies ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
Figure 3. Cobalt complexes used to mediate the reversible deactivation radical polymerization.
Both RDRP and CCTP mediated by cobalt complexes start from the reaction of cobalt(II) complexes and radicals to form the organo-cobalt(III) species (Scheme 1) (23–25). Therefore, an induction period is usually observed in cobalt complexes mediated RDRP and is rationalized as the time required to transform cobalt(II) and radicals to organo-cobalt(III) (6). Although the RDRP directly initiated by organo-cobalt(III) was reported, it needs the synthesis of corresponding cobalt complex in advance (25). When the Co-C bond in organo-cobalt(III) complexes can dissociate via bond homolysis to reversely generate cobalt(II) complexes and radicals that initiate the polymerization without external radical source, the polymerization could be controlled by the reversible termination mechanism (Scheme 1a, RT). If the Co-C bond is too strong for self-dissociation, an external radical source such as AIBN or V-70 is required to initiate the polymerization. The propagating radicals could rapidly exchange with the organic radicals on organo-cobalt(III) so that each radical 338 Matyjaszewski et al.; Reversible Deactivation Radical Polymerization: Mechanisms and Synthetic Methodologies ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
has similar chance to grow. This mechanism controlling radical polymerization by fast interchange of propagating radicals and dormant radicals was known as degenerative transfer (Scheme 1b, DT). These two pathways, RT and DT, can both achieve the reversible deactivation radical polymerization but are distinguished by the source and concentration of radicals. When the radicals are mainly from the bond homolysis of Co(III)-P and the radical concentration is dominated by the equilibrium of Co(II) and Co(III)-P ([P•] = [Co(III)-P]/([Co(II)] × Keq)), the polymerization is controlled mainly by RT mechanism. If the radicals are solely generated from initiator and the radical concentration is determined by the initiator concentration ([P•] = (ki[initiator]/2kt)1/2), the control of polymerization is approached via the DT pathway (19). There is another pathway in which the metal center of organo-cobalt(III) abstracts the hydrogen from the organic group to form the Co(III)-H and corresponding vinyl species (3). It should be noticed that the possibility of coexistence of β-H transfer via intermolecular reaction between cobalt(II) and radicals is not excluded (1, 2, 5, 13, 26). The cobalt hydride is highly active and can react with monomer or other vinyl compounds to generate organo-cobalt(III) species that release radicals. Since the replacement of long propagating radicals (P•) by small radicals (m•) matches the chain transfer reaction in the textbook (1–3, 5, 26) and is catalyzed by cobalt(II) complexes, this pathway was known as catalytic chain transfer (Scheme 1c, CCT).
Scheme 1. Correlation of Reversible Termination (RT), Degenerative Transfer (DT), and Catalytic Chain Transfer (CCT) Mechanisms in Cobalt Mediated Radical Polymerization
One important perspective for polymer synthesis is to combine different polymerization methods such as CCTP, RDRP, or ring opening polymerization to generate new polymeric products (27). The cobalt(II) porphyrin complexes have been reported to mediate CCTP or RDRP depending on the steric effect of the ligand. The cobalt(II) tetra(p-methoxyphenyl)porphyrin (CoII(TAP)) can mediate the CCTP of methyl acrylate with low equivalence of initiator but cobalt(II) 339 Matyjaszewski et al.; Reversible Deactivation Radical Polymerization: Mechanisms and Synthetic Methodologies ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
tetramesitylporphyrin (CoII(TMP)) can control the polymerization of methyl acrylate (5, 23). Herein, we are reporting the cobalt(II) phenoxy-imine complexes (Figure 4) that mediate RDRP or CCTP when the ligand has varied electron donating properties, which could contribute to the hybridization of CCTP and RDRP.
Figure 4. Cobalt(II) phenoxy-imine complexes used to mediate the radical polymerization of vinyl acetate in this chapter.
