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

1567

content. Detailed tacticity studies have been made on the 5,5-dimethyl compound (see Section VIIIA.5). The high-cis syndiotactic polymer has a higher Tg (106 °C) than the atactic polymer (47 °C)498.

12. 5,6-Disubstituted norbornenes

Monomers in this category can be divided into three groups: (i) those in which the two substituents are the same: some recently studied systems are listed in Table 9; (ii) those in which the substituents are different; and (iii) those in which the substituents form part of a ring system.

TABLE 9. ROMP of 5,6-disubstituted norbornenes (both substituents the same)

R		Isomersa		Catalyst	Reference
				typeb
CH3	xx		nn	WC	307
	xx	xn	nn	A,B	323
			nn	WC	113, 114
	xx	(C)xn		WC	442
				MoC	313,314
COOMe			nn	MoC	500
			nn	MoC	341
			nn	WC	467
	xx	xn		MoC	485
	xx	xn	nn	WC	127
		(C)xn		MoC	313
COOEt		xn		B	468
COO(CH2)12H	xx			B	501
COOCH2(CF2)6F	xx			B	501
COOSiMe3		xn		MoC	502
COO(CH2)nOC6H4C6H4CNc		xn		MoC	503
COOCHMeCH2(sty)nBud		xn		MoC	504
COO(pte)e		xn		MoC	505
OCOMe	xx			MoC	485, 506
			nn	MoC	407
O(CO)OMe	xx			MoC	507,508
O(CS)SMe	xx			MoC	507,508
CH2OMe		xn		MoC	483
		xn		MoC	502
		(C)xn		MoC	313
CH2SMe	xxf	xn		MoC	483
CH2Cl			nn	A	509
(CF2)nF (n D 4,6,8)					491
CH2NHCMe3		xn		MoC	510
CH2NHSiMe3		xn		MoC	510
PPh3		xn		MoC	511

axx, xn, nn denote exo,exo-, exo,endo-, endo,endo- respectively; (C) denotes enantiomer.

bA: TiCl4-, MoCl5-, WCl6- or ReCl5-based, or similar; B: Ru-, Osor Ir-based; MtC: metal carbene complexes (MoDC or WDC).

c n D 2 12; both monomer and polymer show liquid-crystalline phases.

dsty D styrene, n D 4 9; gives well-defined comb graft copolymers, containing an average of 4, 7 or 9 styrene units in the side chains.

epte D phenothiazin-10-ylethyl.

fMonomer assumed to be mainly xx.

1568 K. J. Ivin

The metal carbene complex initiators generally give living polymerizations. The carbene proton doublet from the first propagating carbene complex is often resolved (about 0.02 ppm downfield) from that of the longer-chain species, e.g. for R D COOMe. The intermediate transoid metallacyclobutane species can sometimes be detected at low temperature, but not the corresponding cisoid species. With R D Me (endo,endo) the first transoid metallacyclobutane complex formed by reaction with 10 C GaBr3 is particularly stable at 38 °C. It can be produced in 70% yield and its rearrangement followed at higher temperatures. Diastereoisomers (precursors of m and r dyads) in the subsequently formed metallacyclobutanes can also be distinguished113,114.

Absolute tacticities have been determined for polymers of exo,endo-monomers with R D Me, COOMe and CH2OMe, using single enantiomers313. Depending on the initiator one can obtain a high-trans atactic polymer or a high-cis isotactic polymer. Polymers of racemic exo,endo-monomers and of the prochiral exo,exo- and endo,endo-monomers sometimes show m/r splittings in the 13C NMR spectra of both their high-trans and high-cis polymers and more clearly in the spectra of their fully hydrogenated products323.

