Metal-Catalyzed Step-Growth Radical ... - ACS Publications

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Metal-Catalyzed Step-Growth Radical Polymerization of AA and BB Monomers for Monomer Sequence Regulation Kotaro Satoh, Tomohiro Abe, and Masami Kamigaito* Department of Applied Chemistry, Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan *E-mail: [email protected]

Transition metal-catalyzed step-growth radical polymerizations between two α,ω-difunctional monomers were investigated. One of the monomers contained two non-conjugated carbon–carbon double bonds and the other possessed two active carbon–halogen bonds within a single molecule. FeCl2/Pn-Bu3 effectively induced the step-growth radical polymerization of these two bifunctional monomers, which possessed two C=C or C–Cl bonds connected via ester linkages, to produce relatively high-molecular-weight polymers (Mw > 10,000). By varying the initial feed ratio of the monomers, the end-functionalities were controlled to produce telechelic polyesters with C–Cl or C=C groups at the chain ends. Moreover, polyaddition reactions between the two monomers, into which vinyl monomer units were inset, generated various equivalents of sequence-regulated vinyl copolymers, such as abbacc-ordered copolymers, where a, b, and c represent vinyl chloride, styrene, and methyl acrylate, respectively.

Introduction Metal-catalyzed atom transfer radical addition (ATRA), or the Kharasch reaction, is a highly efficient carbon–carbon bond forming radical reaction ((1–3)). Since the discovery of metal-catalyzed living radical polymerizations or atom transfer radical polymerizations (ATRP) in the mid 1990s (4–8), this chemistry has been widely applied to the chain-growth radical polymerization of © 2012 American Chemical Society In Progress in Controlled Radical Polymerization: Mechanisms and Techniques; Matyjaszewski, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.


vinyl monomers, which has begun a new era of precision polymer synthesis for a variety of controlled polymers, such as block, graft, and star polymers (Scheme 1A). More recently, we developed a new class of transition-metal-catalyzed step-growth radical polymerizations or polyaddition using the same catalysts employed in metal-catalyzed chain-growth radical polymerizations (Scheme 1B) (9, 10). The proposed reaction was performed on AB-type monomers possessing a reactive carbon–halogen bond (C–Cl) and an unconjugated carbon–carbon double bond (C=C) within a single molecule, and the reaction proceeded via the activation and deactivation of the carbon–halogen bond. In living chain-growth polymerization reactions, one reactive carbon–halogen bond at the terminus of each polymer chain is reversibly activated to react with the monomer. By contrast, a step-growth reaction occurs between reactive C–Cl and C=C bonds at the termini of monomers, oligomers, or polymers, and propagation is induced by the formation of a carbon–carbon backbone and inactive carbon–halogen pendants (11). Our novel polyaddition reaction was applied to synthesize a series of novel linear polymers. In particular, C–C bond forming reactions were combined with chain-growth living radical polymerizations to achieve unprecedented simultaneous chain- and step-growth polymerizations for the production of random copolymers of vinyl polymers and polyesters (12, 13).

Scheme 1. Polymerizations Based on Metal-Catalyzed Atom Transfer Radical Addition: Chain-Growth or ATRP (A), Step-Growth or Polyaddition (B). The regulation of monomer sequences in synthetic polymers, which is the holy grail of polymer synthesis, has attracted considerable attention. Natural macromolecules such as proteins and nucleic acids must possess a nearly perfect sequence to induce inherent functions (14–16). We developed a novel strategy for the synthesis of sequence-regulated vinyl copolymers via metal-catalyzed step-growth radical polymerization by focusing on the newly formed –CH2–CHCl– unit, which is equivalent to a poly(vinyl chloride) repeating unit (17). A series of well-designed AB-type monomers were prepared from 134 In Progress in Controlled Radical Polymerization: Mechanisms and Techniques; Matyjaszewski, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.


common vinyl monomers and employed in step-growth radical polymerization to produce abc- and abcc-sequence regulated copolymers with perfect vinyl chloride-styrene-acrylate sequences. In this paper, we report the metal-catalyzed radical polyaddition of α,ω-difunctional compounds, i.e., AA- and BB-type monomers. One of the monomers possessed two carbon–carbon double bonds and the other contained two carbon–halogen bonds in a single molecule (A1–A3 and B1–B4 in Scheme 2, respectively). The intermolecular reaction yielded various types of linear polymer, including sequence-regulated vinyl copolymers derived from specifically designed monomers with prearranged united sequences.

Scheme 2. Metal-Catalyzed Step-Growth Radical Polymerization between α,ω-Divinyl and α,ω-Dichloro Compounds.

