Monoalkene Polymerization: Stereospecificity

3 Monoalkene Polymerization: Stereospecificity PAOLO CORRADINI, VINCENZO BUSICO and GAETANO GUERRA Universite di Napoli, Italy 3.1. INTRODUCTION

29

3.2 SELECTED EXPERIMENTAL RESULTS 3.2.1 General Remarks 3.2.2 Heterogeneous Isospecific Ziegler-Natta Catalysts 3.2.3 Homogeneous Syndiospecific Catalysts 3.2.4 Homogeneous Isospecific Catalysts 3.2.5 Summary

30 30 36 38 39

40

3.3 MECHANISMS FOR STEREOSPECIFICITY IN ZIEGLER-NATTA POLYMERIZATION 3.3.1 General Remarks 3.3.2 Heterogeneous Catalysis 3.3.3 Homogeneous Syndiospecific Catalysis 3.3.4 Homogeneous Isospecific Catalysis

40 40

3.4 REFERENCES

48

41 46 47

3.1 INTRODUCTION We will discuss here a few concepts which are useful in the following discussion of stereospecific Ziegler-Natta (ZN) polymerizations of l-alkenes. In a polymerization reaction, a prochirall-alkene molecule will add to a growing polymer chain in one of four possible modes (Scheme I). Addition modes (a) and (b) on the one hand, and (c) and (d) on the other, lead to monomeric units with equal constitution (1,2-addition and 2,1-addition respectively, signifying that in cases (a) and (b) the l-alkene molecule has linked to the growing chain end at C-2, but in cases (c) and (d) at C-I), but opposite configurations at the newly formed tertiary C atom. (I)

(2)

CH 2=CHR

H, ""R

+ -P ------. (a)

c"

"CH; "p

-P=growing polymer chain Scheme 1 Possible addition modes of a l-alkene molecule to a growing poly-l-alkene chain

29

30

Transition-metal Initiated and Related Polymerizations

Macromolecules resulting from successions of (prevailingly) equal additions (either 1,2 or 2,1, independent of the choice of configuration) are said to be regioregular (from a regiospecific polymerization process); if there is regularity in the succession of configurations of the tertiary C atoms, the macromolecules are said to be stereoregular (from a stereospecific polymerization process). In the description of a poly-I-alkene chain stereostructure, two successive monomeric units having the same configuration are said to constitute a meso (m) diad, and two having opposite configurations a racemo (r) diad. A chain ideally resulting from the regular succession of m diads is defined as isotactic, and one from the regular succession of r diads as syndiotactic, but in the absence of stereoregularity, the chain is said to be atactic (Scheme 2).

" c'

R

,/

H R \\\'

c

H

,\\\\\

'CH~ 'CH~

(b)

(a)

(c)

(d)

(e)

Scheme 2 Planar zigzag projection of the backbone carbon atoms of poly-l-alkene chain segments: meso (a) and racemo (b) diads, both in the saw-horse (upper) and in an adapted Fischer (lower) notation; isotactic (c), syndiotactic (d) and atactic (e) sequences in an adapted Fischer notation

In general, ZN catalysts are highly regiospecific in the polymerization of l-alkenes, but differ widely in stereospecificity. Until recently, it was customary to classify stereospecific ZN catalysts into heterogeneous isospecific and homogeneous syndiospecific.t 2 In 1984, the first report.' appeared in the literature of homogeneous catalyst systems which are able to polymerize l-alkenes to isotactic polymers. This was soon followed by others which began to throw light on what is probably the most significant scientific achievement in the field of ZN catalysis since its discovery. In this review, which is perhaps the first to be written after such reports, the new class of homogeneous isospecific ZN catalysts was added to the two traditional ones. This chapter is structured in two parts. In Section 3.2, experimental data on the stereospecificity of polymerization of l-alkenes are presented; these are not intended to provide an exhaustive collection of data on stereospecific ZN catalysts (excellent reviews on this are refs. 1 and 4), but rather a selection of results useful- in the authors' opinion - in the comprehension of the principles of catalyst specificity. In Section 3.3, possible reasons for the origins of regio- and stereo-specificity, and models of catalytic sites are discussed.

3.2 SELECTED EXPERIMENTAL RESULTS 3.2.1

General Remarks

It is widely accepted that, in the presence of ZN catalysts, poly-l-alkene chain growth consists of successive insertions of l-alkene molecules into a metal-earbon bond. 1 , 2 ,4 A cis opening of the l-alkene double bond has been proved conclusively. 3,5-7 An insertion leading to the formation of a M-CH 2-CHR-P sequence (where M = metal; P = growing polymer chain) is said to be primary (1,2 addition of Scheme 1),while one leading to the

M onoalkene Polymerization: Stereospecificity

31

formation of a M-CHR-CH 2-P sequence is said to be secondary (2,1 addition of Scheme 1). Regiospecificity implies the non-equivalence of the two insertion modes, for electronic and/or steric reasons. Stereospecificity, which for a given insertion mode entails the ability of the catalyst to discriminate between the two faces of the prochiral monomer, requires at least one chiral species to participate in the polymerization mechanism. The steric control may be dictated by the configuration of the asymmetric tertiary C atom of the last inserted monomer unit, as well as by the chirality of the organometallic complex of which the active site is part (in case the optically inactive catalyst consists of a racemic mixture of enantiomorphic sites). Both possibilities (e.g. chain end stereocontrol and enantiomorphic site stereocontrol) have been verified, depending on the catalyst system (vide infra). No ZN catalyst is totally stereospecific. Terms such as 'isotactic' and 'syndiotactic', when referred to real poly-l-alkene chains, should be intended as 'mainly isotactic' or 'mainly syndiotactic', to account for the occurrence of failures in the catalyst stereocontrol. The distribution of steric defects along the polymer chains may be indicative of which kind of stereocontrol is operating. Bernoullian statistics have been shown" to be consistent with a chain end stereocontrol, while non- Bernoullian distributions originate from an enantiomorphic site stereocontrol (Table 1).9,10 Table 1 Stereochemical Sequence Distributions for Bernoullian (Chain End) and Enantiomorphic Site Stereocontrol Probability

Probability

(Bernoullian chain end)

( Enantiomorphic site)

Sequence

1-2b(1-b) 2b(1-b) 1-3b(1-b) 2b(1-b) b(l-b) 5b4 - lOb3 + IOb2 - 5b + I -6b 4+ 12b3 -8b 2+2b b4-2b 3+b 2 -6b 4+ 12b3 -8b 2+2b (2b4 - 4b3 + 2b2)+ (2b4 - 4b 3 + 2b2) 2b4 - 4b 3 + 2b2 b4-2b 3+b 2 2b4 - 4b3 + 2b2 - 3b4 + 6b3- 4b2 + b

P

m r

mm mr rr

mmmm mmmr rmmr mmrr mrmm + rmrr rmrm rrrr rrrm mrrm

I-P

p2 2P(1-P) (1_P)2 p4

2p 3(1-P) 2P 2(1_P)2 2p 3(1-P) p 2(1_P)2 +2P(1-P)3 2p 2(1_P)2 2P(1-P)3 p2(1.:..- p)2 (1-P)4

The difference can be readily realized for isotactic propagation. Intuitively, in the case of chain end stereocontrol, an occasional change in the configuration of the last inserted monomer unit tends to be perpetuated (as in Scheme 3a), while in the case of enantiomorphic site stereocontrol the same occasional change, having no effect on site chirality, tends to remain isolated (as in Scheme 3b).2-4

(a)

(b)

Scheme 3 Typical steric defects in a (mainly) isotactic poly-I-alkene chain (adapted Fischer notation)

It should be realized that more than one type of active site may be present (as is the case particularly with heterogeneous catalysts), and that, even for a single type of active site, both kinds of stereocontrol may operate.':" The extent of chain regularity is determined primarily with spectroscopic methods. For a long time IR spectroscopy has been the only available tool. The main drawback of the IR technique, i.e. the lack of generally accepted procedures for the quantitative evaluation of the spectra, has been overcome by the development of NMR spectroscopy, particularly 13CNMR. For most poly-lalkenes of practical importance, the chain stereostructure can be quantitatively determined at least at the triad level. In the following, IR data are reported only in the absence of more reliable 13CNMR results.

