Patai S., Rappoport Z. 1997 The chemistry of functional groups. The chemistry of double-bonded functional groups / 1497 161

			24. Advances in the metathesis of olefins			1537
TABLE 6. Systems giving fully HT-biased polymers
		Monomer	Catalyst	Temp. (°C)	ca	Reference
Me3 Si			W(DCPh2)(CO)5	40	1.0	295
Me			Mo(DCHCMe2Ph)(DNAr)(OCMe2CF3)2	20	1.0	296
		Pr
		Pr	Mo(DCHCMe2Ph)(DNAr)(OCMe3)2	20	0.0	297
		Me
		Me	Mo(DCHCMe3)(DNAr)(OCMe3)2	20	0.0	298
	O		Cr(DCPh2)(CO)5	40	0.5	299
		SiMe2	Mo(DCHCMe2Ph)(DNAr)[OCMe(CF3)2]2	25	0.03	300
	Si
	Me2
			W(DCPh2)(CO)5	50	0.24	301
Me
			W(DCPh2)(CO)5	40	¾0.4	295
Me
			ReCl5	20	1.0	302,303
	Me
			IrCl3	75	0.09	304
	Et
Me		Me
			Mo(DCHCMe2Ph)(DNAr)[OCMe(CF3)2]2	20	0.00	305
		Me
Me		Ph
			Mo(DCHCMe3)(DNAr)(OCMe3)2	20	0.00	306

a c is the fraction of cis double bonds in the polymer.

1538	K. J. Ivin

bias the propagation proceeds through the head species. The increase in HT bias with increasing dilution observed in some cases may be interpreted in terms of two distinct PH (or PT) species: an unrelaxed form of higher energy which is less H/T discriminating, and a relaxed form of lower energy which is more H/T discriminating.

	H	Me
PCH CH	Me	PCH CH	H
	H		H
	C		C
	[Mt]		[Mt]
	PH (69)		PT (70)

FIGURE 4. 62.8 MHz 13C NMR spectrum of an all-cis, all-HT polymer of 1-methylnorbornene. Catalyst: ReCl3025 ,303. Reproduced by permission of the Society of Chemical Industry

24. Advances in the metathesis of olefins

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FIGURE 5. 62.8 MHz 13C NMR spectrum (olefinic region) of a 48% cis polymer of 1-methyl- norbornene. Catalyst: Mo2(OAc)4/EtAlCl2. HT C TH/HH C TT D 2.8. The fine structure arises from double-bond triads, e.g. cct, ccc, tct, tcc302. Reproduced by permission of the Society of Chemical Industry

5. Tacticity

Tacticities have been determined for polymers of derivatives of cyclopentene, norbornene, norbornadiene and 7-oxanorbornene. Three methods have been used: (i) by polymerizing single enantiomers of 5- or 5,5- or endo,exo-5,6-substituted norbornene derivatives and determining whether the polymers have an HT (m) or HH/TT (r) structure, or both (m and r); (ii) by polymerizing 5,6-disubstituted norbornene derivatives in which the substituents contain a chiral centre of a single handedness and determining whether the olefinic protons in the polymer are coupled (m) or not (r); (iii) by polymerizing prochiral monomers and arguing by analogy with the results on closely related chiral monomers. The first method requires some assumptions about the magnitude of 13C NMR substitution parameters, but the second method is absolute. The third method should be safe if applied with caution.

Tacticity determinations by the first method have been carried out using enantiomers of the following derivatives of norborn-2-ene: 1-methyl-302,308, exo-5-methyl-309,310,

1540 K. J. Ivin

5,5-dimethyl-311,312, endo,exo-5,6-dimethyl-313,314, 1,7,7-trimethyl-305, endo-5-methoxy- methyl-315, endo-5-acetate316,317, endo,exo-5,6-dimethoxymethyl313, and endo,exo-5,6- dicarbomethoxy-313; and the following derivatives of 7-oxanorborn-2-ene: exo-5- methoxymethyl-318 and endo-5-methoxymethyl-319.

The extremely slow ROMP of 1,7,7-trimethylnorbornene is initiated by 8 (R D Ph) to give an all-trans, all-HT polymer which is necessarily isotactic when made from a single enantiomer. It is atactic when made from racemic monomer showing that the two enantiomers then add randomly to the growing chain305.