Experimental Materials 3-tert-Butyl-4-hydroxyanisole (Acros), tributylamine (J.T. Baker), tin(IV) chloride (Aldrich, 1.0 M in heptane), paraformaldehyde (Alfa Aesar, 97%), cyclohexylamine (Alfa Aesar, 98+%), cobalt(II) acetate tetrahydrate (Acros, 97%), sodium hydroxide (SHIMAKYU’S), thiophenol (Alfa Aesar, 99+%), tetrabutylammonium perchlorate (TBAP, TCI, 98%), AgNO3 (Acros), and 2,2’-azo-bisisobutyronitrile (AIBN, Showa) were used without any further purification. Toluene were dried by CaH2 before used. Deuterated solvents (Aldrich) were dried over molecular sieves. Vinyl acetate (Merck, 99%) was distilled under reduced pressure and degassed by three freeze pump thaw cycles before use. Measurement The NMR spectroscopy was used to characterize the structures of chemicals and monomer conversion. The spectrum was recorded by a Mercury-400 and Varian-500 spectrometer at 298 K. The chemical shifts in 1H NMR were shown in ppm refer to residual protons in CDCl3 as δ 7.24 ppm. 13C NMR chemical shifts were given in ppm refer to residual solvent in CDCl3 δ 77 ppm. Gel permeation chromatography (GPC) equipped with Ultimate 3000 liquid chromatograph associated with a 101 refractive index detector and Shodex columns (Shodex KF-802, Shodex KF-803, and Shodex KF-805) was used to analyze the polymeric products using THF as the eluent at 30 °C with 1.0 mL min-1 flow rate. The 340 Matyjaszewski et al.; Reversible Deactivation Radical Polymerization: Mechanisms and Synthetic Methodologies ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
calibration was based on narrow linear poly(styrene) Shodex standard (SM-105) ranging in molecular weight form 1.20 × 102 to 2.61 × 106 g mol-1. The Mn, Mw and polydispersity (PDI) of the polymeric products were calculated by DIONEX chromeleon software.
Scheme 2. Synthesis of Aldehyde Precursor Phenol (9.013g, 50.0 mmol) and tributylamine (5.0 mL, 20 mmol) were dissolved in toluene (50.0 mL) and stirred for 10 minutes. The SnCl4 was added dropwise to solution at room temperature under inert atmosphere. After 15 minutes stir, paraformaldehyde (2.078 g, 66.5 mmol) was added in solution. The solution was heated to 100 °C for another 12 hours (Scheme 2). The mixture was then extracted with ether and purified by column chromatography packed with fresh silica gel (hexane/EtOAc = 9/1) to give the product as yellow liquid (73.7%). 1H NMR (400 MHz, CDCl3) : δ (ppm) 11.51 (s, 1H, -OH), 9.79 (s, 1H, CHO), 7.15 (d, J = 1.5 Hz, 1H, Ar-H), 6.78 (d, J= 1.5 Hz, 1H, Ar-H), 3.78 (s, 3H, -OCH3), 1.39 (s, 9H, -tBu). 13C NMR (100 MHz, CDCl3) : δ (ppm) 196.20 (-CH=O), 155.80 (Ar), 151.72 (Ar), 139.76 (Ar), 123.58 (Ar), 119.58 (Ar), 111.48 (Ar), 55.56 (-OCH3), 34.93 (-C(CH3)3), 29.09 (-C(CH3)3).
Scheme 3. Synthesis of Phenoxy-imine (Ligand) Methanol (30 mL) solution of aldehyde precursor (2.941g, 14.0 mmol) and amine (1.611ml for ligand a, 1.737g for ligand b, 14.0 mmol) was reflux under inert atmosphere for 6 hours (Scheme 3). The precipitate was collected by vacuum filtration and washed by iced methanol to give the yellow solid (89.5%) of phenoxy-imine ligand a (28, 29) and the orange solid (63.5%) of phenoxy-imine ligand b (30). 1H NMR of phenoxy-imine ligand a (R = -cyclohexyl)(CDCl3, 400 MHz) : δ (ppm) 8.31 (s, 1H, -HC=N), 6.94 (d, J= 1.5 Hz, 1H, Ar-H), 6.59 (d, J= 1.4 Hz, 1H, Ar-H), 3.75 (s, 3H, -OCH3), 3.15-3.24 (m, 1H, -C=NCH), 1.50-1.90 (m, 10H, -Cy-H), 1.41 (s, 9H, -CCH3). 13C NMR of phenoxy-imine 341 Matyjaszewski et al.; Reversible Deactivation Radical Polymerization: Mechanisms and Synthetic Methodologies ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
ligand a (R = -cyclohexyl)(CDCl3, 100 MHz) : δ (ppm) 162.46 (-CH=N), 154.80 (Ar), 150.82 (Ar), 138.70 (Ar), 117.95 (Ar), 117.61 (Ar), 111.24 (Ar), 67.74 (=NH-C), 55.82 (-OCH3), 35.03 (-C(CH3)3), 34.47 (-C(CH3)3), 29.36 (-Cy), 25.65 (-Cy), 24.57 (-Cy). 1H NMR of phenoxy-imine ligand b (R=-p-methoxybenzyl) (CDCl3, 400 MHz) : δ (ppm) 8.57 (s, 1H, -HC=N), 7.27 (d, J= 4.4 Hz, 2H, Ar-H), 7.00 (d, J= 1.6 Hz, 1H, Ar-H), 6.92 (d, J= 4.4 Hz, 2H, Ar-H), 6.72 (s, 1H, Ar-H), 3.82 (s, 3H, -OCH3), 3.79 (s, 3H, -OCH3), 1.44 (s, 9H, -CCH3). 13C NMR of phenoxy-imine ligand b (R=-p-methoxybenzyl)(CDCl3, 100 MHz) : δ (ppm) 160.65 (-CH=N), 158.41 (Ar), 154.68 (Ar), 151.17 (Ar), 141.13 (Ar), 138.95 (Ar), 122.07 (Ar), 118.68 (Ar), 118.36 (Ar), 114.40 (Ar), 111.61 (Ar), 55.75 (-OCH3), 55.47 (-OCH3), 35.08 (-C(CH3)3), 29.33 (-C(CH3)3).