32

Transition-metal Initiated and Related Polymerizations

Polymer chains differing in regularity have different solubilities in organic solvents (less stereoregular chains being more soluble), and solvent fractionation methods have been proposed to provide indirect estimates of chain regularity. Although it is well recognized that solubility depends on average molecular weight as well (shorter macromolecules being more soluble), these methods are so widely and routinely used that they cannot be ignored. In particular, for polypropene obtained from heterogeneous isospecific catalysts, the weight fraction insoluble in n-heptane at its boiling point is largely identified with the relative amount of isotactic polymer in a sample, and for this reason denoted isotactic index (1.1.). When dealing with the complicated mixture of macromolecules of variable regularity typically resulting from such catalysts (vide infra), this practice is justified by the need of an (admittedly arbitrary) borderline between what is to be considered isotactic polymer and what is not. The specificity of a ZN catalyst system is somewhat dependent on polymerization conditions (temperature, solvent, concentrations of components and of monomer). A detailed description of such conditions for each cited example would have rendered this presentation too burdensome; for details, the interested reader is referred to the original papers. An exception is made for polymerization temperature in the presence of homogeneous catalysts, which is always indicated for the dramatic dependence of stereospecificity on this parameter. Due to the overwhelming importance of polypropene (polypropylene), most literature data on stereospecific ZN polymerizations refer to propene (propylene); in the following, significant results for other l-alkenes will be given when available. 3.2.2 Heterogeneous Isospecific Ziegler-Natta Catalysts In 1954, Natta and co-workers found that the catalyst disclosed by Ziegler in 1953 11 for the low pressure polymerization of ethylene, e.g. the product of the reaction between TiCl 4 and an Al trialkyl in hydrocarbon solvent, is able to polymerize propene to a partly crystalline polymer. 12 An ingenious combination of solvent fractionation and X-ray diffraction methods revealed that the raw polymer was a mixture of atactic and isotactic polypropene (the finding was so revolutionary that even the terms 'isotactic' and 'atactic' had to be coined for the occasionj.P"!" What was defined as 'isotactic polypropene', i.e. the polymer fraction insoluble in boiling n-heptane, amounted to 20-40 % by weight of the total polymer, depending on the experimental conditions."? From later 13CNMR spectra, a mm triad content well over 90% of the total triad distribution was evaluated for this fraction. 18 It was soon recognized that, in polymerization conditions, TiCl 4 and Al trialkyls react to form a brown modification of TiCl 3 (prevailingly p- TiCI 3), which is insoluble in the reaction medium. 19 This prompted Natta and co-workers to explore the catalytic activity of preformed modifications of TiCI 3 , prepared either by reduction of TiCl 4 (e.g. with H 2 or AI) or from different crystalline forms of TiCl 3 by mechanical or thermal treatments. A dramatic improvement in stereospecificity was obtained in propene polymerization using suspensions of a layered (violet) modification of TiCl 3 (a, y or <5) in hydrocarbon medium, in a mixture with Al trialkyls or Al dialkyl halides: 1.I.'s in the range 80-95% were achieved.i? These findings opened the way to a whole generation of TiCl 3-based catalysts for the polymerization of l-alkenes to linear isotactic polymers of wide industrial application. In spite of large differences in catalytic activity, resulting from treatments to increase surface areas and from modifications with organic and/or inorganic compounds.F'
Monoalkene Polymerization: Stereospecificity Table 2

33

Isotactic Index (1.1) and Degree of Isotacticity of the

n-Heptane Insoluble Fraction (0.1.) of Polypropene Samples Obtained in the Presence of Different Heterogeneous Isospecific ZN Catalysts 1.1. (%)

D.I. (0/0)

Ref.

(a) TiCI 4 / AIEt 3 a-TiCl 31AIEt 3 ~- TiCI 3 / AIEt 3 a-TiCl 31AIEt 2CI ~- TiCl 31AIEt 2CI

26 87 75 95 89

91 95b 92b 94b 90

c d d d e

(b) a-TiBr 31AIEt 3 VCI3/AIEt2CI CrCl 31AIEt 2CI

76 61 98

87 80 94

d e e

(c) MgCI 2/TiCI4 - AIEt 3 MgCl 2/EB/TiCl4 - AIEt 3 MgCI 2/TiCI4 - AIEt 3/EB MgCl 2/EB/TiCl4 - AIEt 3/EB

31 50 88 95

91 93 93 93

Catalyst system

8

8

b 8 8

8 8 8 8

Abbreviation: EB = ethyl benzoate. % mm triad as evaluated by l3C NMR. b Evaluated by IR. C Y. Doi, Makrornol. Chern., 1979, 180, 2447. d Y. V. Kissin, V. I. Tsvetskova and N. M. Chirkov, Eur. Polyrn. J., 1972, 8, 529. e C. Wolfsgruber, G. Zannoni, E. Rigamonti and A. Zambelli, Makrornol. Chern., 1975, 176, 2765. f Authors' data.

a

Table 3

Isotactic Index (1.1.), IR Degree of Isotacticity (D.I.) and

Intrinsic Viscosity [11] of Polypropene Samples from ex-TiCI 3/MR n Catalyst Systems 1.1. (0/0)

D.I. (0/0)

91-98 77-80 46-66

93-98 85-92 9{}-95

2-11.8 3-4 0.14>.4

b, c b, c b, c

For the n-heptane insoluble fraction. b A. P. Firsov, B. G. Kashporov, Y. V. Kissin and N. M. Chirkov, J. Polym. Sci., 1962, 62, s-104. C Y. V. Kissin, V. I. Tsvetskova and N. M. Chirkov, Eur. Polym. J., 1972, 8, 529. a

Table 4

Degree of Isotacticity (D.I.) of Different Poly-l-alkene Samples from TiCl 3-based Catalyst Systems

Poly-I-alkene

Catalyst system

Poly-I-butene

y- TiCl 31AIEt 2CI 0-TiCl 31AIBu~

Poly-4-methyl-l-pentene Polystyrene

D.I. (%)

Ref.

ex-TiCI 3 / AIEt 3

0..10 by weight insoluble in diethyl ether. b Evaluated by IR. CJ. Boor, J. Polym. Sci., Part C, 1963, 1, 237. d Y. V. Kissin, 'Isospecific Polymerization of Olefins', Springer-Verlag, New York, 1985, chap. IV. a

A more effective utilization of the catalytically active species has been achieved by a new generation of catalysts, rapidly taking over the market, in which a Ti compound (mostly TiCI 4 ) is supported on a crystalline matrix. It may be of interest to remark that, up to now, stereospecific catalysts have been obtained using supports with a layered crystal structure of TiCI 3-type, primarily MgCI 2 • In the absence of third components (vide infra), average catalyst activities of two orders of magnitude higher than the maximum activity of conventional TiC13-based catalysts can be reached with MgCI 2/TiCI4 catalysts, but their stereospecificity is poor: 1.1. for polypropene is in the range 20-500/0. 2 6 , 2 7 Nevertheless, the polymer fraction insoluble in n-heptane is highly stereoregular, as is the homologous fraction from TjClj-based catalysts (cf Tables 2a and c). This suggests the

34

Transition-metal Initiated and Related Polymerizations

existence, in both cases, of (basically) two classes of active sites - highly stereospecific and nonstereospecific - in variable ratios. It seemed reasonable to suppose that for these two classes of sites- being part of different organometallic complexes-there is a different chemical reactivity. This assumption justified the search for third components able to 'poison' selectively the non-stereospecific sites. Indeed, several families of electron donor compounds were found able to improve the stereospecificity of MgCI 2/ TiCl 4 catalysts, e.g. esters of aromatic carboxylic acids, hindered amines, alkylalkoxysilanes. I.I.'s of 95% and over can be obtained in propene polymerization, with an appropriate choice of electron donors as components of the solid catalyst ('internal' donor) and of the co-catalyst mixtures with Al trialkyls ('external' donor) (Table 2C).27-30 Although the first industrial patents date back to the early 1970'S,31 systematic studies on the mechanism of catalyst modification with electron donors came much later. 27,32 It has been shown that both internal and external donors are necessary to achieve high stereospecificities without depressing catalyst productivities too much. In particular, according to a recent investigation," 7 the internal donor, which is contacted with MgCl 2 prior to TiCI 4, prevents the formation of nonstereospecific sites relatively insensitive to selective poisons (external donors) acting either by complexation or by some other chemical reaction. 27,33 Sound evidence has been presented in favour of the view that the electron donors do not participate in the organometallic complex of which the active site is part. 27,34 Tables 5 and 6 present more detailed spectroscopic data on the regio- and stereo-regularity of polypropene samples obtained in the presence of a number of heterogeneous isospecific catalyst systems. A similar picture emerges for TiClj-based and MgCl 2-supported catalysts. Table 5

Regioregularity (Evaluated by 13C NMR) of Polypropene Samples from Different Heterogeneous Isospeeifie ZN Catalysts

Catalyst system

TiCI 4 / AIEt 3 f>-TiCl3/AIEt 3 VCI 3/AIEt 2CI MgCI 2/TiCI4-AIEt 3

Polymer fraction i7 s7 i7 s7 i7 s7 i7 s7

Amount of regioirregular placements (mol 0/0)

Ref.

n.d.

a, b

n.d.

a, b

n.d.

e

n.d.

d

4-5 1

1-2

~O.1

Abbreviations: i7 = insoluble in n-heptane; s7 = soluble in n-heptane; n.d. = not detected Y. Doi, Makrornol. Chern., 1979,180,2447. by. Doi, Makromol. Chern. Rapid Cornmun., 1982,3,635. c C. Wolfsgruber, G. Zannoni, E. Rigamonti and A. Zambelli, Makrornol. Chern., 1975,176,2765. d Authors' data. a

Table 5 reports, for different polymer fractions, the relative amounts of regioirregular placements of monomer units. These are virtually absent in the isotactic (n-heptane insoluble) fraction; 0.1-5% regioirregular placements, on the other hand, are found in the non-stereoregular (n-heptane soluble) fractions. The spectroscopic analysis of the chain end groups pointed out that, in most cases, the preferred insertion mode is primary.34-37 Table 6 gives the pentad stereo sequence distribution (as determined via 13CNMR) for various polymer fractions. The data for the isotactic (n-heptane insoluble) polymer are consistent with the hypothesis of an enantiomorphic site stereocontrol (cf Table 1).9,10,38 Such a conclusion is independently confirmed by the 13CNMR analysis of isotactic polypropene chains with low amounts of copolymerized ethylene. If the steric control was due to the chirality of the last inserted monomer unit, the insertion of a propene unit following that of an achiral ethylene unit would be non-stereospecific (both cases (a) and (b) of Scheme 4 would be possible), whereas virtually complete stereospecificity has been observed experimentally (case (a) of Scheme 4).2,39 Table 6 deserves a further comment. It is seen from the pentad stereosequence distribution that what is cumulatively denoted as non-stereoregular (atactic) polypropene (i.e. the fraction soluble in n-heptane) is a complicated mixture of macromolecules of variable stereostructure. In all cases, the distribution is not Bernoullian, but short isotactic stereoblocks alternate with atactic sequences and (with a smaller amount of) short syndiotactic stereoblocks.?

