Lewis Acid-Catalyzed Tacticity Control during Radical Polymerization

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Chapter 5

Lewis Acid-Catalyzed Tacticity Control during Radical Polymerization of (Meth)acrylamides Yoshio Okamoto, Shigeki Habaue, and Yutaka Isobe Department of Applied Chemistry, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan

The tacticity control during the radical polymerization of (meth)acrylamides was achieved in the presence of a catalytic amount of Lewis acids such as Y(OTf) and Yb(OTf)3. The conventional polymerization of N-isopropylacrylamide without Lewis acids produced an atactic polymer (meso diad = 42 - 46 %), whereas the polymer prepared using the Lewis acids in methanol had the meso diad of more than 90%. A n analogous effect of the Lewis acids was observed during the polymerization of other (meth)acrylamides, including acrylamide, Ν,Ν-dimethylacrylamide, methacrylamide, N -methylmethacrylamide, N-isopropylmethacrylamide, N -phenylmethacrylamide, and (R)-N-[(methoxycarboxyl) -phenylmethyl]methacrylamide. 3

The tacticity control of a polymer is an important goal for polymer science and industry, because the property and function of a vinyl polymer significantly depend on the stereostructure (1-8). The stereocontrol during radical polymerization cannot be readily attained (5-7), although it has been widely used in industry, and therefore, stereoregular polymers have been mainly produced by ionic and coordinate polymerization processes. A growing freeradical is often very active and electrically neutral, which prevents it from interaction with other agents and makes the control during the propagation difficult. However, in the past decade, a remarkable advance has been made in the living radical polymerization, which enables the synthesis of block and graft copolymers with a defined structure by a radical process (9-12). On the other

© 2003 American Chemical Society

In Advances in Controlled/Living Radical Polymerization; Matyjaszewski, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

59

60 hand, the control of stereochemistry during radical polymerization has slowly advanced (5). The monomers such as dienes (13) and acrylonitrile (14) included in the host crystals afford the stereocontrolled polymers. The solid state polymerization of crystalline diene monomers produces stereoregular polymers (15). The radical polymerizations of the bulky methacrylates (16,17) and acrylamides (18,19) proceed in an isotactic-specific manner. Fluoroalcohols, such as perfluoro-tert-butyl alcohol ((CF ) COH) and l,l,l,3,3,3-hexafluoro-2propanol ( ( C F ^ C H O H ) , can clearly influence the stereochemistry during the radical polymerization of vinyl esters (7,20,21) and methacrylates (7,22,23) through the hydrogen-bonding interaction between the monomers and the fluoroalcohols. Recently, we examined the influence of various Lewis acids on the stereochemistry of the radical polymerization of polar monomers including asubstituted acrylates and (meth)acrylamides, and a clear effect has been observed. Poly[(meth)acrykmide]s are important materials due to their unique properties as gels or hydrophilic polymers (24-26). Among the many Lewis acids, rare earth metal trifluoromethanesulfonates (triflate: OTf) were particularly effective in increasing the isotacticity of various poly(aciylamide)s (27-29) and poly(methaciylamide)s (30,31). Lewis acids have been widely used in organic syntheses. In polymer chemistry, Lewis acids have been used to change the reactivity of monomers during copolymerization, and several alternating copolymers have been prepared (32-34). During the copolymerization of methyl methacrylate ( M M A ) and styrene, the addition of B d resulted in a highly coheterotactic alternating copolymer (35,36). On the other hand, during the homopolymerization, although Lewis acids affected monomer reactivity (37), their influence on the tactieity of the polymers had rarely been reported (5). Matsumoto et al. reported that M g B r was effective in enhancing slightly the isotactic selectivity during the radical polymerization of M M A (38). We also found that Sc(OTf) increased the isotacticity of the poly(methacrylate)s (7,29,39). For α-alkoxymethyl acrylates, the conventional polymerization without Lewis acids produced atactic polymers, but that in the presence of Z n B r 2 and Sc(OTf) resulted in syndiotactic and isotactic polymers, respectively (40,41). Porto* et al. reported that cyclic acrylamides produce isotactic polymers in the presence of rare earth metal triflates (42). Ν,Ν-Disubstituted acrylamides can be polymerized by anionic initiators to produce stereoregular polymers (43,44), but (meth)acrylamides bearing an amide N H proton on the side chain cannot be anionically polymerized, and therefore, title stereocontrol during their polymerization has hardly been studied. In this chapter, we describes the isotactic-specific radical polymerization of (meth)acrylamides, including N-isopropylacrylamide (NIPAM), acrylamide ( A M ) , Ν,Ν-dimethylacrylamide ( D M A M ) , methacrylamide ( M A M ) , N methylmethacrylamide ( M M A M ) , N-isopropylmethacrylamide ( I P M A M ) , N phenylmethacrylamide ( P M A M ) , and (R)-N-[(methoxycarboxyl)phenylmethyI|methacrylamide ( M C P M M A M ) , in the presence of Lewis acids.