The reactions of the monomers with R D COOMe proceed with measurable speed in CD2Cl2 at 25 °C when initiated with a tungsten cyclopentylidene complex. For the endo,endo-isomer the initiation and propagation rate constants are about the same: 5.0 ð

10 3 M 1 s 1127 . The monomer with R D COOSiMe3 has been used to make amphiphilic star-block copolymers in which the trimethylsilyl esters have been converted to carboxylic acids. The monomer with R D COO(pte) has been used to make block copolymers with norbornene, end-capped by reaction with 1-pyrenecarboxaldehyde. Fluorescence emission from the pyrene end-groups is quenched by electron transfer from the phenothiazine group to the excited singlet state of the pyrene group, to an extent which depends on the structure of the copolymer, in particular on the closeness of the two groups in the chain. The polymer made from the monomer with R D OCOMe (exo,exo) is a white powder that can be cast from toluene as a flexible transparent film. On heating at 300 °C the film becomes red-black and insoluble, two equivalents of acetic acid being lost. Similar behaviour is observed with related polymers407,506.

The polymers made from the monomers with R D CH2OMe or CH2SMe form chelation complexes with Zn or Cd compounds; likewise for R D PPh3, with Au or Ag compounds; and polymers can be made from monomers in which R D CH2NHCMe3 or CH2NHSiMe3 are already chelated to Sn or Pb compounds. Such monomers (M1) can be used in living systems to make diblock copolymers with methyltetracyclododecene or norbornene (M2) in which the morphology (lamellar, cylindrical or spherical) can be readily revealed by transmission electron microscopy. The metal component in the M1 blocks can be reacted in various ways without too much effect on the morphology. Thus, Pb and Zn components can be converted to the sulphides by exposure to H2S, while Ag and Au components can be decomposed to the metal at 150 °C resulting in small clusters of metal atoms which reside largely within the original microdomains (20 100 A˚ for Ag, 15 40 A˚ for Au). Further development along these lines can be expected to produce interesting new materials, consisting as they do of conducting or semi-conducting nanoclusters within a non-conducting matrix510 512.

An example of the second group of monomers is trans-5-carbomethoxy-6- ferrocenylnorborn-2-ene. Fluorescence quenching studies have been made on its polymers505,513.

Recent examples of the third group of monomers include compounds 175514,

176409,515, 177, 178409, 179515, 180409, 181516, 182517, 183517, 184518, 185519, 186520,521, 187522 525, 188526, 189517, 190125,501, 191485, 192526, 193527, 194528, 195485,529,530,

196530, 197485,531, 198532. The dicyclopentadienes are discussed in Section VIII.C.16.

	24. Advances in the metathesis of olefins			1569
				X
	Cl			X
	Cl
(175)	(176)	(177)	X = CF3
		(178)	X = CO2 Me
		(179)	X = CN

OMe

MeO

(180)

(181) (endo and exo)

(182)

(183)

(184)

CO2 Me

Me

Me

(185) (main isomer)

(186)

(187)

The ROMPs of 177 and 178 proceed through the opening of the norbornene ring and not the sterically hindered cyclobutene ring. The same is probably true of 179 which gives an insoluble polymer; also of 180.

In much of the early work the monomers used were mixtures of isomers. For example, endo,anti-175 as prepared contains minor amounts of the exo,syn-isomer, a smaller amount of the endo,syn-isomer, but no exo,anti-isomer. Again, 185 is the main isomer (66%) in a mixture with seven others of which only one is present in a significant amount (33%). In general the endo-isomers are less reactive than the exo-isomers, no doubt because of the greater degree of steric hindrance when the endo-isomer approaches the propagating complex. However, endo-isomers that are unreactive with A- or B-type catalysts, as defined in Table 9, have sometimes been found to react slowly when placed in contact with a metal carbene initiator. For example, when 3.2 equiv of exo-190, containing some