Table 6 Pentad Stereosequence Distribution (Evaluated by 13CNMR) for Polypropene Samples from Different Heterogeneous Isospecific ZN Catalysts .. Catalyst system

Polymer fraction

13C N M R pentad stereosequence distribution (%)

0/0 mmm~

mmmr

~90

~4

rmmr

mmrr

mmrm+rrmr

mrmr

rrrr

Ref mrrr

mrrm

~ c ~ c ~

~ ~-TiCI3/AIEt2CI

0-TiCI 3/AIEt2Cl VCI3/AIEt2CI CrC13/AIEt2CI MgCI2/EB/TiCI 4-AIEt 3

i7 s7-i6 s6-i5 s5 i7 s7 i7 s7-i6 s6 i7 s7 i7 s7-i6 s6

47.5 9 12.5 31 91 9 60.5 5.5 34 98.5 1.5 71.7 8.6 19.7

52 24 16

~93

25 74 41 15

~90

32 90 68 28

8 9 11

~3

13 6 9 10

10 ~2 ~3

2 ~2 ~4

9 15 16

~3

17 7 12 13

~4

~4

9 4 10 12

~4

~2

3

11 4 10 16

~2

8 16 16

~2

6 6

6 14 18

~2

13 6 12 23

2

14

9

~1

~2

~1

10 3 11

~2

5

~2

3

13 14 18 3 15

10 10

5 12 9 2 8

a

~3

~1

5

~3

~

~

b,c a

~4

~4 ~2

7 2 4 4

~

c

~4 ~4

~

~

a

~

~

""l

N' ~

c' ~

~

~ ~ ~

d

Abbreviations: i = insoluble; s = soluble; 7 = n-heptane; 6 = n-hexane; 5 = n-pentane a C. Woffsgruber, G. Zannoni, E. Rigamontiand A.Zambelli, Makromol. Chern., 1975,176,2765. b A. Pavan, A. Provasoli, G. Moraglio and A. Zambelli, Makrornol. Chem.; 1977,178,1099. C Y. Doi, Makrornol. Chem., Rapid Commun., 1982, 3, 635. d Y. Doi, E. Suzuki and -T. Keii, Makrornol. Chem., Rapid Commun., 1981, 2, 293.

""l

~

c ~ ~ ~

S; ~

~.

w

Va

Transition-metal Initiated and Related Polymerizations

36

(ajilLilil

Scheme 4

(b)

Possible isotactic poly-I-alkene chain propagation after the copolymerization of an achiral ethylene unit (adapted Fischer notation)

Scheme 5 illustrates recent results of a high resolution 13CNMR investigation''" of the specificity of a b-TiCl 3 catalyst in the first two insertions of the monomer (either propene or l-butene) into an initial M-alkyl bond of a growing chain, the alkyl group deriving from a selectively 13C-enriched Al or Zn alkyl. C

C

1

C

C

C

I I I -c -c--c-c-c-c-c-

(50 /~)

C

I I -c---c--c-C---C-r- DC C

C

1

C

(50%)

C

I I I Jc-c --c-c-c--c--c--c-

(80 °0)

C

-c~---c--t-c-r_13C---C

(20

~Io)

C

Scheme 5 Structure and relative occurrence of polypropene chain ends for polymer samples obtained from the catalyst systems l5-TiCI3/Ale3CH3h/Zn(CH3h (left) and l5-TiCI3/Ale3CH2CH3h/Zn(C2Hsh

The spectra confirm a primary insertion mode, as already discussed. For both propene and l-butene, the first insertion is non-stereospecific when the active site is a M_ 13CH 3 bond, and only partly (ca. 800/0) stereospecific when the active site is a M_ 13CH 2CH 3 bond, while the second insertion is in all cases highly (over 90 % ) isospecific. These results prove once again that stereospecificity is dictated by the intrinsic chirality of the organometallic complex at the active site, and not by the chirality of the growing polymer chain (no asymmetric C atoms being present in ethyl or isobutyl groups), but they also reveal that its extent is dependent on the size of the alkyl group 'bound to the metal in the active site; in particular, stereospecificity appears when the alkyl group has at least two C atoms. The implications of this latter finding on the mechanism of stereocontrol are discussed in Section 3.3. We end with a brief account of the polymerization of chiral l-alkenes. In the presence of heterogeneous isospecific catalysts, it has been shown"! that a racemic mixture of a chiral monomer is converted into a racemic mixture of macromolecules each containing prevailingly one of the two enantiomers (stereoselective polymerization). The degree of stereo selectivity strongly depends on the position of the, asymmetric C atom with respect to the double bond.r? being relatively high (ca. 70-900/0 for 3-methyl-l-pentene in the presence of the catalyst system 8-TiCI3/AIMe3, according to recent 13CNMR investigationst") when this is in the r:J., position, moderate when in the ~ position, and virtually absent when in the }' position or further apart. In principle, stereoselectivity - as with stereospecificity - may arise from the chirality of the last inserted monomer unit as well as from that of the active site. Spectroscopic investigations have pointed out, in this case too, an enantiomorphic site stereocontrol.f'v'" It has also been shown, on the other hand, that stereospecificity and stereoselectivity are independent phenomena, arising from different interactions between the l-alkene monomer and the organometallic complex of which the active site is part.":'

3.2.3 Homogeneous Syndiospecific Catalysts Propene and-very recently-styrene are the only two l-alkenes which have been polymerized with ZN catalysts to syndiotactic polymers with high steric purity. Low amounts of syndiotactic polymer are obtained when polymerizing propene in the presence of some heterogeneous isospecific catalysts.t" even more frequent is the formation of more or less stereoirregular macromolecules containing short syndiotactic stereoblocks.v " This proves the existence in such catalysts of small fractions of active sites having - at least temporarily - a syndiospecific stereocontrol.

37

M onoalkene Polymerization: Stereospecificity

Highly syndiotactic polypropene as a primary product, on the other hand, can be obtained - to our present knowledge-only in the presence of a few V-based catalyst systems 4 7,48 (Table 7) operating in the homogeneous phase at low temperature (below ca. - 50°C, as the polymerization becomes non-stereospecific at higher temperatures). Table 7 Regularity (Evaluated by 13C NMR) of Polypropene Samples from Different Homogeneous Syndiospecific ZN Catalysts Catalyst system

VCI4/AIEt 2CI, -78°C VCl41AIEt 2Br, - 78°C VCl41AIEt 2 I, - 78°C V(acach/AIEt 2CI, -78-°C VCI4/AIEt 2CI, 41 -c-

13C N M R triad distribution OJlo mr 0/0 rr 0/0 mm

Fraction of regioirregular placements (mol%)

Ref

24 25 31 32 26

2.1 5.1 15.0 3.6 4.6

b b b c b

22 46 3 59

76 53 23 65 15

Abbreviations: acac = acetylacetonate. data added for comparison. by. Doi, M. Takada and T. Keii, Makromol. Chem., 1979,180,57; C Y. Doi, Macromolecules, 1979,12, 1012.

a

From Table 7, it appears that both regio- and stereo-regularity are lower than those typical of isotactic polypropene; improvements in syndiospecificity have been obtained by catalyst modification with specific electron donor compounds (typically anisole).47,48 A lot of spectroscopic evidence exists 50,51 that propene insertion is prevailingly secondary. The insertion mode appears to be mainly dictated by steric factors; indeed it has been shown that propene insertion into an initial M-ethyl bond.t! as well as after an ethylene insertion in ethylene/ propene copolymerization.V is substantially non-regiospecific.P:' Regiospecificity would thus arise in consequence of propene secondary insertion. The observed distribution of steric defects (Table 8) obeys Bernoullian statistics. 54,55 This suggests that syndiospecificity results from a chain end stereocontrol (unlike (ul) 1,3 asymmetric induction of the last inserted propene unit). In agreement with this hypothesis, it has been shown that, in ethylene/propene copolymerization, propene insertion after an ethylene insertion is substantially non-stereospecific.P" Table 8 Pentad Stereosequence Distribution (Evaluated by 13C NMR) for a Typical Polypropene Sample from the Homogeneous Syndiospecific ZN Catalyst System VCl 41AIMe 2CljAnisole at -78 o ca Pentad

0/0

mmmm mmmr rmmr mmrr mmrm+rmrr rmrm rrrr mrrr mrrm

1.1 3.3 3.1 3.7 23.9 10.9 25.4 21.7 6.9

a A. Zambelli, P. Locatelli, A. Provasoli and D. R. Ferro, Macromolecules, 1980, 13, 267.