3

3

3

2

3

3



N-Isopropylacrylamide (NIPAM) Table I shows the results of the polymerizations of NIP A M in fhe presence of various Lewis acids in methanol at 60°C. The polymerizations were carried out using Lewis acids in dry nitrogen. The polymerization in the absence of Lewis acids produced an atactic polymer (m / r = 44 / 56), whereas that in the presence of a catalytic amount of Lewis acids proceeded in an isotactic-specific manner. Most rare earth metal triflates except for Sc(OTf)3 exhibited a similar isotacticity-enhancing effect and produced a polymer having a meso (m) value of about 80%. Rare earth metal chlorides were less effective than the corresponding triflates. The effect of solvents was studied in the presence of Y(OTf)3 (Table II). hi the absence of Lewis acids, the influence of the solvents on the tacticity was very small and atactic polymers (m = 42-46%) were produced. The effect of Lewis acids significantly depended on the polymerization solvent, and the highest isotacticity-enhancing effect was observed in methanol. The effect of Lewis acids in alcoholic solvents decreased in the order of methanol > ethanol > isopropanol with increasing bulkiness. Y(OTf) was also effective in tetrahydrofuran (THF), Ν,Ν-dimethylformamide (DMF), chloroform, toluene, 1,4-dioxane and even in water. However, the effect disappeared in D M S O . Figure 1 illustrates the relationship between the Y(OTf) concentration and the m value of the obtained polymers for the polymerization of N I P A M at 60°C, - 2 0 ° C , and - 7 8 ° C A small amount of Y(OTf) was enough to affect the stereostructure of the obtained polymers, and 8 mol% of Y(OTf)i to N I P A M resulted in a polymer having an m value of 90% at -20°C. The highest isotacticity was attained at [Y(OTf) ] = 0.5 mol/1 ([Y(OTf) ] / [ N I P A M ] - 0 . 2 ) at 60°C and - 2 0 ° C (m = 92%), while the further addition of Υ ( Ο Τ ί ^ reduced the m value. Lewis acids were more effective at -20°C than at 60°C and - 7 8 ° C . At - 7 8 Τ ! , the Lewis acids may not catalytically function. Figure 2 shows the *H N M R spectra of the atactic (A) and isotactic poly(NIPAM)s (B) measured at 170°C in D M S O - d . The diad tacticities of these polymers were determined on the basis of the peaks (1.3 ppm-1.8 ppm) of the methylene protons (45). 3

3

3

3

0

3

0

6


0

62

Table I. Effect of Lewis Acids in the Polymerization of NIPAM in Methanol 11


Run 1 2 3 4 5 6 7 8 9 a

Lewis acid Yiela* Ίαώάίβ m/r % None Sc(OTf) Y(OTf) La(OTf) Ce(OTffe Pr(OTfb Nd(OTf) Sm(OTf) Eu(OTf)3 3

