[ACS Symposium Series] Controlled Radical Polymerization Volume 685 || Stereochemical Control of Free-Radical Polymerization of Vinyl Monomers

Chapter 27

Stereochemical Control of Free-Radical Polymerization of Vinyl Monomers

Tamaki Nakano and Yoshio Okamoto

Department of Applied Chemistry, Graduate School of Engineering, Nagoya University, Chikusa-ku, Nagoya 464-01, Japan

This chapter describes stereoregulation methods for free-radical polymerization based on (I) rationally designed monomers, (II) chiral initiators and chain-transfer agents, (III) reaction conditions (monomer concentration, temperature, solvent), and (IV) template molecules. Method I includes polymerization of bulky methacrylates giving highly isotactic polymers. Method II involves menthol-based peroxides and thiols that can control helic-ity in 1-phenyldibenzosuberyl methacrylate polymerization. Method ΙII was effective for triphenylmethyl methacrylate polymerization where isotacticity content of resulting polymer was controllable in the range >99-64%. Method IV was realized for methacrylic acid polymerization in the presence of amine compounds.

Stereoregulation by free-radical polymerization has been achieved only in limited cases. On the other hand, stereochemical control of polymerization is an important topic in macromolecular chemistry because polymer properties are often significantly influenced by main-chain configuration and conformation, and various effective methods and catalysts for stereoregulation have been found for anionic, coordination, and related polymerizations (7,2). However, because free-radical polymerization is applicable to much wider range of monomers and generally less expensive compared with the other polymerizations (J), it wi l l be a powerful and practical tool for producing various types of polymers having ordered structures once effective methods for stereocontrol are developed. This article describes our recent studies on configu-rational and conformational control of free-radical polymerization.

© 1998 American Chemical Society 451

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

452

Control Based on Monomer Design

Monomer structure can affect propagation stereochemistry not only in free-radical but also in the other polymerizations. Although free-radical polymerization of vinyl monomers such as acrylic monomers, vinyl esters, and styrene generally results in atactic polymers or polymers that are moderately rich in syndiotacticity (4,5), use of rationally designed monomers can lead to more ordered structures including highly isotactic main-chain and also can induce configurational or conformational chirality in the main chain.

Bulky or Chiral Monomers. Several bulky, triarylmethyl methacrylates have been known to give isotactic polymers by free-radical polymerization as well as by asymmetric anionic polymerization (4-6). The isotactic specificity in the polymerization of the monomers is related to helix formation of growing radical chain and the degree of stereospecificity is dependent on the structure of ester group. Above all , 1-phenyldibenzosuberyl methacrylate (1) leads to an almost perfectly isotactic polymer by radical mechanism (7,8). The growing radical derived from 1 may have more complete, rigid helical structure compared with those derived from other monomers such as triphenylmethyl ester and its analogues that result in lower isotactic specificity. The higher rigidity of p o l y - Γ s helix may be based on the rigidity of the ester group. Because the two phenyl groups of 1 are tied to each other with an ethylene group, the side group of 1 should have less freedom of internal rotation of bondings compared with triphenylmethyl group.

In the polymerization of chiral 2, use of optically active monomer can lead to a helical polymer with excess right- or left-handed helicity although the isotacticity of poly-2 is lower (-75%) compared with the poly-1 obtained under similar conditions (9). Table I summarizes the conditions and results of polymerization of 2 having various enantiomeric excesses (e.e.'s). The e.e. values of the monomer had little effect on the tacticity of the obtained polymers, suggesting that the helix formation is mainly governed by the bulkiness of monomer but little by chirality of the monomer in this case. The obtained polymers showed optical activity which was opposite in sign to that of the starting monomer. This strongly suggests that the optical activity of the polymers is mainly based on excess helicity of the main chain. The optically pure (+)-2 gave a polymer with large levorotation (run 7), but by anionic polymerization, the same monomer gave a highly isotactic polymer whose optical activity was about twice as large as that of the radically obtained polymer (10). The smaller optical activity of the radically obtained polymer seems to be based on the lower isotacticity which can mean that the polymer has shorter single-handed helical sequences compared with the anionically obtained polymer.