Table 7 shows that syndiospecificity is affected by the choice of the Al alkyl co-catalyst. This has been taken as an indication that the Al alkyl participates in the catalytic complex. 55 A preliminary report has recently appeared in the literature on the first polymerization of styrene to a syndiotactic polymer. 56 An exceedingly high steric purity (rr triad content over 98 0/0) was

38

Transition-metal Initiated and Related Polymerizations

achieved. No detail was given about the catalyst system, generically described as a ZN catalyst comprising of a Ti compound and an organo-Al compound. In a subsequent independent paper.l" the synthesis of syndiotactic polystyrene has been claimed at 50°C in the presence of the catalyst system CP2TiPh 2/methylalumoxane, which is also known to polymerize propene to an isotactic polymer at low temperature (below ca. -40°C, vide infra).

3.2.4 Homogeneous Isospecific Catalysts Homogeneous catalyst systems able to polymerize l-alkenes to isotactic polymers were first described by Giannini et ale in 1970.58 Isotactic polypropene and poly-4-methyl-l-pentene were obtained with catalysts originating from benzyl derivatives of Ti and Zr (typically MBz 4 , MBz 3X with M = Ti or Zr; X = halogen), even in the absence of a non-transition metal organometallic cocatalyst. The exceedingly low catalytic activities discouraged further research on these catalyst systems. Much more impact was produced by a recent (1984) paper by Ewen," describing homogeneous catalysts able to polymerize propene to isotactic polymer with high productivities, and consisting of titanocene compounds in combination with alkylalumoxanes. CP2TiR 2/ AIR~ catalyst systems (Cp = cyclopentadienyl; R = alkyl or aryl; R' = alkyl) for ethylene polymerization have long been known. In the early 1970s, substantial improvements in productivities were obtained by controlled addition of H 20, which reacts with the Al alkyl to yield mixtures of linear and cyclic oligomeric alkyl alumoxanes. At the usual polymerization temperatures (20-70 DC), such catalysts were also known to polymerize propene with lower productivities to purely atactic polymer. 18,59 Quite unexpectedly, Ewen discovered.' that the catalyst system Cp2TiPh2/AIMe3/H20 becomes Increasingly isospecific in propene polymerization with decreasing temperature (Table 9 and Figure 1). Table 9 Diad and Triad Distributions in Polypropene Samples Obtained in the Presence of the Catalyst System CP2TiPh 2/AIMe3 / H 20 at Various Temperatures"

Temperature

50 25 0 -15 -30 -45 -60 -75 -85 a

0/0 m

0/0 r

0/0 mm

0/0 mr

0/0 rr

49 50 67 78 83 85 85 85 84

51 50 33 22 17 15 15 15 16

23 24 45 62 69 73 73 73 72

53 52 44 33 28 25 24 25 25

24 24 11 5 3 2 3 2 3

J. A. Ewen, J. Am. Chern. Soc., 1984, 106, 6355.

The observed Bernoullian distribution of steric defects in the polypropene chains (Table 9) is' consistent with a chain end stereocontrol. 3 The spectroscopic analysis of the chain end groups suggested that chain propagation takes place via primary insertion; no regioirregular placements of propene units were detected by 13CNMR. These conclusions were subsequently confirmed by a 13CNMR analysis. of the stereostructure of the chain end groups resulting from the first propene and l-butene insertions into an initial M-alkyl bond, the alkyl group being derived from a selectively 13C-enriched Al alkyl. 60 Remarkably, this is the first known case of isotactic chain propagation dictated by the chirality of the last inserted monomer unit (like (lk) 1,3 asymmetric induction). An unusual decrease of catalyst stereospecificity has been pointed out"? with increasing size of the alkyl substituent at the double bond in the polymerization of different l-alkenes: the m diad content in the resulting polymer, which is over 80% for polypropene, drops to ca. 60% for poly-l-butene and approaches 50% for poly-4-methyl-l-pentene. This has been taken as a further indication for the existence of a chain end stereocontrol. 57

M onoalkene Polymerization: Stereospecificity

lo)

l b)

39

l c)

Figure 1 Comparison of 13C NMR spectra of the methyl pentad region for polypropylene obtained with CP2TiPh 2/ methylalumoxane at three different polymerization temperatures: (a) 25°C; (b) 0 °C; (c) -45°C (reproduced by permission of American Chemical Society from J. Arn. Chern. Soc., 1984,106,6355)

Ewen also showed 3 that stereorigid chiral titanocenes (e.g. rac-(ethylene)bis(indenyl)TiX 2 with X = halogen, alkyl or aryl, Figure 2), in mixture with alkylalumoxanes, polymerize propene to an isotactic polymer with a non-Bernoullian distribution of steric defects consistent with an enantiomorphic site stereocontrol, as with that observed for the heterogeneous isospecific catalysts. In this case too, chain propagation was shown to proceed via primary insertion with very high regiospecificity. The 13CNMR investigation of the stereostructure of chain end groups resulting from the first propene and l-butene insertions into initial M-( 13C-enriched)-alkyl bonds"! confirmed that the steric control is due to the chirality of the active site, and pointed out a dependence of stereospecificity on the size of the alkyl group bound to the metal very similar to that observed for the heterogeneous isospecific catalysts"? (cf Scheme 5). Noticeably, stereorigid achiral titanocenes (e.g. rneso-Iethylenejbistindenylj'I'Xy), in combination with alkylalumoxanes, polymerize propene to purely atactic polymer.' Ewen's pioneering work has been subsequently extended by Kaminski to homogeneous catalyst systems based on the homologous or strictly related zirconocenes, which turned out to be characterized by similar specificity but much higher catalytic activity in propene and l-butene polymerization.P? Productivities, expressed in moles of polymerized monomer per mole of Zr an hour, analogous to or even higher than those typical of MgCl 2-supported catalysts have been achieved, though a number of still unsolved problems preclude - at the moment - the possibility of short-term industrial applications.

3.2.5

Summary

Table 10 summarizes the main conclusions derived from over three decades of research on the stereochemistry of polymerization of l-alkenes in the presence of stereospecific ZN catalysts.

Figure 2

Molecular structure of racemic ethylene-bis(4,5,6,7-tetrahydro-1-indenyl)titanium dichloride (reproduced by permission of Elsevier from J. Organornet. Chern., 1982,232,233)

Transition-metal Initiated and Related Polymerizations

40

Table 10 Stereochemistry of Propene Polymerization in the Presence of Stereospecific ZN Catalysts

Catalyst type

Stereochemistry of addition to the double bond

Monomer insertion

Type of stereocontrol

cis cis cis

primary secondary primary

enantiomorphic site chain end chain end

cis

primary

enantiomorphic site

Heterogeneous isospecific Homogeneous syndiospecific Homogeneous isospecific, achiral Homogeneous isospecific, chiral

This frame of experimental facts partly obviates the lack of direct observations of the catalytic Ihmr wtncn reasonatne models of such species can be devised, as is discussed in Section 3.3. s~~~J; 0¥0'~1\tliIg a &~li.f

3.3 MECHANISMS FOR STEREOSPECIFICITY IN ZIEGLER-NATTA POLYMERIZATION 3.3.1 General Remarks In this section, we review a number of mechanisms proposed in the literature.jn order to account for the stereospecific behaviour of the Ziegler-Natta catalysts, in the heterogeneous as well as in the homogeneous polymerization of monoalkenes. It is largely accepted, on the ground of specific experimental evidence (cf also Section 3.2) and of basic considerations of organometallic chemistry, that polymer chain growth implies successive insertions into a transition metal-earbon bond of previously coordinated l-alkene molecules, through cis openings of the alkene double bond. Only mechanisms will be considered.which conform to this view and take explicitly into consideration the origin of the stereospecificity, as arising from possible diastereoisomeric situations. The main elements of chirality possibly present in the intermediates and transition states, which can be hypothesized in this framework, are as follows. Firstly, upon coordination, a prochiral l-alkene molecule, such as propene, gives rise to nonsuperposable si and re coordinations'P'P" as indicated in Figure 3. According to the considered mechanisms, an isotactic polymer is generated by a long series of insertions of all si or all re coordinated monomers; a syndiotactic polymer by alternate insertions of si and re coordinated monomers. sl

re

Figure 3 The two possible chiral coordinations to a metal atom (M) of a propene molecule (re and si). The metal atom in the figure is below the propene molecule

A second element of chirality is the configuration of the tertiary carbon atoms in the growing chain, and in particular of that in the last inserted monomer unit. Last but not least, the catalytic site may be chiral itself. An example for the heterogeneous catalysts, first proposed by Arlman'" and common to other models 6 6 - 6 9 is shown in Figure 4: a metal atom is bridge-bonded through halogen atoms to two more metal atoms and two coordination positions are available to the monomer and to the growing chain. The chirality of these sites can be labelled with the symbols A and ~,67 defined for octahedral coordination compounds with at least two bidentate chelating agents.?" In this framework the stereospecificity of polymerization is to be connected with the energy differences between the diastereoisomeric situations which originate from the combination of two or

M onoalkene Polymerization: Stereospecificity

41

A

® metal

atoms

o

chlorine atoms

Figure 4 Chiral model sites for the heterogeneous catalysts constituted by a metal atom bridge-bonded to two other metal atoms. Their chirality can be labelled with the A and L\ notation, defined for coordination compounds. The two positions at the model sites available for the incoming monomer and the growing chain are indicated by arrows

more of the above elements of chirality. All of the literature studies on the subject consider the energy differences between diastereoisomeric intermediates, instead of transition states, since it is easier to assume reasonable internal coordinates. Of course, the relevance of these studies is based on the assumption that such intermediates be energetically and geometrically close (much more than reagents and products) to the transition states; in any case, the comparison between structures of suitable diastereoisomeric intermediates may give an indication of the relative height of the energy barriers to the products.