3

3

3

3

82 86 94 98 89 96 99 87 94

45/55 62/38 80/20 76/24 79/21 81/19 80/20 81/19 82/18

Run 10 11 12 13 14 15 16 17 18

0

Lewis acid Yield* Tacticity m/r % 81/19 99 Gd(OTf) 82/18 97 TtyOTfy 83/17 98 Ho(OTf) 82/18 94 Er(OTf) 84/16 97 Tm(OTf) 82/18 89 Yb(OTf) 84/16 97 LutOTffc 57/43 85 ScCl 67/33 95 YbCl 3

3

3

3

3

3

3

[NIPAM] = 2.40 mol/1. Initiator: AIBN (0.02 mol/1). Temp. = 60°C. Time = 3hr. 0

Hot water-insoluble part. determined by *H NMR (400MHz) in DMSO-4 at 170°C.

Table II. Effect of Solvent in the Polymerization of NIPAM in the Presence ofY(OTf), a

Run

0

Yield

Solvent

1 2 3 4 5 6 7 8 9 10 11

% 82 94 90 92 94 66 75 96 88 96 >99

Methanol" Methanol Ethanol Isopropanol H 0 THF CHC1 1,4-Dioxane Toluene DMSO DMF 2

3

Tacticity" m/r 45/55 80/20 73/27 64/36 57/43 67/33 62/38 53/47 52/48 47/53 60/40

a

[NIPAM]o = 2.40 mol/1. [Y(OTf) ] = 0.20 moM. Initiator: AIBN (runs 1-4, 6-11), Na S03+K2S 08 (run 5) ([initiator] = 0.02 mol/1). Temp.=60°C. Time = 3hr. 3

2

2

0

0

^ o t water-insoluble part. !

determined by H NMR (400MHz) in DMSO-ck at 170°C. d

WithoutY(OTf)3.



63

0.2

0.4

0.6

0.8

[Y(OTf) ] / [NIPAM] 3

Figure 1. Relationship between the [Y(OTf) ] / [NIPAM] ratio and m value of the obtained poly (NIPAM). Conditions: [NIPAM] = 2.40 mol/l Initiator: AIBN(0.02 mol/l) (60°C), AIBNwith UVirradiation (~20°C), (n-Bu) B (0.10 mol/l) with air (-40°C). Time = 3hr (60°C), 24hr (-78°C, -20°C). 3

0

3

—

T

o

—

ζ

jr***>

!

Figure 2. HNMR spectra ofpoly (NIPAM)s prepared (A) in the absence and (B) presence ofY(OTf) (0.50 mol/l) in methanol at -20°C [400MHz, DMSO-d , 170°C]. 3


6

64


Acrylamide (AM) Poly(AM) and its copolymer are widely used for various applications including the clarification of waste water, o i l recovery, and paper manufacture (26). Table III shows the results of the polymerization of A M under various conditions. Although atactic polymers of similar tacticities were obtained in the absence of Lewis acids regardless of temperature, in the presence of rare earth metal triflates, the isotactic polymers with different regularities were obtained depending on the polymerization conditions. The isotactieity increased in the order of Sc(OTf)3 < Y(OTf) < Yb(OTf) . The highest isotactieity (mm = 71) was observed for the pofymerization with Yb(OTf) at - 4 0 ° C . However, the effect of the Lewis acids became negligible at -78°C. The tacticities of the poly(AM)s were determined by the C N M R spectra in D 0 at 80°C on the basis of the peaks of the α-carbon according to the references (46,47). Figure 3 shows the C N M R spectra in the main chain region of the atactic (A) and isotactic poly(AM)s (B). 3

3

3

, 3

2

1 3

Ν,Ν-Dimethylacrylamide (DMAM) The effects of Lewis acids during the radical polymerization of D M A M in methanol are shown in Table IV. The stereocontrol of this monomer has been examined by anionic (43,44) and radical processes (48). The m content of the polymers obtained without Lewis acids slightly increased when the temperature