All resonances in the 13C NMR spectra of all-HT polymers of racemic 1- methylnorbornene are insensitive to tacticity, but C-6 in the spectrum of the hydrogenated polymer does show m/r splitting (0.05 ppm) when the precursor is made with OsCl3 as catalyst. The downfield component (39.12 ppm) is dominant when the experiment is repeated with partially resolved monomer and is therefore assigned to m dyads. The upfield component (39.07ppm) is the only peak observed in the spectrum of the hydrogenated all-cis, all-HT polymer made from racemic monomer with ReCl5 as catalyst, and it may be concluded that this is a syndiotactic alternating copolymer of the two enantiomers. This

structure is forced on the polymer by the steric exclusion of cis HH structures302,308; also see Couturier and coworkers101.

The second method for determining tacticities is essentially an extension of the first, the only difference being that the chiral centre is placed in the side chain(s) of a monomer that would otherwise be prochiral. Thus the chiral diesters 71 and 72 have been prepared and polymerized using several molybdenum carbene initiators. The protons H2 and H3 attached to C2 and C3 are now non-equivalent both in the monomer and polymer (73). In an m dyad the double bonds (cis or trans) will all be of the type C2H2DC3H3 so that if the chemical shift difference is not too small, the signal for H-2,3 will be an AB quartet, split further by coupling to the adjacent ring protons. On the other hand, in a fully syndiotactic polymer the double bonds in the r dyads will be alternately of the types C2H2DC2H2 and C3H3DC3H3 and there will be no coupling between H-2 and H-3. The 1H 1H correlation spectrum (COSY) of the all-trans polymer of 71 made with 74 as initiator, Figure 6(a), shows clearly that the olefinic protons of the trans double bond are not coupled (υ 5.568 and 5.534), and that the polymer is therefore fully syndiotactic. Note also that the couplings to the adjacent protons, H-1 and H-4, are not resolved. In the 13C NMR spectrum the C-1 and C-4 signals are just resolved (υ 46.7 and 46.8), while C-7, being always situated within an rr triad, gives a single peak at υ 37.5. For the all-cis polymer of 71 made with 75 as initiator the 13C NMR spectrum gives a single set of peaks as expected for a tactic polymer. Its COSY spectrum, Figure 6(b), shows that the H-2 and H-3 protons (υ 5.51 and 5.37) are coupled and that the dyads are therefore isotactic (m). In this case there is some observable coupling to H-1 and H-4 and on irradiation of these protons the signal collapses to the expected AB quartet with a coupling constant of 10 Hz characteristic of cis CDC.

	Me	O
CO2 R		O
CO2 R		Me Me
	CHMe2
(71, 72)	(71) R = (−)-menthyl (72) R = (−)-pantalactonyl

			24. Advances in the metathesis of olefins								1541
RCO2		CO2 R		RCO2		CO2 R
CH	•	•	CH	CH	•	•	CH	CH	•	•	CH
2	1	4	3	2	1	4	3	3	4	1	2
				m				r
						(73)		RCO2		CO2 R

FIGURE 6. 1H 1H COSY spectra of the olefinic protons H-2,3 for (a) syndiotactic trans polymer of 71 initiated by 74, and (b) isotactic cis polymer of 71 initiated by 75. Reprinted with permission from Ref. 313. Copyright (1994) American Chemical Society

The third method of determining tacticities may be illustrated by reference to the polymers of anti-7-methylnorbornene. The 13C NMR spectra of all-trans and high-cis polymers are shown in Figures 7 and 8. The first is clearly atactic, all carbons except C-7 being sensitive to tacticity. The assignments are based on a comparison with the spectrum of a polymer made with W(CO)3 (mesitylene)/EtAlCl2/exo-2,3-epoxynorbornane as catalyst and the assumption that, as in the polymer of 5,5-dimethylnorbornene made with this catalyst, the trans double bonds are always associated with m dyads. Likewise the high-cis polymer made with ReCl5 as catalyst is assumed to be syndiotactic as for the polymer of 5,5-dimethylnorbornene made with this catalyst. The spectrum of the hydrogenated atactic polymer shows larger m/r splittings than in the spectrum of its precursor, although C-7 remains insensitive to tacticity. The hydrogenated syndiotactic polymer gives essentially a single set of lines, as expected320.