Results and Discussion Metal-Catalyzed Step-Growth Radical Polymerization between Divinyl and Dichloro Compounds The metal-catalyzed polymerization of α,ω-difunctional ester-linked AAand BB-type monomers (A1 and B1, respectively) was investigated using FeCl2/Pn-Bu3 (18, 19), which is effective for the step-growth polymerization of α,ω-heterofunctional ester-linked AB-type monomers (9). Figure 1 shows the conversion (A), number-average molecular weight (Mn) and molecular weight distribution (MWD: Mw/Mn) as a function of C=C bond consumption (B) and size-exclusion chromatography (SEC) curves (C) of the product of the polymerization of A1 and B1 with FeCl2/Pn-Bu3 in toluene at 100 °C. The consumption of A1 was determined from the SEC curves, which were measured using a UV detector at 255 nm, while the consumption of C=C bonds in A1 was monitored by 1H NMR. However, the conversion of C–Cl bonds in B1 could not be determined due to the overlapping peaks in the 1H NMR spectra. The monomers were smoothly consumed, and the conversion of A1 reached >90%. However, the conversion of C=C bonds was consistently lower than that of the monomer (Figure 1A). Specifically, at 73% monomer conversion, the C=C bond conversion was nearly half of that of the monomer (~40%). Similar results were obtained in the polymerization of ester-linked AB-type monomers (12). 135 In Progress in Controlled Radical Polymerization: Mechanisms and Techniques; Matyjaszewski, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.


The polymer molecular weight progressively increased in later stages of the polymerization and was close to the calculated values, which were based on the assumption that Mn increases inversely to the content of residual functional groups in step-growth polymerization. In addition, as the polymerization proceeded, the peaks of the SEC curves shifted to a higher molecular weight and the peak areas of the monomer decreased. These results indicate that most of the consumed monomer was converted into dimers during the initial stage of the polymerization and that significant undesirable reactions, such as intramolecular cyclization, did not occur. Subsequently, the reactions between oligomers proceeded in a step-growth manner to produce the polymers.

Figure 1. Metal-catalyzed step-growth radical polymerization between A1 and B1 with FeCl2/Pn-Bu3: [A1]0 = [B1]0 = 1.0 M; [FeCl2]0 = 100 mM; [Pn-Bu3]0 = 200 mM in toluene at 100 °C. To identify the optimal conditions for the intermolecular polyaddition of A1 and B1, copolymerization was investigated with a series of catalysts under various conditions (Table I). Although the monomer concentration ([M]total = 4.0 M) did not affect the polymerization rate or the molecular weight of the products, an increase in the catalyst loading ([FeCl2]0 = 200 mM) accelerated the polymerization, leading to a high-molecular-weight polymer (Mw>10,000). In addition, the polymerization proceeded more rapidly at higher temperatures (120 °C). Similar to the polymerization of AB-type monomers, the addition of tin 2-ethylhexanoate [Sn(EH)2] accelerated the polymerization of A1 and B1 in the FeCl2/Pn-Bu3 system, in which the tin additive may act as a reducing agent for the oxidized Fe-species (20). This result suggests that a small amount of the metal catalyst was deactivated during the reaction and that the polymerization can be enhanced by reducing agents. In addition, other metal catalysts based on CuCl and multidentate amine ligands, such as N,N,N′,N′′,N′′-pentamethyldiethylene-triamine (PMDETA) and tris[2-(dimethylamino)ethyl]amine (Me6TREN), were also examined in place of FeCl2/Pn-Bu3 in the AA and BB polymerization (21, 22). Cu-based systems were also effective for the polymerization of A1 and B1; however, the polymerization was less well controlled and yielded lower C=C conversion rates and lower molecular weight polymers compared with using FeCl2/Pn-Bu3. 136 In Progress in Controlled Radical Polymerization: Mechanisms and Techniques; Matyjaszewski, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.

Table I. Radical Polyaddition of A1 and B1 under Various Conditionsa Temp. (°C)

Time (h)

C=C Conv. (%)

Mn

Mw

Mw/Mn

100

1490

85

2400

7400

3.08

FeCl2/Pn-Bu3 ([FeCl2]0 = 200 mM)

100

1030

86

2500

12000

4.78

FeCl2/Pn-Bu3 ([M]total = 4.0 M) b

100

1200

88

2300

7700

3.29

120

240

88

2600

8100

3.11

100

216

87

6300

2.57

b

100

1900

64

1500

3000

2.00

FeCl2/PPh3 b

100

1900

46

950

1600

1.68

System


FeCl2/Pn-Bu3

FeCl2/Pn-Bu3

b

b

FeCl2/Pn-Bu3/Sn(EH)2 b,c FeCl2/PCy3

2500

CuCl/PMDETA

d

100

720

97

1600

5900

3.74

CuCl/PMDETA

d

60

1990

46

1200

2100

1.79

CuCl/Me6TREN

d

60

1600

68

930

1500

1.61

a

[A1]0 = [B1]0 = 1.0 M ([M]total = 2.0 M) in toluene. b [FeCl2]0 = 100 mM, [phosphine ligand]0 = 200 mM. c [Sn(EH)2]0 = 45 mM. d [CuCl]0 = 100 mM, [amine ligand]0 = 400 mM.