Table III. Effect of Lewis Acids in the Polymerization of A M in Methanol Run

Lewis acid

1 2 3 4 5 6 7 8 9 10

None Yb(OTf) None Sc(OTf)j Yb(OTf) Y(OTf) None Yb(OTf)

0

0.10 0.10 0.10 -

3

3

Y(OTf) Yb(OTf)

0.20 0.20 0.20

3

3

3

% 60 84 60 70 50 90 81 95 93 >99

60 60 0 0 0 0 -40 -40 -40 -78

0.10 -

3

Yield*

Temp °C

[LA] mol/l -

8

0

Tacticity mm/mr/rr 24/52/25 47/40/13 22/49/29 40 / 4 3 / 1 7 65/29/6 58/33/9 25/47/29 71/23/6 61/30/8 32/47/21

a

[AM] = 1.00 mol/l. Initiator: AIBN (60°C), or AIBN with U V irradiation (0°C) ([AIBN]o = 0.02 mol/1), (n-Bu) B with air ([(n-Bu) B] =0.10 mol/1) (-78~40°C). Time 24hr. 0

3

3

o

b

Polymers were purified by dialysis with a cellophane tube in water, l 3

determined by C NMR (125MHz) in D Q at 80°C. 2


65


α-carbon

44 42 40 38 36 34 Figure 3. CNMR spectra of poly(AM)s prepared (A) in the absence and (B) presence ofY(OTj) (0.1 mol/l) in methanol at 0°C. [125MHz, D 0, 80°CJ I3

3

2

changed from 60°C to -1S°C. Similar to the polymerization of N I P A M , isotactic polymers were obtained during the polymerization in the presence of rare earth metal triflates, such as Yb(OTf>3, Y(OTf) , and Lu(OTf)3. Sc(OTf) was less effective as well as for N I P A M , and the effect of M g B r andZnBr was not obviously observed. During the polymerization in the presence of Yb(OTf) at 0°C, the m value of the polymer reached 88%. In Table V, the effects of the solvents are summarized for the polymerization in the presence of Yb(OTf) at 60°C. The tacticity of the obtained polymer strongly depended on the solvents. Although the isotacticity of the polymers prepared in the absence of Lewis acids slightly increased in the order of methanol < ethanol < isopropanol < THF < toluene, the isotacticityenhancing effect of Yb(OTf) increased in the reverse order. This suggests that the polarity in an important factor for controlling the stereochemistry. For the Lewis acid to work catalytically, the exchange of the acid between polymer chain and monomer must be essential In nonpolar solvents, this may be slowed down. 3

3

2

2

3

3

3

Methacrylamides The radical polymerizations of methacrylamides also proceeded in an isotactic-specific manner by using a catalytic amount of Lewis acids (Table VI), although the conventional radical polymerization without Lewis acids produced syndiotactic-rich polymers. During the polymerizations of M A M with Yb(OTf) , the isotacticity and heterotacticity of the polymer increased. This 3


66 Table IV. Effect of Lewis Acids in the Polymerization of D M A M in Methanol 8


Run

Lewis acid

1 2 3 4 5 6 7 8 9 10 11

None Sc(OTf)j Yb(OTf) Y(OTf) Lu(OTf)j MgBr ZnBr None Yb(OTfb None Yb(OTf)

3

3

2

2

3

Tacticity m/r 46/54 78/22 8 4 / 16 84/16 85/15 47/53 45/55 49/51 88/12 55/45 65/35

Yield* % 73 76 86 90 85 68 89 81 76 62 76

Temp. °c 60 ' 60 60 60 60 60 60 0 0 -78 -78

a

[DMAM] = 1.00 mol/l. [LA] = 0.10 mol/l. Initiator: AIBN ([AIBN] = 0.02 mol/l) (temp. = 0~60°C), (n-Bu) B with air ([(n-Bu) B] = 0.10 mol/l) (temp. = -78°C). Time = 24hr. 0

0

0

3

3

0

Polymers were purified by dialysis with a cellophane tube in water. l

"Determined by H NMR (400MHz) in DMSO-4 at 170°C.

