Anionic Ring-opening Polymerization: Stereospecificity for Epoxides, Episulfides and Lactones

33 Anionic Ring-opening Polymerization: Stereospecificity for Epoxides, Episulfides and Lactones TEIJI TSURUTA Science University of Tokyo, Japan and YUHSUKE KAWAKAMI Nagoya University, Japan 33.1

INTRODUCTION

489

33.2

SOME STEREOCHEMICAL DEFINITIONS RELATING TO POLYMERS

490

33.3

REGIOSELECTIVITY AND STEREOCHEMISTRY AT THE SITE OF BOND CLEAVAGE

491

33.4

ENANTIOMORPHIC CATALYST SITES MODEL FOR THE STEREOSPECIFIC POLYMERIZATION OF PROPYLENE OXIDE

492

33.5

MOLECULAR LEVEL ELUCIDATION OF THE ENANTIOMORPHIC MECHANISM USING WELL DEFINED ORGANOZINC COMPLEXES

492

33.6

GROWING CHAIN CONTROL MECHANISM IN THE POLYMERIZATION OF t-BUTYLETHYLENE OXIDE

493

33.7

STEREOSELECTIVE AND STEREOELECTIVE POLYMERIZATIONS OF EPISULFIDES: COMPARISON WITH EPOXIDES

494

33.8 STEREOSPECIFICITY IN THE POLYMERIZATION OF LACTONES 33.8.1 OL,(x-Disubstituted Propiolactones 33.8.2 p-Monosubstituted and ft, fi-Disubstituted fi-Propiolactones

496 497 497

33.9

499

33.1

REFERENCES

INTRODUCTION

The first report of the stereospecific polymerization of propylene oxide (PO) was published by Pruitt and Baggett1 in 1955. Their catalyst was a reaction product of iron(III) chloride with propylene oxide. A few years later, various types of stereospecific catalyst for PO polymerization were found and developed almost simultaneously: aluminum isopropoxide/zinc chloride by Price and Osgan;2 zinc alkyl/water (or alcohol) by Furukawa and Tsuruta; 3 aluminum alkyl/water by Colclough, Gee et alf and aluminum alkyl/water/acetylacetone by Vandenberg.5 A zinc alkyl/water system was used by Sigwalt and co-workers in 19656 for the stereospecific polymerization of propylene sulfide. In 1967, Osgan and Teyssie7 reported the synthesis of a bimetallic catalyst, ^-oxoalkoxide (RO) 2 A10ZnOAl(OR) 2 , and noted its high activity. The first example of asymmetric selective polymerization of (RS)-propylene oxide was given by Tsuruta, Inoue et a/. 8,9 using the catalyst system ZnEt 2 -( + )-borneol. A few years later, Sigwalt and Spassky 10,11 and Furukawa et al.12 reported that (#S)-propylene sulfide (PS) also under­ went a stereoelective polymerization with the use of similar organozinc catalyst systems to those for PO. The term 'stereoelective' is a synonym of 'asymmetric selective' as will be discussed later (see Section 33.2). 489

Anionic Polymerization

490

Details of studies carried out in and before the early years of the 1960s have been reviewed elsewhere.13 The enormous progress in methodology for structural analyses of polymers has made it much easier than before to have an insight into mechanistic features of the stereospecific polymerization. In this chapter, studies on stereospecificity in the anionic ring-opening polymerizations of epoxides and episulfides and also of some lactones will be reviewed, with the emphasis on recent progress in this field.

33.2

SOME STEREOCHEMICAL DEFINITIONS RELATING TO POLYMERS

The prerequisite condition for formation of a stereoregular polymer from an epoxide or episulfide is the regioselective ring-opening of monomer molecules in the polymerization process, which results in the formation of a regular polymer. According to the IUPAC documents, 1 4 1 5 a regular polymer is a polymer whose molecules can be described by only one species of constitutional unit in a single sequential arrangement. Regular poly(propylene oxide) (1) is an example. Every constitutional unit shown by the adjacent dashed lines is linked together with its neighboring units in a single sequential (head to tail) arrangement. HvtOCHMe—CH 2 —fOCHMe- -CH 2 —|-OCHMe head

tail

head

tail

head

-CH 2 —j-OCHMe—CH 2 ^w tail

head

tail

(1)

