Asymmetric Atom Transfer Radical Polymerization: Enantiomer

Downloaded by PENNSYLVANIA STATE UNIV on August 8, 2012 | http://pubs.acs.org Publication Date: June 26, 2003 | doi: 10.1021/bk-2003-0854.ch015

Chapter 15

Asymmetric Atom Transfer Radical Polymerization: Enantiomer-Selective Cyclopolymerization of rac-2,4Pentanediyl Dimethacrylate Using Chiral ATRP Initiator Toyoji Kakuchi, Masashi Tsuji, and Toshifumi Satoh Division of Molecular Chemistry, Graduate School of Engineering, Hokkaido University, Sapporo 060-8628, Japan

The enantiomer-selective radical cyclopolymerization of the racemic mixture of (2R,4R-2,4-pentanediyl dimethacryrate (RR-1) and (2S,4S)-2,4-pentanediyl dimethacrylate (SS-1) was achieved using chiral atom transfer radical polymerization (ATRP) initiating systems. RR-1 and SS-1 were predominantly polymerized using methyl 2-bromoisobutyrate /CuBr/(S,S)-2,6-bis(4-isopropyl-2-oxazolin-2-yl)pyridine and methyl 2-bromoisobutyrate/RuCl (PPh ) /(S)- 1,1'-bi-2naphthol, respectively. The enantiomer selectivity ratio was 1.3 ~ 3.2 for the Cu-catalyzed A T R P initiating system and ca. 1.0 ~ 3.4 for the Ru-catalyzed A T R P initiating system. 2

3

3

The synthetic methodology of optically active polymers is of great interest from the viewpoint of the fine stereocontrol of polymerization. Asymmetric polymerization, such as the asymmetric synthesis polymerization, helix-senseselective polymerization, and enantiomer-selective polymerization, has been achieved using chiral ionic and coordination initiating systems (/-J). For the asymmetric radical polymerization, Wulff et al. and we established the radical cyclocopolymerization of a divinyl monomer having a chiral template with an achiral vinyl monomer leading to the main chain chiral vinyl polymer after removal of the chiral template unit from the precursor polymer (4,5). Although Okamoto et al. reported the he fix-sense-selective radical porymerization with enantiomer selectivity using the optically active phenyl-2-pyridyl-o-tolylmethyl

206

© 2003 American Chemical Society

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


207 methacrylate with various enantiomeric excesses, to our knowledge, little is known about the enantiomer-selective radical polymerization of a racemic monomer (6). Atom transfer radical polymerization (ATRP), which has been developed from the Khar ash addition and cyclization and used extensive ry in organic synthesis, has been a major step forward in the "living/controlled" polymerization by Matyjaszewski et ai. and Sawamoto et al. (7,8). Although A T R P has been demonstrated to provide excellent control over the molecular weight and polydispersity and used for the synthesis of various maeromolecular architectures such as block copolymers and star polymers, the stereochemical control of the polymer main chain is still insufficient (9). Thus, of great interest is the endeavor to finely control the stereochemistry of the radical polymerization based on ATRP. In this chapter, we describe the enantiomerselective radical cyclopolymerization of a racemic monomer using a chiral A T R P initiating system, i.e., the polymerization of rac-2,4-pentanediyl dimethacrylate (rac-1) was carried out using the initiating system consisting of methyl 2-bromoisobutyrate (3), metal sources (Cu-1-2 and Ru-1), and chiral ligands (1-1-3) or additives (a-1-5).

Experimental Section ]

13

Measurements. The H and C N M R spectra were recorded using a JEOL JNM-400II spectrometer in deutero-chloroform at 25 °C. The optical rotatory measurements were performed in chloroform at 28 °C using a Jasco DIP 1000 digital polarimeter. The molecular weights were measured by gel permeation chromatography in T H F using a Jasco G P C 900 system equipped with three polystyrene columns (Shodex KF-804L). The number-average molecular weights (M„s) and molecular weight distributions (MJM s) were calculated on the basis of a polystyrene calibration. The chiral high-performance liquid chromatography (HPLC) analysis was performed using a Jasco H P L C system (PU-980 Intelligent HPLC pump and U V 975 Intelligent U V detector) equipped with a Daicel C H I R A L C E L O B - H column (ehient, hexane/2-propanol (vol. ratio, 100/1); flow rate, 0.5 m L min" ). The gas chromatography (GC) analysis was recorded using a Shimadzu GC-17A gas chromatograph equipped with J & W Scientific 30 m DB-1 column. n

!