1 2

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Table I. Radical Polymerization of 2 with ( / -PrOCOO) 2

a t40°C inToluene a

Polymer b

Run E.e. (%) of monomer in feed

Yield (%)

M 365 (deg)

Tacticitv (%)d

m m / m r / I T

E.e.(%) of remaining monomer

1 0 90.4 0 7 4 / 1 9 / 7 0 2 -6.9 98.6 +76 7 3 / 1 9 / 8 -3.4 3 -15.9 88.4 +246 7 2 / 1 9 / 9 -9.0 4 -26.1 84.9 +313 7 2 / 2 1 / 7 -20.6 5 -53.4 95.5 +520 7 5 / 1 9 / 7 -50.3 6 -80.0 90.8 +608 7 5 / 1 9 / 7 -74.6 7 +100 93.7 -617 7 4 / 1 6 / 1 0 +100 8f +100 -1280 9 8 / 1/1 +100

Conditions: [2]/[(/-PrOCOO)2] = 13, time 24 h. bHexane-insoluble part. Degree of polymerization and M w / M n of the polymer were in the range 32-35 and 1.52-1.77, respectively. °In CHCl 3-2,2,2-trifluoroethanol (9/1). Determined by CrPC of P M M A derived from poly-2. determined by chiral

H P L C analysis. Anionic polymerization with n -BuLi in tetrahydrofuran at -78°C. S O U R C E : Adapted from ref. 9.

Figure 1 shows the relation between the e.e. of monomelic unit in the polymer and the optical activity (absolute value) of the polymer based on the data in Table I. The e.e. of monomelic unit was calculated from the polymer yield, the e.e. of the starting monomer, and the e.e. of the recovered monomer. The optical rotation of the polymer was in all cases larger than that expected based on the e.e. of monomelic unit which is indicated as a Une connecting the optical activity values at 0% and 100% e.e.'s of monomelic unit. This observation can reasonably rule out the possibility that the optical activity of the polymer arises simply from the chiral side group of monomelic units. In addition, it can be concluded that an excess helical sense induced by the effect of successive, several (-)-monomeric units based on the major antipode of the starting monomer, can overcome the opposite chiral induction based on the incorporation of (+)-monomeric unit based on the minor antipode of the monomer. In the polymerization of 2 having different e.e.'s, low enantiomer selection was observed. The e.e. of the unreacted monomer was always lower than that of the starting monomer, meaning that the excess antipode was selectively incorporated into the polymer chain.

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700

0 10 20 30 40 50 60 70 80 90 100

e.e. of monomeric units in polymer (%)

Figure 1. Relation between the absolute values of optical activity of the poly-2s and the e.e. of monomeric units in the polymers. (Reproduced from ref. 9. Copyright 1995 American Chemical Society).

Optically active acrylamides 3 also afford isotactic polymers (mm -92%) (77). It has been shown that in the oligomerization of 3, growing radical adds to the vinyl group of monomer selectively from one enantioface by the steric influence of the chiral side group.

° s / ^ R R = -Ph, - B u l , -Pr'

3

Cyclopolymerization of Bifunctional Monomers. Design of birunctional monomers that cyclopolymerize is another approach to stereoregulation. Cyclopolymerization of 4 and 5 gives polymers rich in heterotacticity (poly-4, mm/mr/rr = 14/51/33; poly-5, mm/mr/rr = 12/49/39) (72,73). In the cyclopolymerization of 6, an isotactic polymer (mm 84%) is produced; probably a cyclization effect and helix formation of growing radical are both responsible for the result (14,15). The optically active monomer 7 having two styrene moieties gives polystyrene by copolymerization with styrene followed by removal of the chiral side group (16). In this polymerization, configurational chirality is induced to the main-chain asymmetric centers through cyclization of 7 that forms ( S ^ - d y a d units in a largely atactic polystyrene chain and the resulting polystyrene shows optical activity ([oc] 3 0

3 6 5 -0.5 to -3.5°).

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P M M A derived from the heterotactic-rich poly-5 also shows optical activity ([a] -4.3°) and C D absorptions based on configurational chirality of main chain.