When racemic propylene oxide [(KS')-PO] is polymerized by KOH (or KOR), the ring opening takes place predominantly (~ 95%) at the O—CH 2 bond to form a 'regular' polymer similar to (1). The alkali metal catalyst system, however, cannot recognize the difference between R and S monomer, resulting in the polymer being regular but atactic. The polymerization of (RS)-PO with ZnEt 2 /MeOH (or Zn(OMe) 2 16 " 21 ) as initiator was proved 2 2 , 2 3 to be regioselective (O—CH 2 bond scission) and stereoselective to form the isotactic polymer 23 as shown in Scheme 1. Stereoselective polymerization is defined14 as 'a polymerization in which a polymer molecule is formed from a mixture of stereoisomeric monomer molecules (e.g. (R)and (S)-PO) by incorporation of only one stereoisomeric species (e.g. (R)-PO) into a growing polymer chain'. In Scheme 1, (RS)-PO is polymerized by the optically inactive initiator, so that an equal number of moles of poly[(K)-PO] and poly[(S)-PO] are formed. When an optically active initiator such as the ZnEt 2 -( + )-borneol system 8 ' 9 is used, preferential polymerization of (K)-PO over the S enantiomer takes place, unreacted PO monomer being enriched with the S enantiomer. This is a typical example of asymmetric selective (or stereoelective) polymerization. (S) Me KOR

(*)

Me

(R) H

(R) ij

—CH COCH2COCH2C O—

(S) Me

Me

Me

(S) Me

(S) Me

—CH COCH 2 COCH 2 CCH(S) H Zn(OR)2 or ZnR2/MeOH

—CH

I I J

,COCH 2 COCH 2 0

Scheme

I

I

Me (R)

Me (R)

I

Me (R)

Anionic Polymerization: Stereospecificity for Epoxides, Episulfides and Lactones

491

The IUPAC Macromolecular Nomenclature Commission has recently recommended 25 the terms enantiosymmetric and enantioasymmetric polymerizations for the more specific cases of stereoselective polymerization. The above-mentioned polymerization (see Scheme 1) in which an equal number of moles of poly[(#)-PO] and poly[(5)-PO] are formed is obviously an example of enantiosymmetric polymerization, while asymmetric selective (or stereoelective) polymerization is one of enantioasymmetric polymerization. Stereospecific polymerization 14 ' 24 is 'a polymerization in which a tactic polymer is formed'. A tactic polymer is defined as 'a regular polymer, the molecules of which can be described in terms of only one species of configurational repeating unit in a single sequential arrangement'. The polym­ erizations of (RS)-PO with optically inactive (Scheme 1) and also with optically active organozinc catalyst systems 8 ' 9 are stereospecific because isotactic (or tactic in broader definition) polymers are formed in all of these polymerization systems. It is to be noted that the stereospecific polymerization of propylene cannot be a stereoselective polymerization because the starting monomer, propylene, has no chiral center.

33.3

REGIOSELECTIVITY AND STEREOCHEMISTRY AT THE SITE OF BOND CLEAVAGE

Some previous papers 2 reported that the regioselectivity in the oxirane polymerizations depended strongly upon the nature of the initiator systems. The general conclusion was that anionic and anionic-coordinate catalysts mostly cleaved the CH 2 —O bond (jS-bond in 2) to form regular headto-tail linkages, while cationic catalysts also cleaved the CHMe—O bond (a-bond in 2) concurrently to a significant extent to form irregular polymer chains with head-to-head and tail-to-tail linkages. Most of the catalyst systems proposed for stereospecific polymerization of PO are binary or ternary systems, so that active centers are formed having a varying catalytic nature in terms of regioselectivities and stereoselectivities, which are changeable according to the conditions under which the catalyst systems are prepared. It has long been known that the whole polymer obtained by polymerizing racemic PO with these catalyst systems could be fractionated into crystalline and amorphous polymers. 13 The crystalline fraction was proved to be an isotactic polymer many years 2 6 - 2 8 The structure of the amorphous fraction, however, was not fully elucidated until recently. ago. Owing to progress in the NMR technique, it is now clear that some amorphous poly(PO) prepared by ZnEt 2 /MeOH 2 3 or AKOPrOa/ZnC^ 30 consists of regular head-to-tail linked units, but the percentage of isotactic diads is lower than 60% 2 3 (example of an atactic polymer in the broad sense). Some other amorphous poly(PO) samples prepared with ZnEt 2 /H 2 0 (1:0.5), 30 ' 31 AlEt 3 /H 2 0 (1: l) 3 3 and Al(OPr i ) 3 30 have been shown to have head-to-head and tail-to-tail structures along their polymer chains (example of an irregular polymer). /

\

Me

(2)

It is to be noted in this connection that styrene oxide is polymerized by Al(OPr% under selective cleavage of the O—CHPh bond (a-bond). 34 With the Z n P h 2 / H 2 0 (1:1) system,35 on the other hand, polymerization of styrene oxide proceeds under jS-bond cleavage. Tsuruta et al22,23 prepared poly(trans-propylene oxide-/?-d) (3) with ZnEt 2 /MeOH (1:1.7), and fractionated it into crystalline and amorphous parts. The crystalline fraction was shown to have an erythro-diisot&ctic structure as in (4) and (5). It was concluded, therefore, that the ring-opening took place at the jS-bond with inversion of the configuration at the methylene carbon. The inversion mechanism at the ring cleavage coincides with those reported by Vandenberg 36 " 38 for butene 2-oxide, by Price 3 9 " 4 2 for ethylene oxide and propylene oxide, by Tani 2 9 ' 4 4 ' 4 5 for propylene oxide and by Tsuruta 46 for cyclohexene oxide. Me.