Monomer. rac-2,4-Pentanediyl dimethacrylate (rac-1) was obtained by esterification of rac-2,4-pentanediol with methacryloyl chloride in N-methyl-2pyrrolidinone at room temperature (10).



oô

SS-1

OÔMe

-Br

metal source =

3 =

oô

Cu-1

CuBr

rac-Λ

σ^ο J

3

Cu-2

6

9X\&

3

Ru-1

^Cl

-F

lfPh

soluvent, Δ

3/metal source/ligand or additive

[Cu(CH CN)4]PF

flff-1

Scheme 1


CK^D

polymer 2

O ^ O


a-1

a-2

a-3

a-4


a-5

SO

ο

210 Polymerizations. A typical example of the polymerization using 3/Cu-l/I1 as an initiating system is given follows: In a glovebox under a moisture- and oxygen-free argon atmosphere ( H 0 and 0 < 1 ppm), raoA (500 mg, 2.08 mmol), 3 (3.7 mg, 2.08 χ 10' mmol), Cu-1 (5.9 mg, 4.16 χ 10" mmol), and H (38.3 mg, 8.32 χ 10" mmol) were dissolved in anisole (20.8 mL). Two mL of the mixture was placed in a dry test tube, capped, and then taken out of the glovebox. The test tube content was stirred at 90 °Q. At the end of the polymerization, an aliquot (20 μ ί ) of the reaction mixture was added to hexane (0.5 mL) and filtered through a 025 μηι pore membrane filter. The sample was analyzed for the monomer conversion and enantiomeric excess (e.e.) by H P L C equipped with a C H I R A L C E L O B - H column and for the M and MJM by S E C The residual polymerization mixture was passed through a short alumina column to remove the metal salts, and the solvent was removed under reduced pressure. The residue was poured into hexane and the precipitate was filtered. The obtained powder was purified by reprecipitation with chloroform-methanol and dried in vacuo. 2

2

2

2


2

n

n

Results and Discussion Copper-catalyzed Cyclopolymerization of racA. The atom transfer radical polymerization of rac-l was carried out using an initiating system consisting of 3, C u X , and chiral amine ligands (Table I). A l l the polymerizations homogeneously proceeded and the obtained polymers were soluble in chloroform and tetrahydrofuran. Because the characteristic absorption due to the methacrylate groups was not observed in the C N M R spectrum of the resulting polymer, the polymerization of rac-l proceeded through a cyclopolymerization mechanism to afford the polymer essentially consisting of cyclic constitutional repeating units, i.e., the extent of the cyclization was ca. 100 %. , 3

The number average molecular weights (A/ s) of the resulting polymers ranged from 11,000 to 13,500 and the molecular weight distributions (MJMs) were relatively narrow in the range of 1.21 ~ 1.30. The chiral amine ligand affected the enantiomer selectivity, i.e., the enantiomeric excess (e.e.) of the recovered monomer was 6.8 % ~ 15.3 %. In addition, the enantiomer selectivity changed with the chirality of the amine ligands used, i.e., (2S',45)-2,4-pentanediyl dimethacrylate (SS-l) was predominantly polymerized using 1-JÎ3, whereas (2/?,4/?>2,4-pentanediyl dimethacrylate (ΛΛ-1) using was predominantly polymerized 1-S3. The obtained polymers exhibited optical activity, and the absolute values of the specific rotation ([ocjus) of the obtained polymers were from 18.9° to 38.6°. For porymer 2 prepared using 3/Cu-l/l-»S'3, the SEC chromatograms using R l ( b w e r ) and polarimetric (upper) detectors are shown in Figure 1. n


211

Table I. Enantiomer-selective polymerization of rac-l using coppercatalyzed chiral A T R P initiating system "


CuX/ligand

hr

Cu-1/1-1 Cu-l/l-2 Cu-l/kS3 CU-1/1-JÏ3 Cu-2/1-1 Cu-2/1-2

polymer 2

recovered monomer conv. e.e. ' enriched isomer % % 26.9 6.8 ss-i 18.5 9.4 SS-l 15.3 24.6 SS-l 13.4 22.8 RR-l 4.7 18.5 SS-l 21.3 3.4 SS-l

4 12 6 6 9 24

M (MJMn)" n

13,500(1.23) 11,000(1.30) 12,300(1.25) 11,200(1.23) 10,500(1.21) 12,900(1.25)