A more detailed mechanism of the polymerization was determined using a combination of A1 and B2, which allowed the total consumption of C=C and C–Cl groups in the monomer and polymerized products to be measured by 1H NMR. The polyaddition of A1 and B2 was performed with FeCl2/PBu3 or CuCl/PMDETA in toluene. Figure 2 shows the consumption curves of the two functional groups, which were determined by 1H NMR, along with those of A1, which were obtained using the SEC curves of the reaction mixture. Irrespective of the catalyst, the two functional groups were consumed simultaneously to afford the polymer. Although the molecular weights of the products were slightly lower than those obtained from A1 and B1, the peak of the SEC curves shifted to a higher molecular weight. These results indicate that the polymer was generated via step-growth polymerization and continual ATRA between C–Cl and C=C bonds originating from the monomers. The polyesters obtained from the combination of A1 and B1 or B2 using FeCl2/Pn-Bu3 were purified by preparative SEC to remove residual monomer and catalyst and analyzed by 1H NMR (Figure 3). In the polymer spectra, a series of broad and relatively large peaks (1–8) were observed, which were assigned to the main-chain protons of the repeating units of the expected polyesters. The integral ratio of peaks originating from the α,ω-diene (4) and the dichloride (6) indicates that an equimolar amount of the two compounds was incorporated into the polymer via radical polyaddition. In addition to these signals, small peaks corresponding to double bonds (a–c) and reactive C–Cl bonds (d and e) at the chain ends were observed. These results indicate that the polymers were generated via the expected 137 In Progress in Controlled Radical Polymerization: Mechanisms and Techniques; Matyjaszewski, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.


radical polyaddition reaction and that the chain end structures contained C–Cl or C=C bonds with equal probability. The thermal properties of the polyesters were also evaluated by differential scanning calorimetry (DSC). The glass transition temperatures (Tg) were relatively low (Tg = –8.5 °C [for A1 and B1, Mn = 5400] and –7.6 °C [for A1 and B2, Mn = 3500], respectively), and the melting points were not observed for both polymers, likely due to their low molecular weights, indicating that the products were amorphous polyesters.

Figure 2. Metal-catalyzed step-growth radical polymerization A1 and B2: [A1]0 = [B2]0 = 1.0 M; [FeCl2 or CuCl]0 = 100 mM; [Pn-Bu3]0 = 200 mM; [PMDETA]0 = 400 mM in toluene at 100 °C (for Fe) or 60 °C (for Cu).

Figure 3. 1H NMR spectra (CDCl3, r.t.) of the polymers obtained from A1 and B1 or B2 with with FeCl2/Pn-Bu3. 138 In Progress in Controlled Radical Polymerization: Mechanisms and Techniques; Matyjaszewski, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.


Synthesis of Homo-Telechelic Polymers by Step-Growth Radical Polymerization Using a non-equivalent initial amount of AA and BB monomers, the proposed step-growth process can be used to produce homo-telechelic polymers, i.e., polymers with identical α- and ω-functional chain ends. For the synthesis of homo-telechelic polymers, the step-growth polymerization of A1 and B2 with FeCl2/Pn-Bu3 in the presence of Sn(EH)2 was examined by changing the initial feed ratio of the monomers ([A1]0/[B2]0 ratios of 4/1, 3/2, 1/1, 2/3, and 1/4 (Figure 4)). The reactions proceeded in all cases, even when non-equimolar feed ratios of the monomers were employed. Under these conditions, the conversion of functional groups reached the theoretical values. With an excess amount of A1 ([A1]0/[B2]0 = 4/1 or 3/2), the consumption of C–Cl reached almost quantitative conversion, whereas that of C=C plateaued at approximately 25% and 66%, respectively, and vice versa. The highest polymer molecular weight was obtained when the initial monomer ratio was 1:1. These results also indicate that the polymerization proceeded in a step-growth manner via a 1:1 reaction between C–Cl and C=C bonds and that the terminal structure was dominated by the monomer with the highest content in the initial feed.