iS)y

D

O racemic trans-(RR,SS)-proipy\er\e oxide-p~d (3)

D

I

Me

(S) (R) (5)

Anionic Polymerization

492 33.4

ENANTIOMORPHIC CATALYST SITES MODEL FOR THE STEREOSPECIFIC POLYMERIZATION OF PROPYLENE OXIDE

The reaction mechanism for the stereospecific polymerization of propylene oxide with zinc dialkoxide (the active species in the ZnEt 2 /MeOH system) 1 6 - 2 1 has been satisfactorily explained by the enantiomorphic catalyst sites model, in which the presence of R* and S* sites is assumed to be the origin of the steric control mechanism. The R* sites accept (.R)-propylene oxide in preference to the S isomer, resulting in the formation of—RRR • • • RRR— isotactic sequences, and vice versa (see Section 33.2). Results of copolymerization between R and S monomers were found to be a useful experimental tool for elucidation of the mechanism of stereoregulation. The simple and important copolymerization formula (equation 1) can be derived from the concept of the enantiomorphic catalyst sites model having a symmetrical distribution of R* and S* sites. Equation (1) implies that the relative rate of incorporation of R monomer against S monomer is exactly the same as the ratio of concentrations of the two isomers. More details about phenomenological approaches to the enantiomorphic mechanism were summarized elsewhere47 in 1972. d[H]/d[S]

=

[K]/[S]

(1)

The 1 3 CNMR technique has given information on the triad tacticity of poly(PO). 48 " 50 Matsuzaki and Uryu 51 reported that the triad tacticity of poly(PO) prepared with ZnEt 2 /MeOH catalyst accorded with values calculated on the basis of the enantiomorphic m o d e l 5 2 - 5 4 shown in equations (2)-(4), where parameter o2 is the probability of entering of an R monomer at an R* catalyst site. The triad tacticity data of another poly(PO) sample prepared with Zn(OMe) 2 were also in good agreement with the enantiomorphic model. 55

33.5

7

=

1 -

3
H

=

2a2{\

-

S

=

o2{\

-

-

o2)


o2)

(2) (3)

(4)

MOLECULAR LEVEL ELUCIDATION OF THE ENANTIOMORPHIC MECHANISM USING WELL DEFINED ORGANOZINC COMPLEXES

As stated in the foregoing sections, the mechanism of the stereospecific polymerization of PO could be explained by assuming the presence of R* and S* sites in the catalyst system. No information, however, was available concerning the chiral structure of R* and S* catalyst sites, because none of the active catalysts possessed a well defined structure. To elucidate the stereocontrol mechanism in terms of molecular level considerations, a series of studies 3 2 ' 5 6 - 5 8 were carried out in homogeneous systems using well defined organozinc complexes such as [EtZnOMe] 6 [MeOZnOMe] ( 6 ) , 5 9 - 6 1 [EtZnOCH 2 CH 2 OMe] 6 [Zn(OCH 2 CH 2 OMe) 2 ] (7),62 and [EtZnOCHMeCH 2 OMe] 2 [Zn(OCHMeCH 2 OMe) 2 ] 2 (8)63 as initiator. All of these complexes were isolated in the form of single crystals. Complex (6) consists of two enantiomorphic distorted cubes (i.e. R cube and S cube), which share a six-coordinated central zinc atom. Each cube has three inner methoxy groups and one outer methoxy group. The single crystal is soluble in benzene, and the benzene solution induces the polymerization of PO at 80 °C. Cryoscopic and NMR measurements of the benzene solution have shown that the organozinc complex exists as a monomeric form and retains its structure even in benzene in the temperature range from 5 to 80 °C. Hagiwara, Ishimori and Tsuruta 61 carried out 13 C-PFT NMR analysis of a reaction system in which (6) and (RS)-PO in excess were allowed to react in benzene at 80 °C. They found that all of the observed signals in the reaction system, including those that had newly appeared or disappeared could reasonably be explained in terms of the initiation mechanism by one of the inner methoxy groups. This result indicates that the principal framework of the complex is retained even after the start of the propagation reaction. It was also demonstrated that every complex molecule of (6) consumes only one inner methoxy group for the initiation reaction. The triad tacticities (%) of poly(PO) (unfractionated) formed were found to be in good agreement with those anticipated from the enantiomorphic catalyst sites model (a2 = 0.72). The chiral structure around the central zinc atom in (6) was considered to be responsible for the formation of the R* (or S*) site. Complex (7) has a similar spatial structure to (6) but exhibits inferior steric control ability to that of (6). Complex (8) is a newly found organozinc complex,63 the structure of which was shown to be a 'chair' type in contrast with the 'cube' type for (6) or (7). It was confirmed that (8) exists as a