3.3.2 Heterogeneous Catalysis As discussed in Section 3.2, it is well established that the steric control of the heterogeneous Ziegler-Natta catalysts is due to the chirality of the catalytic site and not to the configuration of the last insetted monomer unit. The hypothesis that the steric control is due to the structure of catalytic sites on the border of crystal layers of TiCl 3 was first put forward by Natta 71 and thoroughly developed by Arlman and Cossee.t": 72." 73 The structure of violet titanium trichlorides, as well as that of other transition metal trichlorides effective as components of Ziegler-Natta catalysts (e.g. VCI3 , CrCI 3 ), is built up of layers of the kind shown in Figure 5, piled,one on top of the other according to a close packing of the chlorine atoms. The various known crystalline modifications 74-76 correspond to different ways in which the piling may be obtained. Within each layer, the metal atoms occupy in an ordered arrangement two thirds of the octahedral positions. As a result, neighbouring metal atoms (bridged by two CI atoms) have opposite chirality.?" which may be designated with the symbols A and Ll.

Figure 5 Schematic drawing of a structural layer of violet TiCI 3 • The titanium atoms (full circles) are each bonded to six chlorine atoms (empty circles) and occupy only two thirds of the octahedral positions; the vacant octahedral positions are indicated by squares. The chirality of some of the metal atoms are also labelled. The dashed lines correspond to the lateral cut of Figure 6

42

Transition-metal Initiated and Related Polymerizations

On the basis of considerations of crystallochemistry and taking into account electron microscopy observations of the surface of crystals upon which some polymer was formed,"? Arlman and Cossee 72,73 concluded that the active sites are located on crystal surfaces different from the basal 001 ones and parallel to the direction a-b of the unit cell defined as in ref. 76. Figure 6 illustrates that, if we cut a TiCl 3 layer parallel to the direction defined above, which corresponds to the line connecting two bridged Ti atoms, electro neutrality conditions impose that each Ti atom at the surface of the cut be bonded to five CI atoms only. Of these, four are bridged to further metal atoms and are then more strongly bonded; the fifth, non-bridged CI atom may be replaced by an alkyl group. The octahedral position which is still left free may n-coordinate an alkene. In general for this kind of model sites, the two positions accessible to the growing chain and to the monomer are nonequivalent: as shown in Figure 6, one (indicated as inward) is further in than the other (indicated as outward) with respect to the lateral cut.

Figure 6 Schematic drawing of a lateral cut of a TiCl 3 layer. The chirality of the Ti atoms on the termination of the layer is indicated. Electroneutrality conditions impose that the Ti atoms at the surface of the cut are bonded to four CI atoms bridgebonded to further metal atoms and one CI atom not bridge-bonded, and that a coordination position is left free (dashed bonds). The two nonequivalent coordination positions, inward and outward, are indicated by i and 0 respectively (see text)

Of the set of crystal faces parallel to a-b of indices (hkl), only the 110 face of C(- TiCl 3 was considered explicitly and an inward coordination of the alkene was supposed by Arlman and Cossee. In these authors' opinion, the environment of the chlorine atoms would force the alkyl group of the alkene (coordinated with the double bond parallel to the Ti-ehain bond, in such a way as to give rise toa primary insertion) to protrude out of the crystal. For a given site, such a condition can be realized for only one of the two 'possible coordinations of the alkene. However, as we shall see, this does not seem to be relevant in the explanation of stereospecificity. The possibility that active sites be located on TiCl 3 layers protruding above the two adjacent ones was discussed by Allegra.P'' For the particular termination of the layer considered by Allegra, both octahedral sites of coordination at a Ti atom are equivalent for alkene complexation because the surface atoms with relevant non-bonded interactions at the catalytic site are locally related by a twofold axis (Figure 7). An argument in favour of such a situation was presented as follows. The Cossee reaction mechanism implies, as originally published, that at the end of each polymerization step the growing chain occupies the coordination site previously occupied by the alkene. Since, on the surface proposed by Cossee, inward and outward coordinations of the chain would prefer opposite

Figure 7 The particular TiCl 3 layer termination, proposed by Allegra/" for which the two coordination positions available for the polymerization reaction (indicated by arrows) are equivalent. In fact they are locally related by a two-fold axis (dashed line)

M onoalkene Polymerization: Stereospecificity

43

coordinations of the alkene,":' an isotactic polymerization would imply a back jump of the Ti-chain bond to the original position before the subsequent reaction step. The equivalence just discussed between both octahedral positions (as in the Allegra protruding layer) allows the back jumping of the Ti-chain bond to be considered an unnecessary step. However, quantum mechanical computations,78.79 as well as chemical intuition, indicate that the best coordination of the growing chain at the catalytic site in the absence of the alkene (as at the end of each polymerization step) should be intermediate between inward and outward positions, so that the equivalence between the inward and outward positions does not appear as a determining argument in favour of a particular catalytic site. Alternative octahedral sites on the basal or lateral surfaces of violet TiCl 3 crystals, presenting two vacancies accessible to the incoming monomer, were also proposed." 80 The main reason for these suggestions was, in the proponents' opinion, that in the Cossee model the access of the monomer to the only vacancy at the metal atom would be largely hindered by the growing chain. These model sites, besides the unlikelihood of two chlorine ligands being removed from the same metal atom, require the helicity of the chain as an element of chirality in order to account for the stereospecificity, which is definitely in contrast with many experimental observations. 1 The model proposed by Arlman and Cossee has been more recently examined in some detail by Corradini, Guerra et al. by calculating the non-bonded energies for all sets of possible internal coordinates of the atoms at the catalytic site."?:81 The main internal coordinates which were varied in the calculations are shown in Figure 8. They are the dihedral angle 00 to be associated with the rotation of the alkene around the axis connecting the metal to the center of the double bond and the internal rotation angles 1 , 2 , 3 , ••• , around the first bonds of the chain.

°°°

Figure 8 The main internal coordinates which were varied in the non-bonded energy calculations for the model catalytic sites by Corradini et al.6 7 , 8 1

According to the calculations, in agreement with the conclusions of Arlman and Cossee, if the catalytic sites are thought to reside on the bulk of lateral (110) Cl- TiCl 3 surfaces, the alkene would prefer to coordinate in the inward position and the steric hindrance readily explains the observed regiospecificity; however, no evidence for the preference of a specific alkene coordination (re or si) at a site of given chirality was found. On the other hand, due to the interaction with the surface, the orientational freedom of the first carbon-carbon bond of the growing chain appeared to be strongly restricted; .this 'chiral' orientation was suggested to result in different activation energies for the enantiomeric coordinated alkenes (re or si) in the successive insertion step. The stereospecificity could thus find an explanation. On the other hand, 'calculations of the non-bonded interactions for the bulk of possible lateral surfaces of y- TiCl 3 showed that the interaction energies are always much higher than those for the corresponding situations in et-TiCI3.81 Hence, if the catalytic sites are thought to reside on plain lateral surfaces, it seems rather difficult to explain the experimental observation that all layered modifications of TiCl 3 have a quite analogous stereospecificity irrespective of the particular stacking of the structural layers (cf Section 3.2). Instead, the computations suggested that, in agreement with previous hypotheses.v?: 82, 83 catalytic sites with similar behaviour for all layered modificiations of TiCl 3 can be located on some kind of terminal layers or on layers protruding above both neighbouring ones. These sites in relief involve

44

Transition-metal Initiated and Related Polymerizations

much lower steric hindrances than sites on plain lateral surfaces and their behaviour is unified by the same general model: lower steric repulsions are obtained for a coordination of the growing polymer chain in the more hindered (inward) position, which, in turn, causes a chiral orientation of the first C-C bond of the chain (for a A site, values for ()1 near +90 would be allowed while values near -90 would be forbidden) (Figure 9). This orientation was identified as an essential factor in determining the isospecificity: indeed, as shown in Figure 9, the alkyl group of the 1-alkene and the second carbon atom of the growing chain, for the si monomer coordination, are on opposite sides with respect to the plane defined by the Ti-C bonds, while for the re coordination they are on the same side. This would determine a lower activation energy and henceforth favour the insertion of the si with respect to the re coordinated alkene on a A site. The opposite, of course, holds for a L\ site. 0

0

A site

Figure 9 Perspective drawing of the two possible chiral coordinations of propene (si and re), suitable for a primary insertion, on a A site of the kind of Figure 6. In both cases the conformation 8 1 ~ 90° of the growing chain corresponds to an energy minimum