Tacticities in polymers of prochiral monomers made with various catalysts have been determined in this way for the following: anti-7-methylnorbornene128,320 322, syn-7-methylnorbornene128,322, endo,endo-5,6-dimethylnorbornene323, spiro(norbornene- 7,10 -cyclopropane)324 and 5,6-bis(trifluoromethyl)norbornadiene325, and the hydrogenated polymers of 4-methylcyclopentene326 and exo, exo-5,6-dimethylnorbornene323.

1542	K. J. Ivin

		Me		Me
		SiMe2 Ph	N
Me2 CH	CHMe2	O	Mo	CHCMe2 Ph
	N	O

Mo CHCMe2 Ph

Me3 CO

Me3 CO

SiMe2 Ph

(74)	(75) (racemic)

FIGURE 7. 13C NMR spectrum of an all-trans polymer of anti-7-methylnorbornene. Catalyst: RuCl3 at 60 °C320. Reproduced by permission of Elsevier Science from Ref. 320

Three broad types of tacticity may be distinguished in polymers made by ROMP:

(i) fully tactic polymers which may be divided into the first four groups c/r, c/m, t/m, t/r listed in Table 7, and a fifth group in which the polymer has intermediate cis content but in which only c/r and t/m structures are found; (ii) completely atactic polymers, which may be of any cis content; and (iii) polymers of intermediate tacticity.

24. Advances in the metathesis of olefins

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FIGURE 8. 13C NMR spectrum of a 90%-cis polymer of anti-7-methylnorbornene. Catalyst: ReCl5. Reproduced by permission of Elsevier Science from Ref. 320

A given catalyst does not always result in polymer of a given type of tacticity: it can vary with the monomer, temperature and dilution. This is to be expected if second-order propagation processes are competing with first-order epimerization or relaxation processes involving a reorganization of the ligand geometry around the metal centre of the propagating species. The catalyst systems WCl6/R4Sn provide good examples of the way in which a change of conditions can give rise to a substantial change in tacticity. Thus the polymer of 5,5-DMNBE is fully tactic (c/r, t/m) when the cocatalyst is Bu4Sn at 20 °C, but if it is changed to Ph4Sn, or the temperature is raised to 100 °C, or the monomer changed to anti-7-MNBE, the polymer formed is atactic or nearly so311.

As in Ziegler Natta polymerization, steric control of the propagation step may involve either the interaction of the monomer with a chiral metal centre (enantiomorphic sites model), or the interaction of the monomer with the chiral centres in the repeating unit(s) adjacent to the metal centre (chain-end model). (The relationship to Ziegler Natta polymerization will be considered further in Section VIII.C.3.)

In the enantiomorphic sites model the propagating species may have leftor righthanded forms (Pl or Pr ), say with octahedral symmetry about the metal centre Mt, and one position vacant for the acceptance of monomer. Assuming that norbornene presents its less hindered exo face to the metal centre and that the MtDC and CDC double bonds approach each other in parallel alignment, it is readily shown that the formation of a cis double bond in the propagation step results in a metal carbene complex in which the chirality of the metal centre is opposite to that in the reacting complex27. This means that, provided that Pl and Pr retain their chiral identity between propagation steps, the

1544		K. J. Ivin
TABLE 7. Some limiting cases of tactic polymers made by ROMP
Monomera	Catalystb	cc		Predominant structure			Reference
			c/r	c/m	t/m	t/r
5,5-DMNBE	ReCl	1.0	p				312
	W-4 5	¾1.0	p				327
anti-7MNBE	ReCl5	¾	p
		¾1.0					320
(š)-1-MNBE	ReCl5	1.0	p				302
7-MNBD	OsCl3	0.97d	p				328
MOMONBE	W-1	0.97	p				319
endo,exo-				p
5,6-DMNBE	Mo-1	0.85					314
5,6-(CO2RŁ 2NBD	Mo-2	0.99		p	p		313
5,5-DMNBEe	W-2	0.15					329
MOMONBE	Ru-1	0.04			p		319
(C)-1-MNBE	OsCl3	0.0			p		302
( )-1,7,7-TMNBE	Mo-3	0.0			p		305
						p
5,6-(CO2RŁ 2NBD	Mo-4	0.06	p		p		313
anti-7-MNBE	W-2	0.45					320
5,5-DMNBE	W-3	0.61	p		p		311

aMNBE, methylnorbornene; DMNBE, dimethylnorbornene; TMNBE, trimethylnorbornene; MNBD, methylnorbornadiene; MOMONBD, endo-5-methoxymethyl-7-oxanorborn-2-ene; (CO2RŁ )2NBD, dicarboalkoxynorbornadiene [RŁ D-menthyl].