Anionic Polymerization: Stereospecificity for Epoxides, Episulfides and Lactones

493

monomeric form in benzene and that the stereochemical structure of the chair framework is retained in benzene at 80 °C. Complex (8) exhibited much higher activity and stereospecificity in PO polymerization than either of complexes (6) or (7). The population of the triad tacticities of poly(PO) prepared with (8) fitted well with those predicted from the enantiomorphic catalyst sites model [
|Zn (4)

endo - coordinated ! \£r J), (5) - CD3OCH2CH(Me)0- j ]\^T^)l I

Figure 1

Zn (I ) ex "coordinated ■ j exo (5) - CD3OCH2CH(Me)0-

A simplified illustration of complex (8)

Another experiment using (S)-PO as monomer showed that the coordination of the S monomer took place at Zn(2), and was attacked by the non-coordinated (K)-methoxypropoxy group. A molecular model study showed that there is formation of a chiral hole which can accommodate only enantiomeric molecules with one sense. The chiral structure of the hole is determined by the chair type Zn—O framework together with the coordinated methoxypropoxy groups. In the PO polym­ erization, the chirality of the non-coordinated methoxypropoxy group has nothing to do with the enantioselection. In the propagation stage, an R* chiral hole receives predominantly (R)-PO, while an S* chiral hole accepts (S)-PO, to form isotactic linkages. Since spatial allowance at the chiral hole decreases with increasing bulkiness of substituent of oxirane monomer, the parameter o2 becomes larger the bulkier the substituent of the monomer molecule is. Actually, c2 was found to be unity for poly(£-butylethylene oxide) which was prepared with complex (8). It is to be noted that the chiral hole of (8) did not recognize the mode of orientation of a cisdisubstituted epoxide such as cyclohexene oxide, which is an achiral monomer. Complex (8) seems to serve as a simple bulky group, which facilitates syndiotactic addition of cyclohexene oxide monomer molecule to the active site, in cooperation with strong steric effects exerted from the terminal and penultimate units of the growing chain. 46

33.6

GROWING CHAIN CONTROL MECHANISM IN THE POLYMERIZATION OF f-BUTYLETHYLENE OXIDE

When racemic PO is polymerized with potassium alkoxide as initiator, R and S monomers are randomly incorporated into a polymer chain to form an atactic polymer (see Section 33.2). Results obtained in copolymerizations between (R)- and (S)-PO were in good agreement with equation (1), no change at all being observed in the optical purity of the monomer phase during the course of the copolymerization. The physical meaning here is obviously different from that of the enantiomorphic catalyst sites model. When the R and S copolymerization of f-butylethylene oxide was carried out with a monomer mixture consisting of R/S = 76/24 with Bu'OK as initiator, R monomer was incorporated into polymer chain preferentially over S monomer. 65 As a consequence, the optical purity in the recovered monomer became smaller than that of the starting mixture as the copolymerization PS 3—Q

494

Anionic Polymerization

reaction proceeded. This was explained by a growing chain control mechanism, the t-butyl group being bulky enough to make the chiral structure of the growing polymer chain able to undergo stereoselection of the chiral monomer. Sato, Hirano and Tsuruta 65 analyzed the curve of R content in unreacted monomer vs. conversion for r-butylethylene oxide by dividing the curve into j-stages. From experimental data at every stage, they estimated the value of a parameter, a,-, defined as equation (5) (d[K]/d[S]),

=

a,([K]/[S]),-

(5)