The analysis by 13CNMR techniques of the chain end groups resulting from the initiation steps on 13C-enriched Ti-alkyl bonds, performed some years later"? and already described in Section 3.2.2, pointed out that the chiral site is able to discriminate between si and re insertions of the monomer only in cases where alkyl groups with at least two C atoms are initially bound to the Ti atom, thus confirming the essential role of the chain in conferring the isospecific behaviour. Indeed specificcalculations, relative to the steric control in the first steps of propene polymerization/'" showed that for the model site proposed by Corradini et ale the insertion of the first monomer unit is nonstereospecific into an initial Ti-Me bond, partially stereospecific into an initial Ti-Et bond and totally stereospecific into an initial Ti-Bu i bond, in agreement with what observed experimentally. At least for single vacancy sites in relief with respect to lateral surfaces, calculations in the presence of a long polypropene chain 8 5 showed that this, though able to discriminate between the two faces of the prochiral alkene, does not hinder severely the access of the monomer to the vacancy, thus overcoming the previously discussed criticism to the single vacancy sites." As recently discussed by Corradini et al., catalytic sites in relief with respect to lateral surfaces of the layered crystals are also suitable to account for the similarity of the general behaviour of TiCI 3based and MgCl 2-supported catalysts.P" In fact, magnesium dichloride has crystal structures very similar to those of violet titanium trichlorides, built up of layers piled one on top of the other according to a close packing of the chlorine atoms, with the magnesium atoms occupying all the octahedral positions within each structural layer (Figure 10); the crystal lattice has also similar dimensions. This dictates the possibility of an epitactic coordination of TiCl 4 and, after reduction, TiCl 3 units on lateral, coordinatively unsaturated, faces of MgCl 2 crystals, giving rise to reliefs crystallographically coherent with the matrix. Possible lateral cuts of MgCl 2 which correspond to 100 and 110 faces are also shown in Figure 10. Both have been observed by optical and electron microscopy on MgCl 2 crystals.t" Electroneutrality conditions impose an average coordination number 5 for Mg atoms on 100 cuts, 4 for Mg atoms on 110 cuts. This causes differences in the surface coordination of single mononuclear TiCl 4 or TiCl 3 units. In any case, calculations suggested that the reduced TiCl 3 species formed after activation of these complexes by Al alkyl would behave as non-stereospecific sites/" ESR studies on MgCl 2-supported catalysts.t" on the other hand, showed that a significant fraction of Ti 3+ is ESR silent. This was explained by the formation of complexes with bridged Ti, e.g. through chlorine atoms as in crystalline TiCI 3. Bridged dinuclear Ti 2Cl 8 species can easily coordinate to the lateral cuts of MgC12 ;6 8 activation by the Al alkyl would change them into Ti 2Cl6 reliefs on the MgC12 support. The epitactic placement

M onoalkene Polymerization: Stereospecificity

45

+::J U

o

Q

(a) TiCI 3 layer

o

CI atoms up

( b) MgCI2 layer • CI atoms down

• metal atoms

Figure 10 Schematic drawing of structural layers of (a) violet TiCl 3 and (b) MgCl z . Possible lateral cuts of the layers are indicated. The bonds of the metal atoms belonging to the lateral cuts are also drawn to point out the different coordination numbers

of a Ti 2Cl 6 unit on the 100 face of MgCl 2 deserves special attention, since it results in reliefs (Figure l lb) very similar to those proposed'" as active sites on lateral 110 cuts of TiCl 3 crystals (Figure lla). It is seen from Figure 11 that the environment of the Ti atoms is chiral. Extensive calculations of non-bonded interactions showed that these sites can have stereo regulating ability, the main factor determining their stereospecific behaviour being the fixed, chiral orientation into which the growing polymer chain appears to be Iorced/" As reported in Section 3.2, in the absence of Lewis bases, the stereospecificity of MgCl 2-supported catalysts is low. This has been explained assuming that, upon coordination of TiCl 4 to MgCI 2 , mononuclear and dinuclear species are formed (as suggested by ESR 87) both at 100 and 110 faces, only dinuclear species on the 100 faces leading to the formation of stereospecific sites/" Possible roles of the Lewis bases in increasing catalyst specificity have been discussed in Section 3.2. The main general drawback of catalytic sites in relief with respect to the lateral surfaces is that no steric hindrance is present in the coordination step which can account for the observed regiospecificity.88 The origin of regiospecificity, however, could be traced back to electronic factors''? and, at least in part, to non-bonded interactions in the insertion step.88 The Cossee mechanism of polymerization, when completed with the chiral orientation of the growing polymer chain, is also able to account for the stereoselective behaviour of the isospecific heterogeneous catalysts."? In particular, the model proved able to explain the experimental results relative to the first insertion of a chiral alkene into an initial Ti-methyl bond.t' i.e. the absence of enantioselectivity (discrimination between si and re insertions) and the presence of diastereoselectivity (preference for S (R) enantiomer upon si (re) insertion). Upon si (re) coordination of the two

l

b)

Figure 11 Detailed view of (a) a Ti zCl6 group in reliefwith respect to the 110 cut of TrCl , and (b) a TijCl, group epitactically placed on a 100 cut of MgCl z . Chiral environments of metal atoms are explicitly labelled

46

Transition-metal Initiated and Related Polymerizations

enantiomers of 3-methyl-l-pentene to the proposed catalytic model, it was calculated that energy minima only occur when the conformation relative to the single C-C bond adjacent to the double bond, referred to the hydrogen atom bonded to the tertiary carbon atom, is nearly anticlinal minus, A - (anticlinal plus, A "), Thus one can postulate the reactivity only of the A- conformations upon si coordination, and of the A + conformations upon re coordination (Figure 12). In other words, upon si coordination, only the Z synperiplanar methyl conformation would be accessible to the S enantiomer and only the (less populated) Z synperiplanar ethyl conformation to the R enantiomer; this would favour the si attack of the S enantiomer with respect to the same attack of the R enantiomer, independently of the chirality of the catalytic site. This result is in agreement with a previous hypothesis of Zambelli and co-workers, based only on the experimental reactivity ratios of the different faces of C-3 branched l-alkenes.t'' It is also worthy to note that, although the mechanism for stereospecificity of the chiral orientation of the chain was developed in the hypothesis of a (J bond between the metal and the first carbon atom of the chain, it would remain valid in the hypothesis of carbenic intermediates.I"

R c

o

+=o

H

H

.s 'U

o

o

u

~

Et

Me

H

c o

+=o

.s

oo

'U

u

Vi

Figure 12 Newman projections of the energy minimum conformations for (R)-3-methyl-l-pentene and (S)-3-methyll-pentene upon coordination on a site of the kind of Figure 6. The energy minimum conformations for the si coordination are shown on the right while those for the re coordination on the left

3.3.3 Homogeneous Syndiospecific Catalysis The problem of the origin of stereoregulation in the syndiospecific polymerization of propene in the presence of homogeneous V-based catalyst systems has been relatively little investigated up to now. As previously discussed (cf Section 3.2.3), there are experimental evidences that the polymerization is not completely regiospecific and that the only syndiospecific step is propene insertion into a metal-secondary carbon bond with formation of a new secondary metal-carbon bond. A chain end stereocontrol is also well established. All the proposed models for syndiotactic propagation suppose that the active center is a vanadium-carbon bond and that the monomer first coordinates to the metal; moreover, all of them attribute the stereospecificity to steric factors. However, different driving forces for the syndiospecificity have been proposed. According to an early model," 3 there are two adjacent accessible positions at the catalytic site, each favouring the coordination of the prochiral monomer with one of its two faces; if the growing polymer chain alternates between the two positions at each insertion step, syndiotactic propagation is ensured. An analogous model had been proposed for the ring-opening polymerization of norbornene derivatives by ReCI 5, where the propagation species is a metallacarbene complex.'" Due to the successive finding of a chain end stereocontrol, this model has to be rejected. Most authors" 93,50 favoured the hypothesis that syndiotacticity arises from steric repulsions between the methyl group of the complexed propene and the last inserted monomer unit of the growing chain. One of these models'" is no more suitable, since it assumed a primary monomer insertion. It was suggested that such steric repulsions could occur in an activated fourcenter complex, resulting from propene secondary insertion into a metal-secondary carbon bond (Figure 13),2,95 though this model has not been subjected to quantitative examination.

M onoalkene Polymerization: Stereospecificity

.,

47

Me

H

/:

M·

\:....

C··

Me

/

C .>

CH

/

;

M···········C .... ,. Me \......

H

/

2

"' ....

