Atom Transfer Radical Polymerization of Methyl Methacrylate: Effect of

Chapter 17

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Atom Transfer Radical Polymerization of Methyl Methacrylate: Effect of Phenols D. M. Haddleton, A . J. Shooter, A . M. Heming, M. C . Crossman, D. J. Duncalf, and S. R. Morsley Department of Chemistry, University of Warwick, Coventry, CV4 7 A L , United Kingdom

Atom transfer radical polymerization of methyl methacrylate initiated by ethyl-2-bromoisobutyrate and catalyzed by 2-pyridinal-alkylimine copper(I) complexes has been demonstrated to be effective in the presence of large excesses of phenolic radical inhibitors. In the absence of phenol an induction period is observed which is reduced on addition of phenol, topanol, 4-methoxy phenol and 2,6-diisopropylphenol. Single crystal x-ray diffraction studies and variable temperature N M R experiments suggest that this is due to the formation of a bridged copper(II) species. The results indicate that propagation in this type of A T R P system does not proceed via a free radical but via a complexed bridging halide atom species.

Atom transfer radical polymerization (ATRP) is emerging as an efficacious method for the controlled polymerization of styrene, acrylates, methacrylates and other vinylic monomers (1-7). The reaction has been developed from the Kharash reaction used for carbon-carbon bond formation in organic synthesis. Two groups, independently, reported this chemistry for Irving polymerization of vinyl monomers. Sawamoto described the use of Ru(PPh ) Br in conjunction with an alkyl chloride and an aluminum phenoxide/alkoxide activator for the living polymerization of methyl methacrylate in toluene at 60 °C (5-6). Matyjaszewski utilized copper(I) halides in conjunction with 2,2'-bipyridine as a complexing ligand for the controlled radical polymerization of styrene, methyl and butyl acrylate and methyl methacrylate in a range of solvents at between 80 °C and 130 °C (1 - 4). The role of the 2,2'-bipyridine was originally described as increasing the solubility of the inorganic salt although with 2,2'bipyridine a heterogeneous polymerization ensues. This work has been extended for styrene by use of a 4,4'-dialkyl substituted 2,2'-bipyridine e.g. 4,4'-di-n-heptyl-2,2'bipyridine, which increases the solubility of the catalyst system in hydrocarbon such that living polymerization is observed with Mn up to 10,000 and PDI < 1.10 (3). It is noted that even with this soluble copper(I) species the reaction is approximately first order 3

284

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2

© 1998 American Chemical Society

In Controlled Radical Polymerization; Matyjaszewski, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.

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with respect to copper for both 1-phenylethyl bromide and 1-phenylethyl chloride initiated polymerization o f styrene. Subsequently, Percec has reported that the alkyl halide initiator may be replaced with an arenesulfonyl chloride. Teyssie and Granel have extended the catalyst set to nickel(II) with Ni(II)[C6H3(CH2NMe2)2-2,6]Br (8), although this was previously reported by Percec to decompose under the appropriate reaction conditions (7). Ni(II)[C6H3(CH2NMe2)2-2,6]Br leads to living polymerization o f methyl methacrylate with M n up to 65,000 and P D I = 1.2 in toluene at 80 °C. The role o f the 2,2'-bipyridine (bpy) when used in conjunction with copper(I) halides is not only to solubilize the active species but to stabilize copper(I) relative to copper(II) by accepting electron density into a π * orbital (9). W e have reported that bipyridines may be replaced with other α-diirnines with the N = C - C = N skeleton e.g. 1 and 2 where R , R ' = alkyl, aryl, etc. (10). Both 1 and 2 also have the , R