The parameter a,- was found to become greater as the conversion increased. Since the polymeriz­ ation of t-butylethylene oxide initiated by Bu l OK was proved to form a living system, the increase of the (Xj value should be ascribed to a surplus contribution from a chiral secondary structure of the growing chain, along with the primary chiral structure of the chain end. Price 42 reported previously the IsoSyn mechanism for the stereochemistry of polymerization of (RS)-t-butylethylene oxide (Bu'EO), initiated with Bu*OK. For the formation of IsoSyn poly(Bu'EO), —RRSSRRSSRR-, PSR/R (or PRS/S) should be larger than PRR/R (or PSS/sl where PSR/R means the conditional probability for R monomer to add on a growing chain end which has S and JR units respectively as the penultimate and terminal units. The other probabilities are defined correspondingly. From results of NMR studies 66,67 on racemic poly(BulEO) prepared with Bu'OK as initiator, the conditional probabilities, (PSR/R + PRS;S) a n d (PRR/S + PSS/R) w e r e evaluated to be 0.55 and 0.43 respectively. This result indicates that the terminal unit of a growing chain reacts with an incoming monomer without any significant influence from the penultimate unit, because PSR/R (or PRS/S) is almost equal to PRR/R (or Pss/S). In the above-mentioned R and S copolymerizations the ratio of the initial concentrations of R and S monomer was 76/24, so that there was an enhanced chance to form isotactic linkages, which would result in the formation of a chiral secondary structure. It is to be noted in this connection that Spassky et a/.68 reported a much larger rate of polymerization of (JR)-( — )-BulEO in comparison with that of the RS monomer under identical conditions. The stereochemical behavior of t-butylethylene sulfide (BulES) is in sharp contrast with that of B^EO, no stereoelection being observed in the reaction systems in which (S)-( — )-BulES in excess was copolymerized with the (K)-( + ) isomer in bulk or in dimethyl sulfoxide using Bu'OK as initiator. 69 A similar result was reported 70 for the polymerization of styrene oxide of different enantiomeric compositions using a potassium alkoxide initiator. The stereoelective effect was almost completely suppressed by addition of complexing agents such as crown ethers or cryptands. These observed phenomena were explained in terms of polymer chain effects. Another example 46 of the growing chain control mechanism has already been given in the last part of Section 33.5.

33.7

STEREOSELECTIVE AND STEREOELECTIVE POLYMERIZATIONS OF EPISULFIDES: COMPARISON WITH EPOXIDES

Extensive studies have been carried out by Sigwalt, Spassky and co-workers on the stereoselective and stereoelective polymerizations of episulfides and epoxides. 7 1 - 7 8 The enantiomorphic catalyst sites control mechanism was found to be valid also for the stereospecific polymerization of episulfides. Sigwalt et al. found that (R)-( - )-3,3-dimethyl-l,2-butanediol (DMBD)/ZnEt 2 (1:1) had a high stereoelective nature both in epoxide and episulfide polymerizations. 71 ' 74 Using a 'stepwise procedure' they succeeded in isolating almost optically pure monomer as the unreacted monomer. 73 ' 77 They found that equation (6) was valid for this stereoelective polymerization d[K]/d[S]

=

r([K]/[S])

(6)

Equation (6), however, was not valid for the polymerization of t-butylethylene sulfide with the ZnEt 2 /(#)-(-)-DMBD system. 7 1 ' 7 8 " 8 0 Sigwalt et al. proposed equation (7) for this system d[R]/d[S]

=

p([i?] 2 /[S] 2 )

(7)

To explain the second-order law, they assumed that incorporation of an R monomer molecule into a polymer chain took place only on the site already complexed by another R monomer molecule, and vice versa.

Anionic Polymerization: Stereospecificity for Epoxides, Episulfides and Lactones

495

It appears also that t-butylthiirane gives pure isotactic chains with most of the stereospecific initiators and that when polymerizing an enantiomerically enriched f-butylthiirane with Z n E t 2 / H 2 0 initiator one can separate the polymer into a pure polyenantiomer partly soluble in CHCI3 and a racemic stereocomplex which is insoluble in CHC1 3 . 81 Spassky, Sigwalt et al83,88 revealed also that some cadmium compounds, including simple salts, were excellent initiators for stereoselective polymerization of propylene sulfide. For instance, the percentage of isotactic diad in the poly(propylene sulfide) sample prepared using Cd (#)-tartrate catalyst was more than 95%, in contrast with 69% that was found in a polymer sample prepared using Zn (R)-tartrate as catalyst. Cd (RS)-tartrate also exhibited the same level of stereoselectivity as Cd (R)-tartrate. The superior stereoselectivity of the Cd tartrate was shown also by an experiment in which the more effective separation into fractions having opposite optical rotation was performed in poly(propylene sulfide) prepared by Cd tartrate, compared with that by Zn tartrate. 84 In the case of stereoelectivity, precisely the opposite situation was found in the two tartrates; only very slight optical activity was associated with the poly(propylene sulfide) sample prepared by Cd (R)-tartrate, whereas [ a ] ^ 5 = — 5.8 (in benzene) for the polymer sample prepared by Zn (K)-tartrate. The CdMe 2 /( — )-DMBD system, however, was found to exhibit a stereoelectivity lower than that of ZnEt 2 /( — )-DMBD system in the polymerizations of propylene sulfide, ds-butene sulfide and cyclohexene sulfide, but the elected chirality was opposite to that found with the Zn system. 85,86 Stereoselective and stereoelective polymerizations of propylene sulfide have also been studied in a homogeneous phase using two series of chiral cadmium derivatives of cysteine and methionine. 87 ~ 8 9 The first series (series I) are the cadmium thiolates of cysteine esters, and the second (series II) are the cadmium carboxylates of cysteine and methionine.

