Stereochemical Control of Free-Radical Polymerization of Vinyl

Chapter 27

Stereochemical Control of Free-Radical Polymerization of Vinyl Monomers

Downloaded by STANFORD UNIV GREEN LIBR on September 28, 2012 | http://pubs.acs.org Publication Date: January 8, 1998 | doi: 10.1021/bk-1998-0685.ch027

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 in volves menthol-based peroxides and thiols that can control helicity in 1-phenyldibenzosuberyl methacrylate polymerization. Method ΙII was effective for triphenylmethyl methacrylate polym erization where isotacticity content of resulting polymer was con trollable in the range >99-64%. Method I V was realized for methacrylic acid polymerization in the presence of amine com pounds.

Stereoregulation by free-radical polymerization has been achieved only in limited cases. On the other hand, stereochemical control of polymerization is an important topic i n 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 will be a powerful and practical tool for pro ducing various types of polymers having ordered structures once effective methods for stereocontrol are developed. This article describes our recent studies on configurational and conformational control of free-radical polymerization.

© 1998 American Chemical Society

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

451

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 polymeriza tion 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, 1phenyldibenzosuberyl 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 specific ity. 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.

1

2

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 ef fect 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 polymeriza tion, 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 opti cal activity of the radically obtained polymer seems to be based on the lower isotac ticity which can mean that the polymer has shorter single-handed helical sequences compared with the anionically obtained polymer.


453

T a b l e I. Radical Polymerization of 2 with ( / - P r O C O O ) a t 4 0 ° C inToluene Polymer


Run

1 2 3 4 5 6 7 8f

E.e. (%) of monomer in feed 0 -6.9 -15.9 -26.1 -53.4 -80.0 +100 +100

Yield (%)

M

b

365

(deg)

90.4 98.6 88.4 84.9 95.5 90.8 93.7

2

a

Tacticitv (%) mm/mr/IT

74/19/7 73/19/8 72/19/9 72/21/7 75/19/7 75/19/7 74/16/10

0 +76 +246 +313 +520 +608 -617 -1280

98/

1/1

d

E.e.(%) of remaining monomer 0 -3.4 -9.0 -20.6 -50.3 -74.6 +100 +100

b

Conditions: [2]/[(/-PrOCOO) ] = 13, time 24 h. Hexane-insoluble part. Degree of polymerization and M w / M n of the polymer were i n the range 3235 and 1.52-1.77, respectively. °In CHCl -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 - B u L i in tetrahydrofuran at -78°C. S O U R C E : Adapted from ref. 9. 2

3

F i g u r e 1 shows the relation between the e.e. of monomelic unit i n 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.


454


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

l

R = -Ph, - B u , -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 i n a largely atactic polystyrene chain and the resulting polystyrene shows optical activity ([oc] -0.5 to -3.5°). 30

365


455


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 A s described i n 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 o f right- and left-handed helices. This assumption was supported by chiral H P L C analysis o f the polymer i n which the polymer was resolved into (+)- and (-)-fractions probably corresponding to single handed helical isomers. W e carried out the polymerization o f 1 under chiral reaction conditions i n order to examine the possibility o f production o f single-handed helical poly-1 via free-radical mechanism (17). Polymerization of 1 with Chiral Initiator. The possibility o f 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 o f 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 o f 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 o f 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 o f 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 o f 8. The conclusion was supported by G P C analysis o f the polymer with polarimetric and U V detections. The G P C analysis of the polymer o f run 2 indicated that the polymer consists o f (+)-fraction with higher molecular weight and (-)-fraction with lower molecular weight. This can be explained i n terms of unequal rates of primary radical termination for (+)- and (-)-helical growing radicals having opposite sense of helix.


456

Table IL Helix-Sense-Selective Free-Radical Polymerization of 1 with 8 and 9 as Initiator i n Toluene THF-soL, B/H-insol. part

THF-insol. part


Run

1 2 3

Initiator [l]/[8 or 9]

8 8 9

50 1 1

a

Temp. (°Q

Yield (%)

40 50 50

75 48 59

21

Yield

Yield

[a] (deg)

1 3 12

~0 +40 +20

(%) 69 30 41

b

3 6 5

DP

C

40 44 40

b

Hexane-insoluble products. Measured in T H F . °Deteimined by G P C analy sis 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*

+ RSH

•

poly-l-H

RS* poly-Ι·

+ +

• •

RS-poly-Ι· poly-l-RS

nl RS*

+ RS«

(1) (2) (3)

Reaction (1) is hydrogen transfer from a thiol to a helical growing radical, (2) initia tion or polymerization by the thio radical formed through (2), and (3) coupling ter mination of a helical growing radical with the thio radial formed through (2). The polymerization o f 1 was carried out with (/-PrOCOO) 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 ob tained 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 over all 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 poly2



457

1, indicating the optical activity of the polymers obtained i n the presence o f 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 o f 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 o f 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 ] -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. 365

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

THF-insol. THF-soL, B/H-insol. part part b

ChainRun

1 2 3 4 5 6

Transfer Agent

[1]/[10 or 11] Y i e l d

(+)-10 (+)-10 (+)-10 (+)-10 (-)-10 (+)-ll

82 80 71 18 19 86

0.05 0.1 0.2 0.4 0.4 0.05

a

a

(%)

Yield (%)

73 70 54 ~0 ~0 82

2 3 5 11 10 2

Yield

c

[a] DP (deg)

d

3 6 5

-80 -130 -140 -140 +110 +60

42 41 42 40 51 50

b

Hexane-insoluble products. Degree of polymerization was in the range 84150 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 . d e t e r m i n e d 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

CK (+)-10

-o< ck SH

(-)-10

£H

(+)-ll

Polymerization o f 1 in the presence o f 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.