Control by Chiral Initiator and Chain-Transfer Agent

As described in the preceding section, 1 gives almost perfectly isotactic polymer (7,8). The poly-1 prepared under achiral reaction conditions is considered to be an equimolar amount mixture of right- and left-handed helices. This assumption was supported by chiral H P L C analysis of the polymer in which the polymer was resolved into (+)- and (-)-fractions probably corresponding to single handed helical isomers. We carried out the polymerization of 1 under chiral reaction conditions in order to examine the possibility of production of single-handed helical poly-1 via free-radical mechanism (17).

Polymerization of 1 with Chiral Initiator. The possibility of chiral induction through initiation and the following stages of propagation was tested using chiral initiators 8 and 9 that were synthesized from optically active menthol. Table II shows the conditions and results of polymerization. The polymerization products were mostly insoluble in common solvents but contained a THF-soluble, benzene-hexane (1:1) (B/H)-insoluble fraction. This fraction, which has a degree of polymerization of ca. 40-45 and is free from oligomeric products, was separated from the original products and used for chiroptical property analysis. The polymer obtained using 8 at [l]/[8] = 1 showed dextrorotation though the one obtained at [l]/[8] = 50 did not show significant optical activity (runs 1 and 2). The optically active polymer exhibited C D absorptions whose spectral pattern was similar to that of one-handed helical poly-1 obtained by asymmetric anionic polymerization (18), indicating that the optical activity is based on excess right- or left-handed helicity. The production of the optically active polymer only at the higher [l]/[8] ratio suggests mat the helix-sense selection took place more effectively through primary radical termination rather than through initiation reaction. Because a polymer chain obtained from 1 is considered to have a complete helical structure probably without helix reversals or some defects, helix-sense selection through initiation reaction would give an optically active polymer regardless of the concentration of 8. The conclusion was supported by G P C analysis of the polymer with polarimetric and U V detections. The G P C analysis of the polymer of run 2 indicated that the polymer consists of (+)-fraction with higher molecular weight and (-)-fraction with lower molecular weight. This can be explained in terms of unequal rates of primary radical termination for (+)- and (-)-helical growing radicals having opposite sense of helix.

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Table IL Helix-Sense-Selective Free-Radical Polymerization of 1 with 8 and 9 as Initiator in Toluene

THF- inso l . THF-soL , B/H-insol . part part

Run Initiator [l]/[8 or 9] Temp. Yield 2 1 Yield Yield [ a ] 3 6 5

b D P C

( ° Q (%) (%) (deg)

1 8 50 40 75 69 1 ~0 40 2 8 1 50 48 30 3 +40 44 3 9 1 50 59 41 12 +20 40

aHexane-insoluble products. bMeasured in T H F . °Deteimined by G P C analysis of P M M A derived from poly-1. S O U R C E : Adapted from ref. 18.

(s-4 (s-^l 8 9

The polymerization with 9 also gave optically active polymer (run 3) but the polymer showed a C D spectrum that is more similar to that of 9 itself rather than to that of helical poly-1 obtained by anionic polymerization. The chiroptical properties of poly-1 of run 3 may be principally ascribed to the chiral group attached to the chain terminals originating from the initiator.

Polymerization of 1 with Chiral Chain-Transfer Agent. Chiral thiols as chain-transfer agents can induce single-handed helix through the three possible mechanisms as shown as eqs. (l)-(3) where a dot (·) denotes a radical electron:

poly-1* + R S H • p o l y - l - H + RS« (1)

RS* + n l • RS-po ly - Ι · (2) p o l y - Ι · + RS* • po ly - l -RS (3)

Reaction (1) is hydrogen transfer from a thiol to a helical growing radical, (2) initiation or polymerization by the thio radical formed through (2), and (3) coupling termination of a helical growing radical with the thio radial formed through (2).

The polymerization of 1 was carried out with (/-PrOCOO) 2 in the presence of (+)- and (-)-neomenthanethiol (10) and (-)-menthanethiol (11). The conditions and results are shown in Table III. Under all conditions, optically active polymers were obtained and optical activity of the polymers was larger than that of the polymers obtained using chiral initiators. The use of (+)- and (-)-10 resulted in the polymers with opposite optical activity. Also, a higher concentration of 10 results in a lower overall yield of products and higher optical activity of the polymer. The polymers showed a similar C D spectral pattern to that of anionically obtained single-handed helical poly-

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1, indicating the optical activity of the polymers obtained in the presence of the chiral thiols is based on excess single-handed helicity.