1570	K. J. Ivin

of the endo-isomer, are mixed with W[DC(CH2)3CH2](OCH2CMe3)2Br2 in CD2Cl2 at 25 °C, the exo-monomer reacts first to give the propagating species P1 which then adds further exo-monomer to give Pn (n > 1). P1 and Pn (n > 1) are distinguished by their (OCH2CMe3 2 NMR signals at υ 4.44, 4.39 and 4.46, 4.41 respectively, the two neopentoxy ligands being non-equivalent in each case. The concentration of P1 passes through a maximum after about 10 min and then declines as it is replaced by Pn. The ratio of concentrations of P1 and initiator at this maximum gives the ratio of initiation to propagation rate constants, ki/kp, as 3.8. Once the initiator has all been consumed, the concentration of propagating species is constant and the remaining monomer disappears with a half-life of 74 min, corresponding to kp D 3.15 ð 10 3 M 1 s 1. The carbene proton signals for P1 and Pn are not resolved; both give a doublet at υ 11.245. Towards the end of the reaction, when the exo-monomer peaks have practically disappeared, a second carbene proton doublet appears in very low intensity at υ 11.59 and other weak peaks appear in the upfield part of the spectrum. On addition of pure endo-isomer to the reaction mixture, in amount similar to that of the original exo-isomer, the weak doublet at υ 11.59 grows at the expense of the exo-Pn doublet at υ 11.245, until after 3 h it represents 70% of the tungsten carbene proton signal. The fall in concentration of the endo-monomer is also several times that of the new carbene proton species, indicating that more than one molecule of

Cl

O

O

Cl O

(188)

(189)

O

O

Me

O

O

(190) (endo and exo)

O

O

O

(192) (endo)

O

Me

(191) (exo)

O

NR

O

(193-198)

(endo and exo)

(193) R = H, (194) R = Me, (195) R = Ph,

(196)R = C6 H4 X (o, m, p; X = F, Cl, Br, I),

(197)R = C6 F5, (198) R = CHMeCO2 Me (optically active and racemic monomers)

24. Advances in the metathesis of olefins

1571

endo-monomer has added to the chain125. Likewise endo-190 cannot be homopolymerized by the relatively short-lived WCl6/Me4Sn catalyst but can be incorporated into a copolymer with norbornene. Polymers of exo-190 are readily converted to diester or half-ester derivatives by reaction with alcohols. It is also possible to carry out ROMP of exo-190 and esterification in one operation using RuCl3 or OsCl3 at 70 °C as catalyst501. The polymer of exo-191 is readily converted to the diol derivative by hydrolysis with aqueous trifluoroacetic acid485.

The ROMP of endo-195 gives a 15% yield of a low-molecular-weight product. In contrast the ROMP of exo-195 proceeds to high conversion with the formation of high-molecular-weight polymers529. The 7-isopropylidene derivative of exo-195 gives a high-trans polymer with RuCl3533. The Tg values of the polymers of exo-196 vary with the substituent X: from 199 °C (X D m-I) to 270 °C (X D o-Br)530,534. The ROMP of exo-197 catalysed by MoCl5/Me4Sn at 60 °C gives a high yield of high-trans polymer. Endo-197 fails to homopolymerize but can be copolymerized to some extent with its exo-isomer531.

Potentially the most interesting polymers in the third group are those of 198, made by reacting 190 with (C)- or ( )-alanine methyl ester, and thus containing a chiral substituent. The chirality and molecular recognition capacity of the resulting polymers might ultimately be useful as a template for controlling the architecture of other polymers formed in their presence. Both endo-198 and exo-198 readily undergo ROMP to give high-trans, optically active polymers of narrow MWD532. The synthesis of numerous monomers related to 198 has been reported535.

+ C6 0	∆	(56)
(199)		(200)

The fullerene monomer (200), made by the cycloaddition of quadricyclane (199) to C60 (equation 56), can be copolymerized with an excess of norbornene in the presence of 8 (R D Me) to yield a high-molecular-weight, soluble, film-forming copolymer (86% cis), containing 1% of C60 derivative and exhibiting electronic and electrochemical properties which are typical of the carbon cluster536.