capability o f accepting electron density

R

ι R

I N

into a π * orbital and have been found to be superior to bpy in stabilizing j metals in l o w oxidation states. Ligands R' such as 1 have recently been used coordinated to N i as very effective 1 ethene polymerization catalysts (11). Both 1 and 2 type ligands are very facile to synthesize involving the reaction o f the appropriate aldehyde, or ketone, with a primary amine. The wide range o f R groups available, from primary amines and carbonyl compounds, as well as the possibility o f substitution on the aromatic ring in the case o f 2 gives control over the position o f the Cu(I)/Cu(H) redox couple. Electron donating and withdrawing groups on the ligand stabilize Cu(II) and Cu(I) respectively, as well as allowing control over catalyst solubility. The availability o f catalysts with a range o f solubilities is important in deterrriining the amount o f active species present in solution as well as in the subsequent application o f this chemistry to heterogeneous polymerization, e.g. mini-emulsion. B o t h ourselves (10) and Matyjaszewski (4) have proposed the mechanism shown in scheme 1 as an integral part o f this chemistry, where [I] initiates polymerization via free radical attack on a vinyl monomer. Propagation proceeds as in normal free radical polymerization. The reverse reaction, end-capping the polymer with a bromide atom and regenerating a [Cu(I)] complex, reduces the free radical concentration, and hence the rate o f conventional bimolecular termination.

-Br

r*'/

Ν^ΓΛ

+

- J .

I Scheme 1 [Cu(I)]

[Cu(n)Br]


[1]

286

This mechanism allows for propagation to occur via a free radical process. Indeed, all three metal based A T R P catalysts Cu(I), Ru(II) and Ni(II) have been described as proceeding via a radical process. Sawamoto uses two tests to demonstrate the involvement o f free radicals in the A T R P o f M M A mediated by Ru(II): ( A ) the addition o f radical inhibitors such as galvinoxyl or D P P H immediately stops or prevents polymerization from occurring and ( B ) C N M R indicates the stereochemistry o f the P M M A product to be consistent with a Bernoullian process with stereochemistry similar to P M M A prepared from A I B N in toluene at 60 °C (5, 12). Matyjaszewski also reports that the stereochemistry o f P M M A as prepared with a [Cu(I)] catalyst is similar to that from classical free radical initiators and that galvinoxyl acts as an efficient inhibitor (2) . Again these two pieces o f evidence are used as proof for a free radical process. Teyssie also reports inhibition by galvinoxyl and a persistence ratio o f close to unity with Ν ι ( Π ) which are used as an argument for free radical propagation. It is noted that only in the case o f Ni(II) are the radicals discussed as being temporarily confined within the coordination sphere of the metal. (8)


1 3

One o f the potential advantages o f a radical type living polymerization over an anionic polymerization is robustness towards various functional groups in monomers, solvents and impurities present in the reaction (13). A s such we have been interested in examining the effect o f various functionality's on A T R P . Surprisingly, we have found that certain phenolic compounds enhance A T R P as opposed to inhibiting polymerization as might have been expected. This paper discusses preliminary results from this study. Experimental General: Methyl methacrylate ( M M A , Aldrich) and xylene ( A R grade, Fischer Scientific) were purged with nitrogen for 2 hours prior to use. The initiator, ethyl-2bromoisobutyrate (98%, Aldrich), and C u B r (99.999%, Aldrich) were used as received. The phenols used, phenol (99%, Pronalys A R ) , 4-methylphenol (99%, Aldrich), 2,6diisopropylphenol (97%, Aldrich) and Topanol (2,6-di-ter^butyl-4-methylphenol, 99%, Aldrich) were all used as received. Monomer conversion was measured by gravimetry. Molecular weights and molecular weight distributions were found by size exclusion chromatography using T H F as eluent with one 5 μπι guard and one mixed-E (3000 χ 7.5mm) column (Polymer Laboratories), calibration was against poly(methyl methacrylate) standards. Preparation of 2-pyridinal-pentylimine (1): "Pentylamine (24.4 m L , 0.21 moL, 99%, Aldrich) was added dropwise to Pyridine-2-carboxaldehyde (20.0 m L , 0.21 mol, 99%, Aldrich) with stirring in an ice bath. After complete addition o f the amine approximately 5 g o f dried magnesium sulfate was added and the reaction left for a further 2 hours. The solution was filtered and distilled under reduced pressure. The product was collected at 60 °C, 0.4 mbar (14, 15). H N M R (CDC1 , 250 M H z ): δ = 8.60 (d, 1H), 8.33 (s, 1H), 7.94 (d, 1H), 7.69 (t, 1H), 7.27 (t, 1H), 3.63 (t, 2H), 1.69 (sextet, 2H), 1.31 (overlapping quintets, 2 H each), 0.87 (t, 3H). 2-pyridinal-ethyliniine (2), 2pyridinal-propylimine (3), 2-pyridinal-butylirnine (4) and 2-pyridinal-fer/-butyliniine (5) !