From G P C analysis of the polymer of run 3, it was found that the polymer consisted of (-)-fraction with higher molecular weight and (+)-fraction of lower molecular weight. The (+)- and (-)-fractions collected by G P C fractionation experiments exhibited C D spectral patterns which are symmetrical to each other. This strongly suggests that the opposite sign of optical rotation means opposite excess helicity in the fractions. These results indicated that chiral induction occurred through the reactions of eqs. (1) and/or (3) (termination reactions). However, detailed *H N M R studies of P M M A derived from the poly-1 of run 3 showed that the polymer bears a hydrogen atom and no neomenthyl group at the co-end of the main chain. This observation excludes the possibility of chiral induction through eq. (3). Therefore, it can be concluded that difference in the hydrogen transfer rate in eq. (1) for (+)- and (-)-growing radicals is responsible for the helix-selection.

The G P C fractionation of poly-1 of run 3 gave a (-)-fraction whose optical activity was estimated to be ca. [a ] 3 6 5 -750°. This value corresponds to an enantiomeric excess of ca. 40% that means a mixture of (-)- and (+)-helices in a ratio of 7 to 3.

Table III. Helix-Sense-Selective Free-Radical Polymerization of 1 with ( /-PrOCOO) 2 at 40°C in the Presence of 10 and 11

as Chain-Transfer Agent in Toluene

THF- inso l . THF-soL , B/H- insol . Chain- partb part

Run Transfer [1]/[10 or 11] Y i e l d a Yield Yield [ a ] 3 6 5

c D P d

Agent (%) (%) (deg)

1 (+)-10 0.05 82 73 2 -80 42 2 (+)-10 0.1 80 70 3 -130 41 3 (+)-10 0.2 71 54 5 -140 42 4 (+)-10 0.4 18 ~0 11 -140 40 5 (-)-10 0.4 19 ~0 10 +110 51 6 ( + ) - l l 0.05 86 82 2 +60 50

aHexane-insoluble products. bDegree of polymerization was in the range 84-150 as determined by G P C analysis of P M M A derived from poly-1. M e a s ured in T H F . determined by G P C analysis of P M M A derived from poly-1. S O U R C E : Adapted from ref. 17.

SH SH £H

CK -o< ck (+)-10 (-)-10 (+)-ll

Polymerization of 1 in the presence of optically active menthol or neomenthol also gave optically active, helical polymers. The alcohols appeared to function as chain transfer agents similarly to the chiral thiols.

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Control by Adjusting Reaction Conditions (Monomer Concentration, Temperature, and Solvent )

Free-radical stereochemistry is generally recognized to be almost independent on the reaction conditions such as substrate concentration, temperature, and solvent. H o w ever, we have found that the reaction conditions remarkably affect the stereochemistry of polymerization of triphenylmethyl methacrylate (12) and tacticity of the polymer can be controlled in a wide range by simply adjusting the three factors in reaction conditions (8).

Controlled Stereochemistry of Polymerization of 12. The conditions and results of polymerization of 12 are shown in Table IV along with those of 1 and M M A . In the polymerization of 12, higher polymerization temperatures and lower [ M ] 0 gave higher isotacticity (runs 1-6). Tetrahydrofuran (THF) and chloroform as solvent were found to lead to higher isotacticity of the products than toluene. Polymerization in T H F under the same conditions as those for run 3 gave a polymer with a triad tacticity of mm / mr / rr = 99.1 / 0.9 / ~0. The isotacticity of poly-12 obtained by free-radical polymerization was previously reported to be 64% (19) but thus we have achieved nearly perfect isotacticity that is comparable to that of poly-12 obtained by asymmetric anionic polymerization (20,21). In contrast, the effect of reaction conditions on the propagation stereochemistry was not seen for 1 and M M A polymerizations giving an almost perfectly isotactic polymer (runs 7-10) and a polymer rich in syndiotacticity (runs 11-13), respectively.