Figure 4. Metal-catalyzed step-growth radical polymerization A1 and B2: : [A1]0 + [B2]0 = 2.0 M; [FeCl2]0 = 100 mM; [Pn-Bu3]0 = 200 mM; [Sn(EH)2]0 = 45 mM in toluene at 100 °C. 1H NMR analyses of the products isolated by preparative SEC clearly showed

the formation of homo-telechelic polymers (Figure 5). Polymers obtained from an equimolar amount of A1 and B2 exhibited the characteristic peaks of C=C (a–c) and C–Cl termini (d and e) (A). By contrast, at different initial feed ratios ([A1]0/ [B2]0 = 3/2 and 2/3), only one structure was predominant at the termini of the 139 In Progress in Controlled Radical Polymerization: Mechanisms and Techniques; Matyjaszewski, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.


polymers (B and C). Namely, α,ω-divinyl polymers were produced at [C=C]0 > [C–Cl]0, and vice versa. Thus, the polymerization proceeded via a controlled process to produce a homo-telechelic polymer bearing well-defined α,ω-difunctionalized termini, and significant side reactions did not occur. Terminal C–Cl and C=C functionalities can be further utilized for various reactions such as transition-metal-catalyzed living radical polymerization (for C–Cl) and metathesis or thiol-ene reactions (for C=C), respectively.

Figure 5. 1H NMR spectra (CDCl3, r.t.) of the polymers obtained from A1 and B2 in the same experiments as for Figure 4. Sequence-Regulated Vinyl Copolymers from AA and BB Monomers Step-growth radical polymerization is accompanied by the formation of a –CH2–CHCl– unit, which is equivalent to a poly(vinyl chloride) repeating unit. As a result, sequence-regulated vinyl copolymers were synthesized via the metal-catalyzed radical polyaddition of AB-type monomers, which was prepared from common vinyl monomers as building blocks (17). Here, α,ω-difunctional AA- and BB-type monomers (A2, A3, B3, and B4) were also designed to create novel families of sequence-regulated vinyl copolymers via intermolecular step-growth radical polymerization. The AA- and BB-type monomers were polymerized using Fe- and Cu-based systems under various conditions. Although appropriate conditions that enabled the synthesis of high-molecular-weight polymers with quantitative monomer conversions were not fully optimized, polymeric products were obtained using the CuCl/Me6TREN system. Figure 6 shows the 1H NMR spectra of the products after purification by preparative SEC (Mn = 630 [for A2 and B3], 880 [for A2 and B4], and 4100 [for A3 and B4], respectively), in which the characteristic peaks of the main chain protons 140 In Progress in Controlled Radical Polymerization: Mechanisms and Techniques; Matyjaszewski, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.

of styrene, methyl acrylate, and vinyl chloride repeating units were observed. Specifically, the polymerization of A3 and B4 resulted in the formation of a complex abbacc-sequenced copolymer, where a, b, and c represent vinyl chloride, styrene, and methyl acrylate, respectively. Thus, by designing and combining various monomers, the proposed metal-catalyzed radical polyaddition reaction can be used to produce complicated sequence-regulated vinyl copolymers.


Conclusions In conclusion, metal-catalyzed radical step-growth polymerization of AAand BB-type monomers was successfully performed to produce novel polymer structures including various polyesters and sequence-regulated vinyl copolymers. The proposed method produces synthetic vinyl polymers possessing periodically located functional groups, which leads to vinyl polymers with a highly ordered structure.

Figure 6. 1H NMR spectra (CDCl3, r.t.) of the sequence-regulated polymers obtained by metal-catalyzed step-growth raidcal polymerization between A2 or A3 and B3 or B4 with CuCl/Me6TREN: [AA monomer]0 = [BB monomer]0 = 2.0 (for A2/B3 and A2/B4) or 1.0 M (for A3/B4); [CuCl]0 = 100 mM; [Me6TREN]0 = 400 mM in toluene at 60 (for A2/B3 and A2/B4) or 100 °C (for A3/B4). 141 In Progress in Controlled Radical Polymerization: Mechanisms and Techniques; Matyjaszewski, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.