bW-1, W(DCHCMe3)(DNAr)[OCMe(CF3)2]2 (Ar D 2,6-diisopropylphenyl); W-2, W(CO)3 (mesitylene)/EtAlCl2/ exo-2,3-epoxynorbornane; W-3, WCl6/Bu4Sn/20 °C; W-4, W(DNC6H3-Me2-2,6)(Cl)3(OArO)(OEt2)/Et2AlCl; Mo-1, Mo(DCHCMe3)(DNAr)[OCMe(CF3)2]2; Mo-2, Mo(DCHCMe2Ph)(DNAr)[OC(CF3)3]2; Mo-3,

Mo(DCHCMe2Ph)(DNAr)[OCMe(CF3)2]2; Mo-4, Mo(DCHCMe2Ph)(DNAr)[OCMe3]2; Ru-1, [RuCl( -Cl) ( 3: 3-

C10H16)]2 (C10H16 D 2, 7-dimethyloctadienediyl). c Fraction of double bonds with cis configuration.

dMainly anti repeating units. e[M] D 0.4 M.

enchained rings in an all-cis polymer will have alternating configurations, i.e. the polymer will be syndiotactic (c/r). At the other extreme, an all-trans polymer will be isotactic (t/m) and, if Pl, and Pr propagate independently, the result will be a racemic mixture of polymer molecules.

When the propagating metal carbene complex does not have a predetermined vacant ligand position, but is instead trigonal bipyramidal or tetrahedral, it may still behave like the octahedral model provided that the ligands other than the carbene offer an asymmetric environment which controls the direction of approach of the monomer. If this is not the case there will not be a favoured direction of approach unless the chain-end effect comes into play.

Barriers to rotation about MtDC have been measured by observation of NMR coalescence temperatures123,330,331. In some cases these are sufficiently high that epimerization by rotation about MtDC is unlikely to be important, but in other cases such a process may be as fast as or faster than the propagation step. More detailed considerations show that when both cis and trans double bonds are formed in accordance with the enantiomorphic sites model then the cis junctions will always be associated with r dyads, and trans junctions with m dyads27,332. This model thus correctly predicts the observed tacticities in the first, third and fifth groups of results listed in Table 7. Cases of intermediate tacticity can also be interpreted in terms of this model if it is modified to include partial epimerization of Pl and Pr between propagation steps.

This leaves the second (c/m) and fourth (t/r) groups in Table 7 to be explained in a different way. In these cases it must be the stereochemistry of the polymer chain itself that

24. Advances in the metathesis of olefins

1545

is controlling the mode of addition of the next monomer unit. We have already seen that in the formation of high-cis polymers of norbornene the c/t selectivity depends not only on the configuration of the previously formed double bond but, when this is trans, also on the one before that. It is therefore very reasonable to expect that in high-cis polymers made using carbene initiators with symmetrical tetrahedral ligand geometry, any m selectivity must arise through the influence of the configuration of the previously added monomer unit or units. While one cannot argue in the same way for high-trans polymers, where the c/t selectivity is generally independent of the configuration of the previously formed double bond, nevertheless the m/r selectivity with such initiators may still be sensitive to the configuration of the previously added unit, so accounting for the comparatively rare case of predominantly t/r structure (Table 7).

B. Monocyclic Alkenes

In this section are summarized some of the recent findings for monocyclic alkenes of various ring sizes.

1. Four-membered rings

The ROMP of cyclobutene initiated by 7W (R D Me) gives a polymer with a relatively broad MWD (Mw/Mn > 2). This is because propagation is much faster than initiation and only a very small fraction of initiator is consumed (kp/ki ca 1000 at 60 °C). If PMe3 is added to the system an equilibrium is established between free PMe3 and PMe3 bound to both initiating and propagating species. However, the binding to the propagating species at 25 °C (K ca 105 M 1) is much stronger than to the initiating species (K ca 500 M 1) because it is sterically less hindered; and in the presence of sufficient PMe3 (10 equiv) the propagation reaction is slowed down relative to initiation to such an extent that all the initiator is consumed and the MWD becomes very narrow (Mw/Mn D 1.03). The reaction then shows well-defined kinetics and all the characteristics of an ideal living system. Similar results are obtained when the reaction is initiated with the corresponding molybdenum carbene complex 7. The polymers may be hydrogenated to give very well defined samples of polyethene333,334.