Table V. Effect of Solvent in the Polymerization of D M A M in the Presence ofYb(OTf) a

3

Run 1 2 3 4 5 6 7 8 9 10

Solvent Methanol Methanol Toluene Toluene THF THF Ethanol Ethanol Isopropanol Isopropanol

Lewis acid None Yb(OTf) None Yb(OTf) None Yb(OTf) None Yb(OTf) None Yb(OTf)

[LA], mol/l

3

0.10

3

0.10

3

0.10

3

0.10

3

0.10

-

-

0

Yield % 73 76 96 84 80 71 73 60 21 66

Tacticity m/r 46/54 84/16 53/47 55/45 52/48 65/35 47/53 80/20 49/51 70/30

a

[DMAM] = 1.00 mol/l. Initiator: AIBN ([AIBN] = 0.02 mol/l). Temp. = 60°C. Time = 24hr. 0

0

b

Polymers were purified by dialysis with a cellophane tube in water, l

determined by H NMR (400MHz) in DMSO-dô at 170°C.


67 polymer was insoluble in the common solvents and soluble only in strong acids such as concentrated sulfuric acid. The polymerization of ( R ) - M C P M M A M gave the polymers with a variety of tacticities. The polymerization o f (R)M C P M M A M without a Lewis acid at low temperature (-78°C) produced a syndiotactic polymer (rr = 85%), whereas the polymerization in the presence of Yb(OTf) and M g B r gave the isotactic (run 13, mm = 82%) and heterotactic polymers (run 14, m r = 63%), respectively. The former isotactic polymer was obtained in the presence of equimolar Yb(OTf) to the monomer. The effects of the Lewis acids significantly depended on the monomers and solvents. For the polymerization of M M A M and I P M A M , methanol was a better solvent to attain a higher isotactieity than THF, while for P M A M and ( R ) - M C P M M A M , T H F was a better solvent.


3

2

3

Table VI. Effect of Lewis Acids in the Polymerization of Various Methacrylamides* Run

Monomer

1 2 3 4 5 6 7 8 9 10 11 12 13 14

MAM MAM MMAM MMAM MMAM MMAM IPMAM IPMAM PMAM PMAM (RHV1CPMMAM (R)MCPMMAM (RHMCPMMAM (R)MCPMMAM

Lewis acid Solvent None Yb(OTf) None Sc(OTf) Y(OTf) Yb(OTf) None Yb(OTf) None Yb(OTf) None Yb(OTf) Yb(OTf) MgBr

Methanol Methanol Methanol Methanol Methanol Methanol Methanol Methanol THF THF THF THF THF CHC1

3

3

3

3

3

3

3

3

3

2

0

Temp. Yield" Tacticity mm/mr/rr °C % 7/39/54 34 60 36/50/14 84 60 2/29/69 97 60 2 8 / 5 5 / 17 92 60 46/40/14 91 60 46 / 4 4 / 1 0 89 60 -0/20/80 16 20 63/33/4 54 20 4/30/66 20 0 55/38/7 0 55 -0/15/85 33 -20 68/22/10 59 -20 82/16/2 34 -20 24/63/13 70 60

a

[Monomer] = 2.4 mol/1 (runs 1, 2), 2.0 mol/1 (runs 3-6), 1.0 moM (runs 7-10), 1.1 mol/1 (runs 11-13), 0.54 mol/1 (run 14). [LA] = 0.20 mol/1 (runs 2, 6-8,12), 0.10 mol/1 (runs 8, 10, 14), 1.10 mol/1 (run 13). Initiator: AIBN (0.02 mol/1) (runs 1-10, 14), (n-Bu) B (0.1 mol/1) (runs 11-13). Time = 24 hr (runs 1-10,14), 48 hr (runs 11-13). THF-insoluble part (runs 1-6), hot water-insoluble part (runs 7, 8), water-methanolinsoluble part (runs 9,10), methanol insoluble part (runs 11-14). 0