Experimental


Materials Methyl acrylate (Tokyo Kasei, >99%) and styrene (Wako Chemicals, >98%) were distilled from calcium hydride under reduced pressure before use. FeCl2 (Aldrich, 99.99%), CuCl (Aldrich, 99.99%), and RuCl2(PPh3)3 (Aldrich, 97%), were used as received. All metal compounds were handled in a glove box (VAC Nexus) under a moisture- and oxygen-free argon atmosphere (O2, 98%), PCy3 (KANTO, >97%), PPh3 (KANTO, >98%), and Sn(EH)2 (Aldrich, ~95%) were used as received. PMDETA and Me6TREN were distilled from calcium hydride before use. Synthesis of Monomers Diallyl terephthalate (A1), ethylene bis(2-chloropropanoate) (B1), dimethylethylene bis(2-chloropropanoate) (B2) were synthesized from the corresponding acid chlorides and alcohols; i.e. terephthaloyl chloride (Tokyo Kasei; >99%) and allyl alcohol (Tokyo Kasei; > 99%) (for A1), 2-chloropropionyl chloride (Aldrich; > 97%) and ethylene glycol (Tokyo Kasei; > 99.5%) or 2,3-butanediol (Tokyo Kasei; > 97%, mixture of stereoisomers) (for B1 and B2, respectively), and purified by distillation. Diallyl toluene (A2) was prepared by the reaction of benzal chloride (Tokyo Kasei; > 95%) and allylmagnesium bromide (Aldrich; 2.0 M in THF) and purified by distillation. A diallyl styrene dimer (A3) was prepared by RuCl2(PPh3)3-catalyzed Kharasch addition between benzal chloride and styrene followed by TiCl4-catalyzed allylation of the C–Cl bond adjacent to the styrene unit with allyltrimethylsilane (Tokyo Kasei; > 98%) (23) and purified by column chromatography on silica gel and distillation. Methyl dichloroacetate (B3) was distilled from calcium hydride before use. A dichloro MA dimer (B4) was prepared by the ruthenium-catalyzed Kharasch addition between B3 with methyl acrylate and purified by column chromatography on silica gel and distillation. Metal-Catalyzed Radical Polyaddition of AA- and BB-Type Monomers Polymerization was carried out under dry nitrogen in baked glass tubes equipped with a three-way stopcock. A typical example for the polymerization is given below. A mixture of FeCl2 (50.7 mg, 0.40 mmol) and Pn-Bu3 (0.20 mL, 0.80 mmol) in toluene (2.54 mL) was stirred for 24 h at 80 °C to give a homogeneous solution of FeCl2(Pn-Bu3)2 complex. After the solution was cooled to room temperature, A1 (8.0 mmol) and B1 (8.0 mmol) were added. The solution was evenly charged in seven glass tubes, and the tubes were sealed by flame under a nitrogen atmosphere. The tubes were immersed in thermostatic oil bath at 100 °C. In predetermined intervals, the polymerization was terminated by cooling the reaction mixtures to –78 °C. The conversion of A1 was determined from the concentration of residual monomer measured by SEC using UV detector 142 In Progress in Controlled Radical Polymerization: Mechanisms and Techniques; Matyjaszewski, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.

operating at 254 nm (1490 h, 97%). The quenched reaction mixture was diluted with toluene (30 mL), washed with dilute citric acid and water to remove complex residues, evaporated to dryness under reduced pressure, and vacuum-dried to give the product polymers (Mn = 2400, Mw = 7400, Mw/Mn = 3.08). Measurements


1H

NMR spectra were recorded in CDCl3 at room temperature on a JEOL ESC-400 spectrometer, operating at 400 MHz. Conversion of A1, number-average molecular weight (Mn), weight-average molecular weight (Mw), and molecular weight distribution (Mw/Mn) of the product polymers were determined by SEC in THF at 40 °C on two polystyrene gel columns [Shodex K-805L (pore size: 20–1000 Å; 8.0 mm i.d. × 30 cm) × 2; flow rate 1.0 mL/min] connected to a Jasco PU-980 precision pump, a Jasco 970-UV detector, and a Jasco 930-RI detector. The columns were calibrated against eight standard poly(MMA) samples (Shodex; Mp = 202–1,950,000; Mw/Mn = 1.02–1.09). Glass transition temperature (Tg) of the polymers were recorded on Q200 differential scanning calorimetry (TA Instruments Inc.). Samples were first heated to 100 or 150 °C at 10 °C/min., equilibrated at this temperature for 0 or 10 min, and cooled to –100 °C at 10 °C/min. After being held at this temperature for 20 min, the samples were then reheated to 200 °C at 10 °C/min. All Tg values were obtained from the second scan, after removing the thermal history.

Acknowledgments This work was supported in part by a Grant-in-Aid for Young Scientists (S) (No. 19675003) by the Japan Society for the Promotion of Science, a Grant-in-Aid for Scientific Research (No. 23107515) on the Innovative Areas: “Fusion Materials” (Area no. 2206) from the Ministry of Education, Culture, Sports, Science and Technology, Japan, and the Global COE Program “Elucidation and Design of Materials and Molecular Functions.”

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