........p

.... , :'

H

C~CH2

H

Me

(0)

( b)

Figure 13 Supposed four-center activated complexes for secondary insertion of a propene molecule (reproduced by permission of Springer-Verlag from Adv. Polym. Sci., 1974,15,31)

A detailed catalytic model, with a pentacoordinated metal (V) atom was proposed by Zambelli and Allegra.?" able to account for several experimental observations concerning the homo- and copolymerization of propene and other l-alkenes in the presence of the syndiospecific catalyst systems under consideration. However, the authors pointed out that the steric repulsions between the methyl group of a coordinated propene molecule and the last inserted propene unit of the growing chain do not generate energy differences between diastereoisomeric situations in the hypothesized catalytic complex, so that steric interferences with the growing chain, during monomer approach to the catalyst, were suggested as the source of the syndiospecific control. More recently a further model of the origin of the syndiospecificity has been advanced/'? consisting of a hexacoordinated metal (V) atom surrounded by four chlorine atoms assumed to be bridge bonded to other metal (i.e. AI) atoms (Figure 14).The catalytic site is chiral and analogous to that proposed for the isospecific heterogeneous catalysts by the same research group.P" but, unlike the heterogeneous model site, interconversion between enantiomeric complexes is assumed to be possible, after each insertion step, when the metal atom is pentacoordinated. The analysis of the nonbonded interactions at the catalytic site showed that a si insertion of the last monomer unit (which generates what was called asi chain) favours the formation of the A complex, which in turn favours the re coordination and insertion of the successive monomer unit, this assuring the syndiospecific propagation of the chain. In summary, the chirality of the growing chain (e.g. the configuration of the last added unit) influences the chirality of the coordinated monomer (hence the configuration of the successive inserted unit) by influencing directly the chirality of the metal atom. None of the last three models can be discarded on the basis of the available experimental data. si chain

E

E+b.E

Figure 14 Possible diastereoisomeric catalytic complexes for the homogeneous syndiospecific catalysts, with a si chain, presenting suitable orientation of the chain and of the alkene. The corresponding minimum energies are indicated below the models

3.3.4 Homogeneous Isospecific Catalysis As already discussed (cf Section 3.2.4),a class of homogeneous isospecific Ziegler-Natta catalysts has recently been disclosed, based on titanocenes and zirconocenes in combination with alkylPS 4--C

48

Transition-metal Initiated and Related Polymerizations

alumoxanes. The stereocontrol has been attributed in some cases to the chirality of the catalytic complex and in some other cases to the chirality of the last inserted monomer unit in the growing chain, depending on the ligands of the metal. A possible model for the chiral site-controlled catalysis has already been proposed.?" It refers to stereorigid chiral complexes, with an ethylenebis(l-indenyl) ligand (Figure 15). The coordination to the metal of three more ligands in addition to the bis(indenyl) ligand was regarded as unlikely on the basis of calculations of non-bonded interactions. In consequence, it was assumed that the catalytic complex presents only two ligands (coordinated monomer and growing chain) besides the indenyl groups in the stage preceding monomer insertion. This is possible for instance if the active catalyst has an ion pair nature, in analogy to the cationic character of the catalytic species assumed for similar systems: 96-99 [bis(indenylligand)Ti(propene)(Pn )] + [R(AIMeO)x]-

-+

[bis(indenylligand)TiPn +1]+ [R(AIMeO)x]-

Figure 15 A possible model site for the homogeneous isospecific catalysis: a sterically rigid chiral complex, in which, besides the monomer and the growing chain, an ethylenebis(4,5,6,7-tetrahydro-l-indenyl) ligand is present. The broken lines indicate the conformation of the growing chain which is forbidden, and the corresponding favoured coordination of the monomer

The calculations indicated that the chiral environment of the metal atom forces the growing chain to a chiral orientation (for instance, in Figure 15 the orientation of the chain indicated with full lines is allowed, that indicated with broken lines is forbidden). This in turn favours the coordination of the prochiral monomer with one of its two faces (for the sketched complex, the re coordination would be favoured). The similarity between this model and that for the heterogeneous isospecific catalysts proposed by the same research group'" is pointed out by a comparison between Figure 9 and Figure 15. In both cases the chiral environment of the metal atom forces the growing chain to assume a chiral orientation, which in turn discriminates between si and re coordinated alkenes. The predicted behaviour of this model catalytic site seems to be in agreement with the available experimental data. In particular, the model is nonstereospecific for the monomer insertion into a metal-methyl bond (cf Section 3.2.4). Moreover, the model is in agreement with the elegant results of the analysis and optical activity measurements on the saturated propene oligomers obtained, under suitable conditions, with this kind of catalysts, for which the re insertion of the monomer has been verified to be favoured in the case of the (R,R) chirality of coordination of the bis(indenyl) ligand.'?" At the moment, no model has yet been proposed for the isospecific homogeneous Ziegler-Natta catalysts with a chain end stereocontrol.

3.4 REFERENCES 1. 1. Boor, Jr., 'Ziegler-Natta Catalysts and Polymerizations', Academic Press, New York, 1979. 2. A. Zambelli and C. Tosi, Fortschr. Hochpolym.-Fortsch., 1974, 15,31. 3. J. A. Ewen, J. Am. Chem. Soc., 1984,106,6355. 4. Y. V. Kissin, 'Isospecific Polymerization of Olefins', Springer-Verlag, New York, 1985. 5. G. Natta, M. Farina and M. Peraldo, Chim. Ind. (Milan), 1960,42,255. 6. T. Miyazawa and T. Ideguchi, J. Polym. Sci. Part B, 1963,1,389. 7. A. Zambelli, M. G. Giongo and G. Natta, Makromol. Chem., 1968,112, 183. 8. F. A. Bovey and G. V. D. Tiers, J. Polym. Sci., 1960, 44, 173.

M onoalkene Polymerization: Stereospecificity 9. 10. 11. 12. 13.

49

R. A. Sheldon, T. Fueno, T. Tsunetsugu and 1. Furukawa, J. Polym. Sci., Part B, 1965,3,23. Y. Doi and T. Asakuru, Makromol. Chern., 1975,176,507. K. Ziegler, Belg. Pat. 533362 (1953). G. Natta, P. Pino and G. Mazzanti, (issued to Montecatini), US Pat. 3715344 (1973) (Chern. Abstr., 1973, 78, 148866). G. Natta, P. Pino, P. Corradini, F. Danusso, E. Mantica, G. Mazzanti and G. Moraglio, J. Am. Chern. Soc., 1955, 77,1708. 14. G. Natta, P. Pino and G. Mazzanti, Chim. Ind. (Milan), 1955, 37, 927. 15. G. Natta and P. Corradini, Atti Accad. Naz. Lincei, Ct. Sci. Fis., Mat. Nat., Rend., 1955, 4 (Ser. 8), 73. 16. G. Natta, P. Pino and G. Mazzanti, Gazz. Chim. Ital., 1957,87,528. 17. G. Natta, P. Pino, G. Mazzanti and P. Longi, Gazz. Chim. Ital., 1957, 87, 549. 18. P. Pino and R. Muelhaupt, Angew. Chern., Int. Ed. Engl., 1980,19,857. 19. G. Natta, P. Corradini, I. W. Bassi and L. Porri, Atti. Accad. N az. Lincei, Cl. Sci. Fis., Mat. Nat., Rend., 1958,24 (Ser. 8), 121. 20. G. Natta, P. Pino and G. Mazzanti, (issued to Montecatini), Ital Pat. 526101 (1954). 21. E. Tornqvist, W. Seelbach and A. W. Langer, Jr., (issued to Esso Res. Eng. Co.), US Pat. 3032510 (1962) (Chern. Abstr., 1964, 60, 14630). 22. Mitsubishi Petrochem. Corp., Br. Pat. 1092390 (1966) (Chern. Abstr., 1968, 68, 13557). 23. R. L. McConnel, M. A. McCall, G. O. Cash, Jr., F. B. Joyner and H. W. Coover, Jr., J. Polym. sa.. Part A, 1965,3,2135. 24. 1. P. Hermans and P. Henrioulle, Ger. Pat. 2256789 (1973); J. P. Hermans and P. Henrioulle, (issued to Solvay), Ger. Pat 2213086 (1972) (Chern. Abstr., 1973, 78, 16791). 25. R. Colton and 1. H. Canterford, 'Halides of the First Row Transition Metals', Wiley, London, 1969. 26. P. Longi, U. Giannini and A. Cassata (issued to Montedison), Belg. Pat. 774600 (1971). 27. V. Busieo, P. Corradini, L. De Martino, A. Proto, V. Savino and E. Albizzati, Makromol. Chern., 1985, 186, 1279; V. Busieo, P. Corradini, L. De Martino, A. Proto and E. Albizzati, M akromol. Chern., 1986, 187, 1115. 28. U. Giannini, E. Albizzati and S. Parodi, (issued to Montedison) US Pat. 4277589 (1976)(Chern. Abstr., 1978,89,25087); G. Cecchin and E. Albizzati, (issued to Montedison), US Pat. 4294721 (1977) (Chern. Abstr., 1979,90,138426). 29. L. Luciani, N. Kashiwa, P. Barbe and A. Toyota, (issued to Montedison and Mitsui Petrochemical), US Pat. 4226741 (1975) (Chern. Abstr., 1977,87, 68893). 30. A. W. Langer, T. 1. Burkhardt and J. 1. Steger, Proceedings of the MMI International Symposium on 'Transition Metal Catalyzed Polymerization: Unsolved Problems', Midland, MI, 1981, Part A, p. 97. 31. U. Giannini, A. Cassata, P. Longi and R.Mazzocchi, (issued to Montedison), Belg. Pat. 785332 (1972) (Chern. Abstr., 1973, 98, 98293). 32. 1. C. W. Chien, 1. C. Wu and C. J. Kuo, J. Polym. Sci., Polym. Chem. Ed., 1982,20,2019; 1. C. W. Chien and J. C. Wu, J. Polym. Sci., Polym. Chem. Ed., 1982, 20, 2445, 2461; J. C. W. Chien, J. C. Wu and C. 1. Kuo, J. Polym. Sci., Po/ym. Chern. Ed., 1983, 21, 725. 33. A. Zambelli, L.Oliva and P. Ammendola, Gazz. Chim. ltal., 1986, 116,259. 34. A. Zambelli, P. Locatelli, M. C. Sacchi and I. Tritto, Macromolecules, 1982,15,831. 35. P. Longi, G. Mazzanti, A. Roggero and M. P. Lachi, M akromol. Chern., 1963, 61, 63. 36. Y. Takegami, T. Suzuki and T. Okazaki, Bull. Chern. Soc. Jpn., 1969,42, 1060. 37. A. Zambelli, P. Locatelli and E. Rigamonti, Macromolecules, 1979,12, 156. 38. A. Zambelli, G. Gatti, M. C. Sacchi, W. O. Crain, Jr. and 1. D. Roberts, Macromolecules, 1971,4,475; A. Zambelli, P. Locatelli, G. Zannoni and F. A. Bovey, Macromolecules, 1978, 11,923; A. Zambelli, G. Bajo and E. Rigamonti, M akromol. Chern., 1978, 179, 1249. 39. W. O. Crain, Jr., A. Zambelli and 1. D. Roberts, Macromolecules, 1971,4,330. 40. A. Zambelli, M. C. Sacchi, P. Locatelli and G. Zannoni, Macromolecules, 1982, 15,211. 41. P. Pino, Adv. Polym. Sci., 1965,4,363; P. Pino, F. Ciardelli and G. Montagnoli, J. Polym. Sci., Part C, 1968,6,3265; P. Pino, F. Ciardelli, G. P. Lorenzi and G. Natta, J. Am. Chern. Soc., 1962,84, 1487; P. Pino, G. Montagnoli and E. Benedetti, Makromol. Chern., 1966, 93, 158. 42. P. Pino, F. Ciardelli, G. Montagnoli and O. Pieroni, J. Polym. Sci., Part B, 1967, 5, 307; F. Ciardelli, E. Benedetti and O. Pieroni, Makromol. Chern., 1967,103, 1. 43. A. Zambelli, P. Ammendola, M. C. Sacchi, P. Locatelli and G. Zannoni, Macromolecules, 1983, 16, 341; A. Zambelli, P. Ammendola, P. Longo and A. Grassi, Gazz. Chim. Ital., 1987, 117,579. 44. F. Ciardelli, P. Locatelli, M. Marchetti and A. Zambelli, Makromol. Chern., 1974,175,923; P. Pino, F. Ciardelli and G. P. Lorenzi, J. Polym. Sci., Part C, 1964,4,21. 45. G. Natta, I. Pasquon, P. Corradini, M. Peraldo, M. Pegoraro and A. Zambelli, Atti. Accad. Naz. Lincei. Cl. Sci. Fis., Mat. Nat., Rend., 1960,28 (Ser. 8),452; G. Natta, P. Corradini, I. Pasquon, U. Pegoraro and M. Peraldo, (issued to Montecatini), US Pat. 3258455 (1966); E. J. Addink and J. Beintema, Polymer, 1961,2, 185. 46. Y. Doi, Makrornol. Chem., Rapid Commun., 1982,3,635. 47. G. Natta, I. Pasquon and A. Zambelli, J. Am. Chern. Soc., 1962, 84, 1488. 48. A. Zambelli, G. Natta and I. Pasquon, J. Polym. Sci., Part C, 1963,4,411. 49. A. Zambelli, I. Pasquon, R. Signorini and G. Natta, Makromol. Chern., 1968,112, 160. 50. Y. Takegami and T. Suzuki, Bull. Chern. Soc. Jpn., 1969,42,848; 1970,43, 1484. 51. Y. Doi, F. Nozawa, M. Murata, S. Suzuki and K. Soga, Makromol. Chern., 1985,186, 1825. 52. A. Zambelli, C. Tosi and M. C. Sacchi, Macromolecules, 1972,5,649. 53. Cf. ref. 2; primary propene addition to a growing chain ending with an ethylene unit is slightly favoured. 54. A. Zambelli, P. Locatelli, A. Provasoli and D. R. Ferro, Macromolecules, 1980,13,267. 55. Y. Doi, Macromolecules, 1979, 12, 248. 56. N. Ishihara, T. Seimija, M. Kuramoto and M. Uoi, Macromolecules, 1986, 19,2465. 57. P. Ammendola, C. Pellechia, P. Longo and A. Zambelli, Gazz. Chim. Ital., 1987, 117,65. 58. U. Giannini, U. Zucchini and E. Albizzati, J. Polym. Sci. Polym. Lett. Ed., 1970, 8, 405. 59. H. Sinn and W. Kaminski, Adv. Organomet. Chern., 1980, 18, 99. 60. A. Zambelli, P. Ammendola, A. Grassi, P. Longo and A. Proto, Macromolecules, 1986, 19, 2703. 61. P. Longo, A. Grassi, L. Pellecchia and A. Zambelli, Macromolecules, 1987,20, 1015.