13. Other polysubstituted norbornenes

Various polymerizable polysubstituted norbornenes have been reported, among them

201537, 202305, 203324,538, 204350, 205539, 206485; also see elsewhere526,540,541.

1,7,7-Trimethylnorbornene (202) was originally thought to be unpolymerizable320 but it yields to initiator 8 (R D Ph). In CD2Cl2 at 20 °C the head alkylidene adduct P1H, in which the CDC bond is trans, is formed after a few hours, reaching a maximum of more than half the original initiator concentration, and then declines very slowly as more monomer adds to P1H to give an all-trans, all-HT polymer. The first-order rate constant for the decay of initiator in the presence of excess monomer M shows a complex dependence on [M], tending towards first-order at low [M] and zero-order at high [M].

1572		K. J. Ivin
	Me	Me
CH2
		Me
CH2
(201)		(202)	(203)
			OAc
F2		F2
F(CF3 )		F(CF3 )	OAc
(204)		(205)	(206)

This is interpreted in terms of a mechanism involving an equilibrium between the syn and anti rotamers of the initiator, in which the conversion of the dominant syn rotamer into the minor anti rotamer is rate-determining at high [M], with k D 6.1 ð 10 5 s 1, while at low [M] the addition of M to the anti rotamer becomes rate-determining305. The value of k agrees well with the value determined directly in toluene by photochemical displacement of the equilibrium between the rotamers121,122. The tacticity of this polymer is mentioned in Section VIII.A.5.

The spiro compound 203 is more readily polymerizable than syn-7-methylnorbornene (157); the cyclopropyl group evidently offers less steric hindrance than the syn-methyl group in the propagation reaction. The pattern of the fine structure of the 13C NMR spectrum of the high-trans polymers of 203 is similar to that of the high-trans polymers of anti-7-methylnorbornene (156) and m/r assignments can be made on this basis324.

For 205 (55% exo-CF3, 45% endo-CF3) the exo-isomer is more reactive so that if polymerization is incomplete, the residual monomer is enriched in the other isomer539. Polymers of 206 have a narrow MWD if initiated by 7 in THF. The polymer has a Tg of 110 °C and decomposes at 300 °C losing two molecules of acetic acid per repeat unit. It is also readily hydrolysed to the polydiol, which is soluble in CF3COOH/CHCl3 and degrades with loss of water at 300 °C485.

14. Norbornadiene and its monosubstituted derivatives

Monomers in this category that undergo ROMP are 207103,278,328,542,543, 208328, 210544, 211545, 212546 and 213547. For ease of comparison between these monomers and those

derived from norbornene we shall adopt the numbering system shown below, with C2DC3 always unsubstituted. This sometimes differs from the IUPAC numbering system.

Where one (or both) of the 5,6-positions are substituted, ROMP invariably occurs only by cleavage of the unsubstituted double bond. For monomers substituted at the 7-position only (208 210), less active catalysts favour cleavage of the less hindered C2DC3 double bond but more active catalysts show little discrimination between C2DC3 and C5DC6.

With norbornadiene (207) the first double bond to react can open in one of four ways, as for norbornene, to give either a cis or trans double bond, located within an m or r ring

			24. Advances in the metathesis of olefins						1573
	7			X			7
			7				1
		1			(208)	X = Me		(211)	Y = SiMe3
				1
							2
2					(209)	X = Ph		6	Y = SiHMe2
		6 2		6				(212)
					(210)	X = OCMe3	4	(213)	Y = CF3

4	4	3	5
3	5	5	Y
	3
(207)	(208− 210)	(211− 213)

dyad. Polymers have been produced in which the cis contents of the double bonds between the rings range from 90% (OsCl3 catalyst) to 37% (MoCl5/Bu4Sn catalyst). Provided that [M]0 < 0.2 M and hex-1-ene is used as chain transfer agent (40% of [M]0), the polymers are soluble, allowing well-resolved 13C and 1H NMR spectra to be obtained which can be fully assigned in terms of the structures represented in 207P103,278. However, if the concentration of C5DC6 bonds in the polymer solution exceeds 0.2 M, this double bond also opens with the formation of a cross-linked polymer; see Section VIII.B.2.