[a] 5 * 43

-18.9° -35.5° -36.5° +38.6° -19.6° -12.3°

" [rac-l] = 0.1 mol L•'; [root ] /p]o/[CuX]D/pigand]o = 200/1/2/4; solvent, anisole; temperature, 90 °C. 0

0

b

Determined by HPLC equipped with CHIRALCEL OB-H column.

c

Enantiomeric excess of recovered monomer.

d

Determined by SEC in THF using polystyrene standards.

e

c = 0.3 in chloroform at 28 °C

Polarimetric-detecting

RI-detecting I ι ι ι I ι ι ι I ι ι ι ; ι ι ι I ι ι ι I ι ι ι I ι ι ι I

16

18

20

22

24

26

28 30 volume / mL

Figure 1. SEC chromatograms using RI (lower) andpolarimetric (upper) detectors for polymer 2 prepared using 3/Cu-l/l-S3.


212 The peak shapes of the two chromatograms were almost similar, suggesting that the chirality of polymer 2 should be due to the excess amount of the RR-l units in the copolymer composition. Figure 2 shows the relations between the M and MJM of the obtained polymers and the monomer conversion for the polymerization of rac-l using 3/Cu-l/l-%S3. The M values increased with the increasing monomer conversion, and the MJM values were relatively narrow in the range of 1.26 - 129. Figure 3 shows the kinetic plots for the polymerization of rac-l using 3/Cu-l/ln$3. The apparent polymerization rates of SS-l and RR-l were both first order with respect to the monomer concentration, indicating that the polymerization of rac-l using 3/Cu-l/I-»S3 was living-like. The consumption rate of RR-l was faster than that of S S - l . The enantiomer-selectivity ratio (r) was calculated using eq. 1, the monomer conversion and the e.e. value of the recovered monomer (72), n

n


n

l n r

"

(1 -conversion/100)(1 - e.e./lOO) (1 - conversion/100)(1 + e.e. 1100)

According to eq. 1, for the polymerization of rac-l using the 3/Cu-l/ligand, the r value was calculated as 3 2 for M 3 , 1.6 for 1-1,1.3 for 1-2, and 3.1 for Ι-Λ3. Figure 4 shows the change in the e. e. of the recovered monomer and the specific rotation of the obtained polymers with the monomer conversion for the polymerization of rac-l using 3 / C u - l / k S 3 . The e.e. value of the recovered monomer increased with the increasing monomer conversion. On the other hand, the specific rotation ([a] 3s) of the resulting polymers decreased with the increasing monomer conversion, which was caused by the optical purity (o.p.) of the resulting polymers decreasing with the increasing monomer conversion. In general, the o.p. value of the resulting polymer is given by eq. 2 (13), 4

o. ~ p

(2)

For example, the o.p. value of the resulting porymer was calculated to be 56.3 % at a 13.1 % monomer conversion and 8.0 % at a 80.8 % monomer conversion. This change in the o.p. was in good agreement with that of the specific rotation, indicating that the optical activity of the polymer was attributable to the excess amount of the RR-l units in the obtained polymer. These results indicated that the chiral Cu-complex affected the addition of rac-l to the growing end, in which the SS-l enantiomer of rac-l was predominantly polymerized, i.e., the enantiomer-selective radical polymerization. Copper-catalyzed Copolymerization and Homopolymerization of RR-l and SS-l. The enantiomer-selective polymerization of rac-l using the chiral A T R P initiating system can be treated as the usual cyclocopolymerization of RR-l and SS-l in the following reactions,



ο Ο

20

40 60 80 conv. / % Figure 2. Dependence of M„ (open circle) and MJM (open square) on monomer conversion for polymerization of rac-l using 3/Cu-l/l-S3. n



214

conv. / % Figure 4. Dependence of e.e. ofrecovered monomer (open circle) and [a]435 of polymer 2(open square) on monomer conversion for polymerization of rac-l using 3/Cu-l/l-S3.

Figure 5. Mole fractions of the RR-l units in polymer 2 (ft) vs. mole fraction of RR-l in the feed (Ft). The circles are experimental results, and the solid line is thefitof the Mayo-Lewis equation to the data with r$ = 0.296 and rR = 1.49.


215 2

+ SS-l

—SS-S-S* C u B r / L

2

+ RR-l

-S-S-R-R' C u B r / L

2

(4)

+ SS-l

~»R-R-S-S* C u " B r / L

2

(5)

I!

Cu Br /L 2

n

^R-R* C u B r / L 2

2

n

Asymmetric Atom Transfer Radical Polymerization: Enantiomer

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