.1? CH 2

S^

Cd

/NH 2 —CHCOR

ROCCH—NH2

i*

S

MeS(CH2)„CH

CH

/

C

X

NH2

(9) Series I

°2\

Cd

yNHl\ CH(CH2)„SMe

NlO-f

(10) Series II

Poly(propylene sulfide) samples prepared by the series I Cd compounds (thiolates) are highly isotactic and optically active though the value of the optical rotation is not large. Polymer samples from the series II compounds (carboxylates) are also isotactic but not optically active. The lack of stereoelectivity of the carboxylates should be noted in connection with the behavior of Cd (#)tartrate. Further studies 89 with the Cd thiolate catalyst (R = Pr1) revealed that stereospecificity in a wide sense (both stereoselection and stereoelection) appeared only for molecular weights higher than 6000 and also depended on the temperature of propagation: with decreasing temperature, the isotacticity increased and stereoelection could be inverted. Spassky and his co-workers 90 ' 91 reported recently that an optically active atropoisomeric system exhibited a remarkable stereoelection almost one order of magnitude higher than that obtained with the best previously known initiator. For instance, zinc (S)-binaphtholate polymerized methyl- or ethyl-substituted episulfide with very high stereoelectivity, ks/kR being 15-20, where ks and kR denote the rate constants for polymerizations of S and R monomer respectively. Sepulchre et al.92 reported the formation of disulfide linkages when using the atropoisomeric catalyst and the particular role of additions of tetrahydrothiophene which allows the most crystalline, isotactic products to be obtained in the case of most substituted thiiranes including ds-butene sulfide. It was found also that the resolution efficiency decreased significantly when the episulfide ring carried a bulky group such as r-butyl. 90 ' 91 On the basis of the results obtained from stereoelective polymerizations of epoxides and episulfides with a variety of chiral catalysts, Spassky et al.12,11,85 proposed configurational rules governing the stereochemical control of chiral initiators in the ring-opening polymerization. They Me Ho'

*BU'

(11) ( + )-(Ry 3,3-dimethyl-2-butanol

CH2OH H O '

*BU'

(12) (-)-(/?)3,3-dimethyl-l,2-butanediol

/-CH2 N

S*

\le

(13) ( + )-(K)propylene sulfide

Anionic Polymerization

496

classified substituents into three categories, (i) CH 2 OH, CH 2 SH, CH 2 S; (ii) OH, OR, SH, SR; (iii) Me, Bul, and any alkyl group. For instance, molecules (11)—(13) were considered to have the 'same' spatial configuration. When initiator systems prepared from these chiral alcohols with ZnEt 2 were used, the enantiomer having the same configuration as the alcohol was preferentially chosen for the polymerization. More generally, the chiral initiator preferentially polymerizes the enantiomer with the same spatial configuration. Spassky et al. confirmed that this rule could be extended to a number of experimental results. They named this type of polymerization as 'homosteric' stereoelective polymerization.93 It is to be noted that all of the homosteric initiators were chiral zinc systems, the composition of which was expressed as [RZnOR*] JC [R*OZnOR*] y (x/y < 1). In contrast with the homosteric initiator, an initiator derived from the reaction between CdMe 2 and (— )-DMBD polymerized preferentially (S)-( — )-propylene sulfide which is the enantiomer with the opposite spatial configuration to that of the chiral diol in the initiator system. This was named by Spassky et al as 'antisteric' stereoelection. Some examples of antisteric stereoelection were also found in polymerization systems with ZnEt 2 -( — )-DMBD as initiator. A common compositional feature of the 'antisteric' initiator [RMtOR*] x [R*OMtOR*] y is x/y > 2, where Mt is Zn or Cd. The nature of homosteric and antisteric stereoelection has not yet been fully elucidated at the molecular level because the structure of the operating species and the reaction mechanism with these initiator systems are not clearly established.

33.8 STEREOSPECIFICITY IN THE POLYMERIZATION OF LACTONES There are a great number of papers available which deal with lactone polymerizations (see Chapter 34). Stereospecificity in the polymerizations, however, has been discussed only in relatively few papers compared with those for epoxides and episulfides. This section will concentrate on stereochemical behavior of mono- and di-substituted jS-propiolactones in the polymerization reactions induced by anionic or coordinate anionic initiators. The mode of ring cleavage of lactones with anionic as well as coordinate anionic initiators was discussed in Chapter 34. From an overview of results so far reported in the literature, it may be possible to deduce the following criteria regarding the mode of ring cleavage: (i) the more free anionic character the attacking reagent possesses, the higher the regioselectivity observed at the /?-carbon for the reaction site which causes the C—O bond cleavage to form a carboxylate anion; while (ii) the more important the role of the coordination process of the ester group of the lactone onto the metallic moiety of initiator, the higher is the regioselectivity observed at the carbonyl carbon which causes the 0 = C — O bond cleavage to form an alkoxy group. (i) Anionic type attack