				c/t							c/t		7		c/t
[ CH •	4				CH 1•		•	4			4		1		1		• CH ]
	•	CH						CH				CH •	• CH			CH •
		3		2		7		3			3		2		2
											5		6
				m						r					r
								(207P)

Several other features are worthy of note in the ROMP of norbornadiene. First, OsCl3 gives a polymer with a much higher cis content (90%) than for the ROMP of norbornene (40%); and RuCl3 fails to give any polymer at all, yet with norbornene it gives a high- trans polymer. These differences are ascribed to the di-endo chelation of one molecule of norbornadiene to the metal carbene centre, acting as a spectator ligand. For OsCl3 the resultant crowding of the reaction site favours the approach of the monomer leading to the formation of a cis double bond, while for RuCl3 the crowding is such as to prevent reaction altogether. Secondly, like the polymers of norbornene, the high-cis polymers of norbornadiene tend to have a blocky cis/trans distribution278. Thirdly, the polymer may be dehydrogenated to give a black lustrous rigid solid which is strongly paramagnetic (g D 2.0027) and may be presumed to contain some units of the type shown in equation 57543.

[ CH •	• CH ]	I2 /CCl4
		reflux

[ CH	CH ]	(57)

Like 207, the 7-substituted norbornadienes 208 210 give high-cis polymers with OsCl3, containing 7%, 11% and 30% syn units respectively328,544,548. With 8 as initiator both 208 and 210 give high-cis polymers containing approximately 50% syn units549. Some of the possible dyad sequences are illustrated in 214. The higher proportion of syn units when the substituent is t-butoxy suggests that the lone pairs of electrons on the oxygen atom may assist the approach of the monomer to the C5DC6 double bond on the exo face. This might happen through simultaneous coordination of the metal centre to the oxygen atom and the syn double bond (syn exo chelation)328.

Polymers of 208 can be made with cis contents ranging from 97% (OsCl3 catalyst) to 20% (MoCl5/Me4Sn/Et2O catalyst); a somewhat narrower range (80 33%) is observed for polymers of 209. The 13C NMR spectra of the polymers of 208, containing mainly

1574

K. J. Ivin

R

R

[ CH •

• CH

CH •

•

CH CH •

•

CH CH •

• CH ]

•

•

R

R

anti

anti

syn

syn

r

m

r

(214)

anti units, show a similar c/t and m/r splitting pattern to that observed in the spectra of polymers of 156. Tacticity assignments have been made by comparing the spectra of the fully hydrogenated polymers of 208 and 209 with those of 156. ReCl5 gives an 80% cis polymer of 208 in which the cis-centred dyads are r and the trans-centred dyads are m. Very high syndiotacticity is also observed in the 97% cis polymer made with OsCl3 as catalyst. In contrast, the 42% cis polymer made with WCl6/Me4Sn as catalyst is essentially atactic. RuCl3 is ineffective for 208 and 209, as for 207328.

Monomers 211 213 are all readily polymerized by WCl6/Me4Sn to give unbiased, approximately 50% cis, polymers545 547. A 95% cis unbiased polymer of 213 is obtained with 8 (R D Ph) as initiator490.

15. Polysubstituted norbornadienes

The 5,6-dicarboxylic chiral esters (71, 72) have already been mentioned in connection with the determination of tacticity; see Section VIIIA.5. Other polysubstituted norborna-

dienes, not containing fused aromatic rings, which undergo ROMP are 21510,126,325,550,

217551, 218551, 21910,325,552 555, 220539, 221551, 222121,550,551 but not 21610.