A proposed mechanism for the stereocontrol is based on postulated existence of at least two types of propagating radicals with different monomer addition stereochemistries that are interchangeable and of different thermodynamic stability. The propagating radical in T r M A polymerization has been assumed to take helical conformation (6,19). However, the helical conformation may have some flexibility in the chain terminals and the proposed two propagating radicals may have conformational differences in the vicinity of the active end of the chain. The stereochemical characteristics described above can be interpreted using these models as shown in Figure 2. On monomer addition, the thermodynamically less stable radical (conformation) having lower meso monomer addition probability is formed and this can stereomutate to the other radical having a higher meso monomer addition probability at a rate comparable to monomer addition. The stereomutation of growing radical can be faster than propagation at a lower [ M ] 0 and a higher reaction temperature can facilitate the stereomutation. Solvent may also affect the conformation of growing radial and its mutation.

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Table I V . Free Radical Polymerization of Methacrylates in Toluene under Various Reaction Conditions 3

Run Monomer [ M ] 0 Temp Tacticity b

(M) ( ° Q m m / m r / r r

1 12 0.95 60 63.6/24.1 / 12.3 2 12 0.34 60 8 2 . 6 / 1 3 . 1 / 4 . 3 3 12 0.18 60 9 3 . 4 / 5 . 2 / 1 . 4 4 12 0.12 60 9 8 . 2 / 1 . 7 / 0 . 1 5 12 0.18 40 8 1 . 7 / 1 3 . 4 / 4 . 9 6 12 0.18 30 69.9 / 20.2 / 9.9 7 1 0.17 60 9 9 . 9 / 0 . 1 / - 0 8 1 0.17 40 99.8 / 0.2 / ~0 9 1 0.17 30 99.7 / 0.3 / ~0 10 1 0.40 60 99.5 / 0.3 / 0.2 11 M M A 6.3 40 2.5 / 30.8 / 66.7 12 M M A 0.82 40 2 . 5 / 3 1 . 3 / 6 6 . 2 13 M M A 0.23 40 2 . 6 / 3 1 . 0 / 6 6 . 4

initiator: A I B N for the polymerizations at 60°C, (/-PrOCOO) 2 for the polymerizations at 30°C and 40°C. [Monomer]/[Initiator] = 50 or 25. determined by *H N M R as P M M A . S O U R C E : Adapted from ref. 8.

hel ical growing radical

v/v\A/vv\yvrv/* . higher meso monomer addition probability

monomer addition

lower meso monomer mutation i l addition probability

Figure 2. A proposed mechanism for the stereochemical characteristics in the polymerization of 12.

The growing radical derived from 1 also takes helical conformation but the helix may be too rigid to allow the two types of propagating species assumed for the polymerization of 12. The growing species in the M M A polymerization may have only the thermodynamically stable conformation because the conformational dynamics of M M A growing radical is reasonably assumed to be much faster than that of the growing radical of 12 under the reaction conditions shown in Table I V .

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Control Based on Template Molecule

As described in the first section of this chapter, proper design of monomer can lead to controlled stereostructure of polymer by free-radical polymerization. In such type of polymerization, the side group of monomer can be considered as "template" that controls reaction stereochemistry of the vinyl moiety through steric repulsion. However, this technique has an inherent limitation that synthesizing polymers with various tac-ticities requires polymer reactions involving cleavage of the covalent bonding between the template moiety and polymer main chain as in the conversion from poly-1 to poly(methacrylic acid) by hydrolysis. In order to improve this method, we have searched for reaction systems involving a relatively weak interaction between monomer and template.

Methacrylic Acid Polymerization in the Presence of Amines. We found that stereochemistry of methacrylic acid ( M A A ) polymerization can be regulated in the presence of various amine compounds including racemic and optically active 1,2-diaminocyclohexane (13) (22). Table V summarizes the conditions and results of polymerization of M A A using α,α'-azobisisobutyronitrile (AIBN) .