Of the cyclobutene derivatives 76 87 all except 87 undergo clean ROMP. The polymer of 87, obtained in low yield using WCl6/Me4Sn as catalyst, is a black insoluble powder of reduced chlorine content, caused by loss of HCl and development of conjugation335.

Polymers of 76 generally have a high proportion of cis double bonds but with varying degrees of HT bias296,336. The initiator Mo(DCHCMe2Ph) (DNC6H3-i-Pr2-2,6) (OCMe2CF3)2 is the only one to give an all-cis, all-HT, polymer, identical with natural rubber, cis-1,4-polyisoprene. This remarkable result clearly stems from a fine balance between the electrophilicity of the metal centre, and the steric interactions during the approach of the monomer to the initiator. Very little of the initiator is consumed and the propagating species cannot be detected by 1H NMR, indicating a high value of kp/ki. The yield of polymer is 78% (MW ca 20,000), but the MWD is inevitably broad (Mw/Mn D 2.5). Propagation can be slowed down relative to initiation by addition of 10 equiv of PPh2Me to the initiator. About half the initiator is then consumed during

polymerization and the carbene	proton in the propagating species can be detected as
a triplet, corresponding to the	tail species [Mo]DCHCH2CH2C(Me)DCHPn (υ 13.2,
J D 6.3 Hz)296.

The behaviour of 3-methylcyclobutene (78) with various catalysts resembles that of cyclobutene. With 7 as initiator the propagating species gives a 1H NMR spectrum

1546	K. J. Ivin

R

		Me		R	R	R
				R
(76)	R = Me	(78)	(79)	R = Me	(81)	R = CHMe2
(77)	R = SiMe3		(80)	R = Pr	(82)	R = CH2 OMe
					(83)	R = CH2 OCH2 Ph

RCO2	CO2 R Cl	Cl
(84)	R = Me	(87)
(85)	R = Et
(86)	R = CH2 Ph

containing two carbene proton doublets (υ 11.95, J D 6.6 Hz; υ 11.50, J D 9.5 Hz), assigned to the anti and syn rotamers of the head species [Mo]DCHCHMeCH2CHDCHP in which the polymer chain P0 in [Mo]DCHP0 points away from (anti) or towards (syn) the (DNAr) ligand. This polymer contains 84% cis double bonds and the methyl substituents are randomly oriented with respect to both cis and trans double bonds298. In contrast 3,3-dimethylcyclobutene (79) with the same initiator gives the ‘tail’ propagating species [Mo]DCHCH2CMe2CHDCHP, characterized by a carbene proton triplet (υ 8.53, J D 5.9 Hz), and an all-trans, all-HT polymer298. Likewise the ROMP of 3,3- dipropylcyclobutene (80) initiated by 74 also gives an all-trans, all-HT polymer. The 13C NMR spectrum of the hydrogenated polymer contains four signals from methylene carbons confirming that it has an all-HT structure. On the other hand, the initiator 8 (R D Ph) gives a largely cis HT polymer, and catalysts such as WCl6/Et3Al give polymers which are neither stereospecific nor regiospecific297.

The monomers 82 86 all give living polymers when initiated by 7 and related complexes. The ROMP of 85 shows well behaved second-order kinetics in C6D6 at 25 °C. The rate constant is about 10 times lower than for the reaction of 5,6- dicarbomethoxynorbornadiene, no doubt due to the greater deactivating effect of the COOR groups in 85. Polymers of 84 and 85 have 45 93% cis content depending on

the initiator337.

The ROMP of 88 can be effected using [Ti(DCH2)Cp2] sources as catalyst338, also with WOCl4/EtAlCl2 in chlorobenzene at 78 °C, admitting the monomer as a gas339 (equation 50). The polymer forms as a transparent highly soluble material that becomes conductive (10 3 ohm 1 cm 1) when doped with iodine. The undoped polymer shows a single absorption peak at 278 nm (ε D 20, 000 M 1 cm 1 per triene unit), the position of which shows that the -system of the polymer chain is segregated into triene segments. This is the result of steric interactions that force the polymer backbone into a non-planar

conformation.

For work on 77 and 81 see Katz and Shippey340 and Brunthaler and coworkers335, respectively.