3


287


were prepared in a similar manner replacing /7-pentylamine with the appropriate primary amine. T y p i c a l polymerization. In a typical reaction, C u B r (0.134 g; [Cu] : [Initiator] = 1 : 1 ) was placed in a predried Schlenk tube which was evacuated and then flushed with nitrogen three times. Methyl methacrylate (10 m L ) followed by 2-pyridinal-pentylimine (0.17 m L , [ligand] : [Cu] = 2 : 1 ) was added with stirring and witfiin a few seconds, a deep, brown solution formed. Xylene (20 m L ) and , where appropriate, substituted phenol were then added and the flask heated in a constant temperature oil bath to 90 °C. When the solution had equilibrated ethyl-2-bromoisobutyrate (0.14 m L , [Monomer] : [Initiator]=100 : 1) was added. Samples were taken 30, 60, 120, 180, and 240 minutes after initiator was added. Crystals for single crystal x-ray diffraction study. [ C u ( C H C N ) ] [ B F ] , (7), was added to a solution o f 5 in degassed methanol under nitrogen. The solution was filtered and red crystals suitable for a single crystal x-ray diffraction study recovered at 5 °C. Copper (I) bromide was added to a solution o f 5 (two fold excess) in degassed methanol under nitrogen. A n equimolar amount phenol with respect to 5 was then added and the solution cooled to 5 °C. After 24 Hrs green crystals were isolated. 3

4

4

Results a n d Discussion Polymerization of M M A by Cu(I)Br/2,2'-bipyridine/ethyl-2-bromoisobutyrate (6) i n the presence of phenol. Polymerization o f M M A in xylene solution at 70 °C with [ M M A ] : [Initiator] = 100:1 (i.e. Μ η = 10,000) in the presence o f C u B r and bpy proceeds to approximately 50% conversion after 180 mins, with P D I narrowing over the course o f the reaction (Table I). The addition o f a 5 mole equivalent o f phenol dramatically increases the rate o f polymerization reaching 76% conversion over the same time period. Increasing the amount o f phenol by a factor o f two results in the rate o f polymerization increasing such that greater than 97% conversion is achieved in 180 mins. The increase in rate is seen by the increase in the gradient o f a semi first order plot (gradient = k [Pol*], where P o l * is the active chain end). Each reaction shows an induction period, where the number o f active species increases, prior to a relatively linear region indicating that the active species is not destroyed in the reaction. A value for kp[Pol*], i.e. k(apparent), can be calculated and this is found to increase from 1.2 χ 10" s" to 5.5 χ 10" s" on addition o f phenol. Thus phenol accelerates the rate o f polymerization and does not inhibit the reaction as might have been expected for either a radical or anionic propagation step. It is noted that in this set o f reactions the P D I is considerably greater than would be expected for a living polymerization. Λ 6 0 Γ

p

3

P

1

3

1

l

*H N M R study; role of phenol. Figure l a shows the variable temperature H N M R spectrum o f Cu(I)Br/2,2'-bipyridine in d - D M S O . A t room temperature we see four peaks from pseudo-tetrahedral Cu(bpy)2 . A t higher temperature, corresponding to 6

+


288


Table

I. Polymerization o f M M A , with Cu(I)Br/2,2'-bipyridine/ethyl-2isobutyrate 100 [ M M A ] : 1 [In] : 1 [CuBr] : 3 [bpy] at 70 °C. [Phenol] /[Inl 0 0 0 0 5 5 5 5 10. 10 10 10

il mins

Mn

Mw

PDI

%Conv.

30 60 120 180 30 60 120 180 30 60 120 180

4984 6390 9287 12392 4444 6841 9877 12038 5754 8978 10587 14173

8816 9630 14402 18240 6974 10523 15426 18527 9182 13420 17535 23064

1.77 1.51 1.55 1.47 1.57 1.53 1.56 1.54 1.6 1.5 1.66 1.63

2.4 10.1 30.4 52.7 12.3 23.4 58.4 76.1 26.8 47.1 78.8 97.6

(a) 25°C final 85°C 65°C 45°C 25°C initial

9.:

i.i

9/

Î4

75

8C

7.2

(b)

9f

9/

a

-:

Atom Transfer Radical Polymerization of Methyl Methacrylate: Effect of

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