458


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 ] gave higher isotacticity (runs 1-6). Tetrahydrofuran ( T H F ) 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 m m / 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. 0

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 ] and a higher reaction temperature can facilitate the stereomutation. Solvent may also affect the conformation of growing radial and its mutation. 0


459

T a b l e I V . Free Radical Polymerization of Methacrylates


in Toluene under Various Reaction Conditions Run

Monomer

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

12 12 12 12 12 12 1 1 1 1 MMA MMA MMA

[M] (M) 0.95 0.34 0.18 0.12 0.18 0.18 0.17 0.17 0.17 0.40 6.3 0.82 0.23

0

3

b

Temp (°Q

Tacticity mm/mr/rr

60 60 60 60 40 30 60 40 30 60 40 40 40

6 3 . 6 / 2 4 . 1 / 12.3 82.6/13.1/4.3 93.4/5.2/1.4 98.2/1.7/0.1 81.7/13.4/4.9 69.9 / 20.2 / 9.9 99.9/0.1/-0 99.8 / 0.2 / ~0 99.7 / 0.3 / ~0 99.5 / 0.3 / 0.2 2.5 / 30.8 / 66.7 2.5/31.3/66.2 2.6/31.0/66.4

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

m o n o m e r addition helical growing radical

lower meso monomer mutation i l addition probability v/v\A/vv\yvrv/* . higher meso monomer addition probability

F i g u r e 2. A proposed mechanism for the stereochemical characteristics i n the polymerization o f 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 i n 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 o f the growing radical o f 12 under the reaction conditions shown i n T a b l e I V .


460


Control Based on Template Molecule A s 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 con trols reaction stereochemistry of the vinyl moiety through steric repulsion. However, this technique has an inherent limitation that synthesizing polymers with various tacticities 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 mono mer and template. Methacrylic Acid Polymerization in the Presence of Amines. W e 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 1,2diaminocyclohexane (13) (22). Table V summarizes the conditions and results of polymerization of M A A using α,α'-azobisisobutyronitrile ( A I B N ) .

MAA

13

14

Degree of polymerization (DP), molecular weight distribution ( M w / M n ) , and tacticity of the products were determined by the analysis of P M M A derived from the obtained p o l y ( M A A ) . 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 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 . The effect was more obvious for the polymerization in C H C 1 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 ob served in the presence of amines especially for the polymerization in C H C 1 . 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 ob served. The interaction between M A A and the added amine in C H C 1 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 i n C D C 1 was found to be [MAA]/[13] = ca. 3/2 by *H N M R analysis. In contrast to 1 3 , 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 can not completely rule out Η-bonding effects. 3

3

3

3

3

3


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


a

b

Run

Solvent

Amine

1 2 3 4 5 6

MeOH MeOH CHC1 CHCI3 CHCI3 CHCI3

(R,R)-13 none (R,R)-13 CH (CH ) NH cyclohexyl-NH none

3

3

2

4

Tacticity mm / mr / rr 4.0/34.6/61.4 3.9/29.1 /67.0 16.3/48.8/34.9 12.9/46.2/40.9 12.3/47.0/40.7 8.1/41.0/50.9

2 2

a

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

0

0

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 af fording optically active poly(MAA) is known though this method requires fairly com plicated 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 ef fective 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; El sevier: 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.


462


12) Nakano, T.; Sogah, D. Y. J. Am. Chem. Soc. 1995,117, 534. 13) Kakuchi, T.; Kawai, T.; Katoh, S.; Haba, O.; Yokota, K. Macromolecules 1992, 25, 5545. 14) Nakano, T.; Okamoto, Y.; Sogah, D. Y.; Zheng, S. Macromolecules 1995, 28, 8705. 15) Wulff, G.; Gladow, S.; Kühneweg, B.; Krieger, S. Macromol. Symp. 1996, 101, 355. 16) Wulff, G.; Dhal, P. K. Angew. Chem., Int. Ed. Engl. 1989, 28, 196. 17) Nakano, T; Shikisai, Y.; Okamoto, Y. Polym. J. 1996, 28, 51. 18) Nakano, T.; Matsuda, Α.; Mori, M.; Okamoto, Y. Polym. J. 1996, 28, 330. 19) Yuki, H.; Hatada, K.; Niinomi, T.; Kikuchi, Y. Polym. J. 1970, 1, 36. 20) Nakano, T.; Okamoto, Y.; Hatada, K. J. Am. Chem. Soc. 1992, 114, 1318. 21) Okamoto, Y.; Suzuki, K.; Ohta, K.; Hatada, K.; Yuki, H. J. Am. Chem. Soc. 1979, 101, 4763. 22) Nakano, T.; Ishigaki, Y.; Okamoto, Y. Polym. Prepr. Japan 1996, 45(2), 126. 23) Krakovayk, M. G.; Anufrieva, Ε. V.; Sycheva, Ε. Α.; Sheveleva, T. V. Mac romolecules 1993, 26, 7375. 24) Lando, J. B.; Semen, J.; Farmer, B Macromolecules 1970, 3, 524. 25) Kataoka, S.; Ando, T. Kobunshi Ronbunshu 1980, 37, 185. (Chem. Abstr. 1980, 92, 198833q). 26) Kataoka, S.; Ando, T. Polym. Commun. 1984, 25, 24.


Stereochemical Control of Free-Radical Polymerization of Vinyl

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