Scheme 4. Synthesis of CoII(Phenoxy-imine)2 (Mediator) The methanol solution (20 mL) of cobalt acetate tetrahydrate (0.126g, 0.49 mmol) and NaOH (0.043g, 1.07 mmol) was added dropwise to a methanol solution (10 mL) of phenoxy-imine (0.310g for a, 0.335g for b, 1.07 mmol). The mixture was refluxed for 6 hours (Scheme 4) then the precipitate was collected by vacuum filtration and washed by ice methanol to give the orange powder (89.5%) complex a (R = -cyclohexyl) and the orange-red powder (73.6%) complex b (R = -p-methoxybenzyl). Mass spectrum (FAB, m/z) for complex a was 635, calc. exact mass C36H52CoN2O4 635 and for complex b was 683, calc. exact mass C38H44CoN2O6 683. Polymerization The desired amount of cobalt(II) complexes and AIBN were mixed in a 50 mL Schlenk flask with three vacuum/nitrogen cycles to remove oxygen. Dry, degassed vinyl acetate was subsequently injected into the Schlenk flask by syringe under inert atmosphere. The mixture was stirred and heated to 60 °C and the monomer conversion was followed by 1H NMR.
Results and Discussion The polymerization of vinyl acetate was mediated by cobalt(II) phenoxyimine of complex a or b in bulk at 60 °C with the condition of [Co(II)]0/ [AIBN]0/[VAc]0=1/10/700 (Table 1). An induction period, which was rationalized as the time required to transform the cobalt(II) complexes and radicals to 342 Matyjaszewski et al.; Reversible Deactivation Radical Polymerization: Mechanisms and Synthetic Methodologies ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
organo-cobalt(III) species (6), was observed in the polymerization mediated by complex a, followed by the linear increased conversion with time to reach 34% monomer conversion within 110 minutes (Figure 5 diamond). Increasing the equivalent of AIBN from 10 to 20 (Table 1, entry 2) shortened the induction period and elevated the polymerization rate to approach 39% conversion within 80 minutes (Figure 5 circle). These observations match the typical phenomena of cobalt complexes mediated RDRP (31).
Table 1. Polymerization of VAc Mediated by CoII(Phenoxy-imine)2 in Bulk at 60 °C Entry
Complex
Condition
Time (mins)
Conv. (%)a
Mnb
Mn,thc
PDIb
1
a
1/10/700
110
34
59000
20500
1.80
2
a
1/20/700
80
39
46700
23500
1.94
3
b
1/10/700
30
33400
18100
1.70
480 1H
Conversion was measured by NMR. Mn was determined by gel permeation chromatography (GPC) with polystyrene as standard. c Mn,th=([VAc]0/[CoII]0)×(M.W. of VAc)×Conv. a
b
Figure 5. The plots of conversion versus time for VAc polymerization mediated by complex a at 60 °C in bulk under the condition of [complex a]0/[AIBN]0/[VAc]0=1/10/700 (diamond) and [complex a]0/[AIBN]0/[VAc]0=1/20/700 (circle).
The smoothly shifted GPC traces demonstrated another feature of RDRP (Figure 6). The molecular weight of poly(vinyl acetate) showed a moderate linear relationship to monomer conversion (Figure 7). The gap between the measured molecular weight and theoretical one implied that the polymerization may be 343 Matyjaszewski et al.; Reversible Deactivation Radical Polymerization: Mechanisms and Synthetic Methodologies ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
mainly controlled by reversible termination mechanism, which was supported by the observation of decreased molecular weight with increased equivalent of AIBN because the RT mechanism is based on the equilibrium of Co(II), radicals, and organo-Co(III) so that the theoretical molecular weight calculated by the assumption of one chain per cobalt would be lower than real molecular weight and the increasing of radicals raises the concentration of organo-cobalt(III) and thus decreases the molecular weight (23). However, the relatively high PDI values of 1.80 and 1.94 demonstrated the experimental condition could be further improved for a better control process.