(n) Coordination type attack

»

a,a-disubstituted /?-propiolactone ► /VAA/VW

0>

'-substituted and /?,/?-disubstituted 0-propiolactone

preferential site of attack preferential site of ring cleavage Scheme 2

It is understandable from points (i) and (ii) above that /?- and /^-substituted propiolactones are generally not polymerized with typical anionic initiators owing to the steric environment around the jS-carbon. On the other hand, coordination type initiators should be successfully applied to /?-, /},/?- and a,a-substituted propiolactones (substituted PLs). It is to be noted that a-monosubstituted PL is basically not an appropriate monomer for stereospecific polymerization, because steric configuration at the a-carbon is liable to be racemized during the polymerization process owing to its active hydrogen. Most studies on the stereochemistry of polymers from a- and ^-substituted PLs have been carried out, first by Tani et a/., and by Spassky, Leborgne and co-workers.

Anionic Polymerization: Stereospecificity for Epoxides, Episulfides and Lactones 33.8.1

497

a,a-Disubstituted Propiolactones

Copolymerizations of (#)-( + )- and (S)-( — )-a-ethyl-a-methyl-/?-propiolactone (MEPL) using a 1:1 MeC0 2 K/dicyclohexyl-18-crown-6 complex as initiator were carried out with different enantiomeric compositions. 94 The optical rotation of the poly(MEPL) obtained was found to be linear with the optical purity of the starting monomer. If p = [ £ ] / [ # ] is defined as the enantiomeric ratio in the initial monomer with [5] + [R] = 1, the Bernoullian statistics predict triad populations as shown in equations (8) and (9) /

H,

=

Hs

= =

(P3 S

+ =

D/(P p(p

+ +

l) 3 l)/(p

(8) +

l)

3

(9)

where /, H{, Hs and S are triad contents of ([RRR] + [SSS]), (IRRS] + [SSK]), ([KSS] + [SKK]) and ([RSR] + [SKS]) respectively.96 A study with high field NMR showed that triad as well as tetrad populations observed in poly(MEPL) were in good agreement with the Bernoullian statistics (cf. ref. 47). Since the initiator attacks exclusively at the j8-carbon of the monomer (see above) to break the CH 2 —O bond, the monomer molecules are incorporated into polymer chains with the steric configuration unchanged. In consequence, it is concluded that the rate constant kRS for reaction of S monomer with a growing chain end having an R monomeric unit should be equal to the rate constant kRRi defined correspondingly. This situation is just the same as that obtained from R,S copolymerization of propylene oxide with potassium alkoxide as initiator (see Section 33.6). The Bernoullian statistics are also applicable to the triad as well as the tetrad populations in poly(a-methyl-a-rc-propyl-jS-propiolactone) [poly(MPPL)], which was prepared with the MeC0 2 K initiator complexed with 18-crown-6. Racemic MEPL gave only an atactic polymer (J = 0.25; Hs = 0.26; Hj = 0.26; S = 0.24) even when the ZnEt 2 /MeOH system was used as initiator. 96 In other words, the enantiomorphic catalyst sites (Section 33.4) in the zinc-coordinated initiator were not able to recognize the chirality of MEPL, a fact which forms a sharp contrast with the case of racemic propylene oxide (see Section 33.2). It was reported that some irregularities, not yet identified, appeared in the spectra of poly(MEPL) prepared with the zinc-coordinated initiators. Polymerization 95 of racemic MEPL using the ZnEt 2 /(R)-( — )-DMBD system was found to obey the first-order law (see Section 33.7, equation 6) to form an optically active polymer. The stereoelection was a homosteric type, though the kR/ks ( = rR) value was very low (1.02-1.07). A stereoelective polymerization 98 of racemic MPPL using ZnEt 2 /(K)-( — )-DMBD gave a similar result (homosteric, rR = 1.25) to that of MEPL. The high field NMR analysis showed a slight enrichment of isotactic tetrads (ca. 7%) in the stereochemical sequence of the poly(MPPL) formed. This result is compatible with the previous finding97 that the optically active poly(MPPL) had a higher equilibrium melting point than the racemic polymer prepared with an anionic initiator. Antisteric type stereoelective polymerization of racemic MPPL was also reported 98 when CdMe2/(JR)-( —)-DMBD was used as initiator (see Section 33.7).