Degree of polymerization (DP), molecular weight distribution (Mw/Mn) , and tacticity of the products were determined by the analysis of P M M A derived from the obtained po ly (MAA) . A simple procedure consisting of removal of polymerization solvent, addition of small amount of H C l - M e O H , methylation of diazomethane in benzene, and removal of benzene insoluble part (HC1 salt of amine) readily gave the P M M A (benzene solution) from the polymerization mixture. The polymerizations in M e O H and C H C 1 3 without using amines exhibited different stereochemical features which are consistent with the previous report (23). The presence of amines resulted in increased iso- and heterotacticity (mm and mr) at the expense of syndiotacticity ( I T ) in both M e O H and C H C 1 3 . The effect was more obvious for the polymerization in CHC1 3 than in M e O H , suggesting that polar interaction between the amines and M A A has a role in the stereoregulation. Also , higher M n ' s and broader M w / M n ' s were observed in the presence of amines especially for the polymerization in C H C 1 3 . Among the amines examined in this study, 13 had the most significant stereoeffect although the effect of chirality of 13 ((±)-trans-, (/?,#)-trans-, or cis-) was not clearly observed. The interaction between M A A and the added amine in C H C 1 3 was suggested to be based on both Η-bonding between 13 and M A A and ionic interaction between ammonium cation of 13 and M A A anion by IR analysis. The stoichiometry of the interaction in C D C 1 3 was found to be [MAA]/[13] = ca. 3/2 by *H N M R analysis. In contrast to 13 , Af-acetylated 13 (14) did not show clear effects on polymerization stereochemistry. Therefore, ionic interaction may be more important than H-bonding in inducing stereochemistry of the main chain although the acylation experiment cannot completely rule out Η-bonding effects.

M A A 13 14

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Table V. MAA Polymerization Using AIBN in the Presence and the Absence of Amine Compounds at 60°Ca

Run Solvent Amine Tacticityb

mm / mr / rr

1 MeOH (R,R)-13 4.0/34.6/61.4 2 MeOH none 3.9/29.1 /67.0 3 CHC13 (R,R)-13 16.3/48.8/34.9 4 CHCI3 CH 3(CH 2) 4NH 2

cyclohexyl-NH2

12.9/46.2/40.9 5 CHCI3

CH 3(CH 2) 4NH 2

cyclohexyl-NH2 12.3/47.0/40.7 6 CHCI3 none 8.1/41.0/50.9

a [MAA] 0 = 0.1 M, [-NHJ 0 = 0.1 M, [AIBN]0 = 0.004 M. determined by Ή NMR as PMMA.

Previously, several methods of stereoregulation for MAA polymerization were studied. Syndiotactic specificity of MAA polymerization in 2-propanol was reported to be enhanced up to rr = 95% by lowering reaction temperature to -78°C (24). Concerning increasing isotactic specificity, polymerization of MAA complexes affording optically active poly(MAA) is known though this method requires fairly complicated procedures of polymer synthesis and isolation (25,26). Thus, the use of amines provides a simpler way of enhancing isospecificity of MAA polymerization. Also, although stereochemistry of MAA polymerization can be affected by changing polymerization solvent (23), the method using amines is advantageous in mat it is effective even at much lower concentration of amine (0.05M) compared with solvent concentration.

Literature Cited

1) Macromolecular Design of Polymeric Materials; Hatada, K.; Kitayama, T.; Vogl, O., Eds.; Plastic Engineering 40; Marcel Dekker: New York, NY, 1997. 2) Catalysis in Precision Polymerization; Kobayashi, S., Ed.; Wiley: Chishester, Sussex, 1997. 3) Moad, G.; Solomon, D. H. The Chemistry of Free Radical Polymerization; Elsevier: Oxford, 1995. 4) Hatada, K.; Kitayama, T.; Ute, K. Prog. Polym. Sci. 1988, 13, 189. 5) Yuki, H.; Hatada, K. Adv. Polym. Sci. 1979, 31, 1. 6) Okamoto, Y., Nakano, T. Chem. Rev. 1994, 94, 349. 7) Nakano, T.; Mori, M.; Okamoto, Y. Macromolecules 1993, 26, 867. 8) Nakano, T.; Matsuda, Α.; Okamoto, Y. Polym. J. 1996, 28, 556. 9) Okamoto, Y.; Nishikawa, M.; Nakano, T.; Hatada, K. Macromolecules 1995, 28, 5135. 10) Okamoto, Y., Yashima, D.; Hatada, K. J. Polym. Sci., Part C: Polym. Lett. 1987, 25, 297. 11) Porter, Ν. Α.; Allen, T. R.; Breyer, R. A. J. Am. Chem. Soc. 1992, 114, 7676.

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