Figure 6. The GPC traces of VAc polymerization mediated by complex a at 60 °C in bulk under condition of [complex a]0/[AIBN]0/[VAc]0=1/20/700.
Figure 7. The plots of Mn and PDI versus conversion for VAc polymerization mediated by complex a at 60 °C in bulk under the condition of [complex a]0/[AIBN]0/[VAc]0=1/10/700 (diamond) and [complex a]0/[AIBN]0/[VAc]0=1/20/700 (circle). 344 Matyjaszewski et al.; Reversible Deactivation Radical Polymerization: Mechanisms and Synthetic Methodologies ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
The VAc polymerization mediated by complex b also displayed an induction period followed by a linearly increased monomer conversion with time but the polymerization was slow and took 480 minutes to reach 30% conversion (Figure 8). However, given by the GPC results, the polymerization illustrated the features of catalytic chain transfer polymerization rather than reversible deactivation radical polymerization. The molecular weight approached to 37000 at the early stage of polymerization and stayed constant (Figure 9). The PDI values increased gradually with conversion from 1.49 to 1.70. To clarify the mechanism, the concentration of complex b was varied in the VAc polymerization and the molecular weight at similar monomer conversion around 8% was recorded (Figure 10). The Mayo plots showed that the degree of polymerization declined properly with the increased concentration of complex b and provided the solid evidence for the catalytic chain transfer mechanism. The observation of induction period indicated the formation of organo-cobalt(III) species before the CCTP and thus suggested that complex b should mediate the polymerization via an intramolecular β-H transfer reaction.
Figure 8. The plots of conversion versus time for VAc polymerization mediated by complex b at 60 °C in bulk under the condition of [complex b]0/[AIBN]0/[VAc]0=1/10/700.
The methyl acrylate polymerization mediated by CoII(TAP) and CoII(TMP) demonstrated the steric effect of the ligand to the polymerization mechanism (5, 23). The more bulky ligand has better chance to block the β-H transfer reaction and leads to the reversible deactivation radical polymerization. In our study, the cobalt(II) phenoxy-imine complexes of a and b altered the VAc polymerization mechanism by electronic effect of the ligand. The cobalt complex with more electron donating ligand prefers to mediate the polymerization via the catalytic chain transfer pathway. The polymerization results of complex a and b could contribute to the development of mediators for the combination or the switch of different polymerization mechanisms and thus novel synthetic methods for advanced materials. Further study of how other factors such as temperature, and radical concentration affect the polymerization mechanism and theoretical calculation for quantitative understanding of these results are being processed. 345 Matyjaszewski et al.; Reversible Deactivation Radical Polymerization: Mechanisms and Synthetic Methodologies ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
Figure 9. The plots of Mn and PDI versus conversion for VAc polymerization mediated by complex b at 60 °C in bulk under the condition of [complex b]0/[AIBN]0/[VAc]0=1/10/700.
Figure 10. The plots of DP versus [Co]/[VAc] (×10-3) for VAc polymerization mediated by complex b at 60 °C in bulk under different quantity of complex b. The molecular weight was recorded at the monomer conversion at 8%. 346 Matyjaszewski et al.; Reversible Deactivation Radical Polymerization: Mechanisms and Synthetic Methodologies ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
Conclusion The cobalt(II) phenoxy-imine complexes of a and b have been developed to mediate the radical polymerization of vinyl acetate. As expected, complex a showed a moderate control efficiency, demonstrated by the linearly increased molecular weight with conversion and smoothly shifted GPC traces, to the VAc polymerization. According to the molecular weight deviation and the correlation between concentration of AIBN and molecular weight of PVAc, the control mechanism was proposed to be reversible termination. However, complex b, a cobalt(II) phenoxy-imine complex similar to a with the more electron donating ligands, mediated the VAc polymerization by catalytic chain transfer mechanism. Associated with the results of methyl acrylate polymerization mediated by cobalt(II) porphyrins (5, 23), the more bulky, less electron donating ligands should be able to suppress the catalytic chain transfer pathway and promote the control efficiency of cobalt complexes to the radical polymerization.
Acknowledgments We thank the research funding supported by Ministry of Science and Technology, Taiwan, (MOST 104-2113-M-007-012-MY3).
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