0

3

b

c

13

Determined by C NMR in D S 0 at 60°C (runs 1, 2), *H NMR in DMSO-dé at 170°C (runs 3-6), C NMR in DMSO-d at 80°C (runs 7-14). 2

4

13

6


68


I o.oio

o.ooo

a 0 0.2 0.4 0.6 0.8 1 [Y(OT£) ] / [Y(OTf) ] + [NIPAM] 3

3

Figure 4. Job 's plot based on the chemical shifts of NCHproton of NIPAM in methanoUd where [complex] was calculated with the following equation: [complex] = [NIPAM] χΔδ/Δό} [Δό} = δ(Ν1ΡAM saturated by Y(OTf) ) bffree NIPAM)]. [NIPAM] + [Y(OTf) ] = 0.1 mol/l. 4t

3

3

time (min) Figure 5. Time-conversion plots of the polymerization of NIPAM in the absence or presence ofY(OTf) in methanol at 60°C. Conditions: [NIPAM] = 0.5mol/l. [AIBN]ο = 0.02mol/l [Υ(ΟΤβ ] = 0.20mol/l. 3

Q

3 0

Mechanistic Study of Catalytic Function of Lewis Acid As mentioned above, a small amount of Lewis acids resulted in a sufficient effect in increasing the isotactic selectivity during the polymerization of the (meth)aciylamides. In the H N M R measurement of the N I P A M solution in l


69 methanol-cU, the signal of the N C H proton shifted downfield with the addition of Y(OTf) . In order to determine the stoichiometry of the interaction between the Lewis acids and the monomers by Job's method (49), the H N M R titration experiment was carried out for the M P A M - Y ( O T f ) system (Figure 4). These results suggest that a nearly 1:1 complex is formed between N I P A M and Y(OTf) . Figure 5 shows the time-conversion plots for the polymerization of N I P A M in the absence and presence of Y(OTf) in methanol at 60°C. The acceleration of the porymerization by Y(OTf) was clearly observed. The accelerating effects of other Lewis acids during the radical polymerization have already been reported (57). This monomer activation effect of the Lewis acid appears to play a very important role in the catalytic action of the Lewis acids. Figure 6 illustrates a plausible mechanism for the stereocontrol during the polymerization by Lewis acids. A monomer must be activated by the coordination of a Lewis acid (A). The activated monomer is preferentially polymerized and therefore, the Lewis acid is incorporated into the propagating chain end. The Lewis acid controls the conformation of the chain end for the polymerization to proceed in an isotactic-specific manner (B). The Lewis acids inside the polymer chain must weakly interact with the chain to readily be transferred to the monomer and activate it again (C). 3

!

3


3

3

3

Η Ï

CH(CH ) 3

2

CH(CH ) NIPAM 3

^ (A) Activation of Monomer

2

Ο Release

NHCH(CH ) 3

r 2

O,

NHCH(CH ) 3

2

(CH ) CHNH 3

(CH ) CHNH 3

2

2

^

CH

2

NHCH(CH ) 3

2

(C) Transfer of Lewis Acid

(B) Isotactic Propagation

- Lewis acid

Figure 6. Plausible mechanism of the stereocontrol in the polymerization of acrylamides by a catalytic amount of Lewis acids. In Advances in Controlled/Living Radical Polymerization; Matyjaszewski, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

70 Acknowledgements


This work was supported in part by the New Energy and Industrial Technology Development Organization (NEDO) under the Ministry of Economy, Trade and Industry, Japan, through the grant for the "Nanostructure Polymer Project" in the "Nanotechnology Materials Program" (2001-).

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C., 14. 15. 16. 17. 18. 19. 20. 21. 22.

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71 23.


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