33.8.2

/?-Monosubsthuted and /?,/?-Disubstituted ^-Propiolactones

Agostini, Lando and Shelton" polymerized (KS^/J-butyrolactone (jS-BL) by using initiator sys­ tems derived from zinc and aluminum alkyls. Poly(jS-BL) prepared with AlEt 3 /H 2 0 (1:1 in molar ratio) as initiator was fractionated by chloroform. The chloroform insoluble fraction showed, in the X-ray diffraction pattern, the practically identical crystalline structure to that of the naturally occurring poly[(JR)-j8-hydroxybutyrate]. In comparison with this, polymers prepared with the Z n E t 2 / H 2 0 system were less crystalline. Leborgne and Spassky reviewed a number of initiator systems so far proposed in terms of their stereospecificity.100 They concluded that in most cases Al coordinate type initiators gave higher values of the index of stereospecificity (IS), defined by Tani et al. as the percent crystalline fraction with respect to the unfractionated polymer. The IS was generally evaluated by the solvent fractionation method. Among the initiators reviewed, the highest IS value (72%) was found for the AlEt 3 /H 2 0/epichlorohydrin system which was prepared via a high-vacuum drying process. 101,102 It is to be noted that the proposed mechanism of polymerization of j?-BL by the AlEt 3 /H 2 0 initiator was assumed by Shelton et al103 to involve an oxonium ion as the propagating species, as previously proposed by Yamashita et al.104" This is considered to relate to the strong acidic character

498

Anionic Polymerization ^vi—o—AI: 0+

H ►Me

(

°
of aluminum, and initiation may be written as shown in Scheme 3. In contrast with this, it has been confirmed that Z n E t 2 / H 2 0 (or ZnEt 2 /MeOH) systems possess much less cationic character than aluminum systems (see Section 33.3). Araki, Tani et a/.102 reported that diad and triad tacticities of /?-alkyl- or /?-chloroalkyl-PL are difficult to observe in 100 MHz ^ N M R , but diad tacticity may be observed in 1 3 CNMR. Leborgne et a/.105 succeeded in the stereoelective polymerization of (RS)-p-BL by using ZnEt 2 -{R)(— )-DMBD as initiator. Unreacted monomer was enriched in S enantiomer (enantiomeric excess, e.e. = 46%) at 84% conversion. This corresponds to kR/ks ( = rR) = 1.6, which is not too far from the value observed in the stereoelective polymerization of propylene oxide (rR = 1.8). The stereoelec­ tion is of the homosteric type. The whole polymer was fractionated into two parts by methanol. The methanol insoluble fraction (25% by weight) was shown by 1 3 CNMR to contain 72% isotactic diads. The stereoselective behavior of ZnEt2-(R)-( — )-DMBD contrasted with that of Z n E t 2 / H 2 0 (or ZnEt 2 /MeOH) system, the latter of which had no stereoselectivity as stated above. According to these results, it may be suggested that the mechanisms of stereoselection or stereoelection of the lactones must fit the concept of the enantiomorphic sites model (Section 33.4). The effect of substituents on stereospecific polymerization of monosubstituted /?-alkyl-PL and /?-chloroalkyl-PL was studied extensively by Araki, Tani et al.102 The values of IS (%) for polymers obtained with (EtA10)„ as initiator were 66% for methyl, 73% for ethyl, 44% for isopropyl, 0% for r-butyl, 22% for chloromethyl, 48% for dichloromethyl and 0% for trichloromethyl substituents. The non-formation of crystalline polymers from t-butyl and trichloromethyl-substituted PL was attributed to the presence of highly crowded substituents. It is to be noted, however, that the IS values may not always parallel the tacticities of the relevant macromolecules. Organozinc initiator systems again gave only amorphous polymers from these substituted PLs. Recently, Prud'homme, Spassky et al106 compared polymerizations of the optically active and racemic /?-chloroalkyl monosubstituted and disubstituted monomers (CC13-PL; CF 3 ,Me-PL; and CF 3 ,Et-PL). They found in the polymerizations of optically active CC13-PL and CF 3 ,Me-PL, with tetraphenylporphyrin/AlEt 2 Cl and Z n E t 2 / H 2 0 initiators respectively, that (i) the rotatory power of the polymer varies linearly as a function of the enantiomeric excess of the monomer, and (ii) the specific rotation of the residual monomer is equal to that of the starting monomer. These results indicate that the R,S copolymerization of these lactones takes place similarly to that of
CH 2 —C=0

Scheme 4

Anionic Polymerization: Stereospecificity for Epoxides, Episulfides and Lactones

499

y-Ray polymerization of racemic /?-dichloromethyl-PL in the solid phase was found to result in the formation of a syndiotactic polymer. On the other hand, /?-trichloromethyl-PL formed an isotactic polymer under the same conditions. These results were explained by the particular molecular packing in the monomer crystals. 108

33.9 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

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