50

Transition-metal Initiated and Related Polymerizations

62. W. Kaminski, S. Kuelper and S. Niedoba, Makromol. Chem. Macromol. Symp., 1986,3,377; W. Kaminski, Preprints of International Symposium on 'Transition Metal Catalyzed Polymerizations', Akron, 1986. 63. P. Corradini, G. Paiaro and A. Panunzi, J. Polym. Sci., Part C, 1967,16, 2905. 64. K. R. Hanson, J. Am. Chem. Soc., 1966,88,2731. 65. E. J. Arlman, J. Catal., 1964, 3, 89. 66. G. Allegra, Makromol. Chem., 1971, 145,235. 67. P. Corradini, V. Barone, R. Fusco and G. Guerra, Eur. Polym. J., 1979, 15, 133. 68. P. Corradini, V. Barone, R. Fusco and G. Guerra, Gazz. Chim. Ital., 1983,113, 601. 69. P. Corradini, G. Guerra and R. Pucciariello, Macromolecules, 1985,18,2030. 70. Nomenclature of Inorganic Chemistry, Pure Appl. Chem., 1971,28,77. 71. G. Natta, J. Inorg. Nucl. Chem., 1958, 8, 589. 72. E.1. Arlman and P. Cossee, J. Catal., 1964, 3, 99. 73. P. Cossee, in 'The Stereochemistry of Macromolecules', ed. A. D. Ketley, Marcel Dekker, New York, 1967, vol. 1, chap. 3. 74. G. Natta, P. Corradini, I. W. Bassi and L. Porri, Atti. Accad. Naz. Lincei, Cl. Sci. Fis., Mat. Nat., Rend., 1958,24 (Ser. 8), 121. 75. G. Natta, P. Corradini and G. Allegra, J. Polym. Sci., 1961, 51, 399. 76. G. Allegra, Nuovo Cimento, 1962, 23,502. 77. B. Hargitay, L. Rodriguez and M. Miotto, J. Polym. Sci., 1959, 35,599. 78. D. R. Armstrong, P. G. Perkins and 1. J. P. Stewart, J. Chem. Soc., Dalton Trans., 1972, 1972. 79. G. Giunchi, E. Clementi, M. E. Ruiz-Vizcaya and O. Novaro, Chem. Phys. Lett., 1977,49,8. 80. Yu. V. Kissin and N. M. Chirkov, Eur. Polym. J., 1970, 6, 525. 81. P. Corradini, G. Guerra, R. Fusco and V. Barone, Eur. Polym. J., 1980,16,835. 82. L. A. Rodriguez and A. Gabant, J. Polym. Sci., Part C, 1964,4, 125. 83. J. Boor, Jr., J. Polym. Sci., Part C, 1963, 1, 257. 84. P. Corradini, V. Barone and G. Guerra, Macromolecules, 1982,15, 1242. 85. P. Corradini, V. Barone and G. Guerra, Eur. Polym. J., 1984, 20, 1177. 86. U. Giannini, G. Giunchi, E. Albizzati and P. C. Barbe, 'Frontiers in Polymerization Catalysis and Polymer Synthesis', NATO Advanced Research Work, Feb. 1-6, 1987, in press. 87. J. C. W. Chien and 1. C. Wu, J. Polym. Sci., Polyrn. Chern. Ed., 1982,20,2461. 88. P. Corradini, V. Barone, R. Fusco and G. Guerra, J. Catal., 1982,77,32. 89. J. 1. Eisch, 'Proceedings, International Symposium on Transition Metal Catalyzed Polymerization', Akron, 1986, in press. 90. P. Corradini, G. Guerra and V. Villani, Macromolecules, 1985, 18, 1401. 91. K. J. Ivin, J. J. Rooney, C. D. Steward, M. L. H. Green and R. Mahtab, J. Chem. Soc., Chern. Commun., 1978,604. 92. 1. G. Hamilton, K. 1. Ivin and 1. J. Rooney, J. Mol. Catal., 1985,28,255. 93. J. Boor, Jr. and E. A. Youngman, J. Polym. Sci., Part A-l, 1966,4, 1861. 94. A. Zambelli and G. Allegra, Macromolecules, 1980,13,42. 95. P. Corradini, G. Guerra, M. Vacatello and V. Villani, Gazz. Chim. Ital., in press. 96. F. S. Dyachkowsky, A. 1. Shilova and A. E. Shilov, J. Polym. Sci., Part C, 1967, 16, 2333. 97. 1. W. Lauher and R. Hoffman, J. Am. Chem. Soc., 1976, 98, 1729. 98. E. Giannetti, G. M. Nicoletti and R. Mazzocchi, J. Polym. Sci., Polyrn. Chern. Ed., 1985, 23, 2117. 99. 1. 1. Eisch, A. H. Piotrovsky, S. K. Brownstein, E. 1. Gabe and F. L. Lee, J. Am. Chem. Soc., 1985, 107, 7219. 100. P. Pino, P. Cioni.and 1. Wei, J. Am. Chem. Soc., 1987, 109, 6189.