Polymers formed by ROMP of 219 range from all-trans (polymer A), made with 7 (R D Me or Ph) as initiator, to all-cis made with 8 (R D Ph) or 223 as initiator, (polymers B and C respectively)10,550. The 13C NMR spectrum of the all-trans polymer A gives single lines for C-2,3 (133.50), C-1,4 (49.80) and C-7 (37.20 ppm). However, while the all-cis polymer C gives a single C-7 line at υ 38.38, the all-cis polymer B gives three lines: at υ 38.38, 37.61 and 36.44, in order of diminishing intensity. This fine structure can only be due to m/r splitting. Polymer C is therefore fully tactic, while polymer B is partially tactic in the same direction (74/26), but one cannot say whether the bias is towards m or r. Nor can one be sure that polymer A is tactic since no all-trans polymers have been made which show fine structure for C-7 (or any other carbon). The fact that the trans polymer is semi-crystalline with a melting point at 200 °C, and can be fibred and cold-drawn, lends support to the belief that it is in fact highly tactic325,554. The trans polymer of 219 also has an unusually high relaxed dielectric constant which accords with a predominantly syndiotactic structure, but the relatively low value for the cis polymer can be interpreted in terms of either a syndiotactic structure557 or an isotactic structure313.

A marked temperature dependence of the cis content of the polymer formed from 219 using 224 as initiator in THF (100% at 35 °C, 24% at 60 °C) has been interpreted in terms of an equilibrium between syn and anti rotamers, with cis CDC formed mainly by addition to the THF-free syn rotamer, and trans CDC formed mainly by addition to the THF-free anti rotamer556; cf Section III.B.5.

The reaction of 219 with 7 (R D Me) proceeds mainly via the less abundant but much more reactive anti rotamer of the initiating and propagating species. The first four propagating species may be distiguished in the 1H NMR spectrum; for the longer chains the anti and syn rotamers are just resolved, in the ratio of 1:6. The corresponding 13C

24. Advances in the metathesis of olefins

1575

R2

R1

CO2 R	CO2 Me				CF3
			R1	R2
		216	Me	Me
CO2 R	CO2 Me 217		Me	Ph	CF3
(215)	(216− 218)	218	Ph	Ph	(219)
	Ph		Ph	Ph
	Me
CF3		CF3			CF3
CF3	CF3				CF3
(220)	(221)			(222)

Me		Me	CMe3	CMe3
SiMe2 Ph	N			N
O Mo		CHCMe2 Ph Me3 C	O Mo CHCMe2 Ph
O			O
	SiMe2 Ph			CMe3
			Me3 C
(223) (racemic)			(224)

NMR signals are also resolved: υ 253.1 and 252.6. In benzene at 22 °C, kp/ki D 0.72 and kp D 0.057 M 1 s 1. The reactivity of 219 is some 30 times less than that of 215 (R D Me). This may be ascribed mainly to a lower electron density at the unsubstituted double bond in 21910,96.

The ROMP of 219 initiated by a mixture of 7 (R D Ph) and 8 (R D Ph) in trifluorotoluene might have been expected to give a mixture of all-trans and all-cis polymer. Instead the cis and trans double bonds are distributed throughout the chains, as shown by the C-7 fine structure557. This is because the alkoxy ligands in 7 and 8 undergo rapid exchange to form an equilibrium mixture with the complex containing one of each type of alkoxy ligand, the rates of exchange of alkoxy ligands being much faster than the rates

1576 K. J. Ivin

of addition of monomer. A polymer of any desired cis content can thus be prepared from 219 by mixing with appropriate proportions of the two initiators555.

The ROMP of 215 (R D Me) initiated by 7 (R D Me) in C6D6 gives a mainly trans (ca 95%) polymer, with Mw/Mn D 1.06. The initiator is fairly quickly consumed when there is an excess of monomer (kp/ki D 3)10,325,490.

The all-cis polymers in the series 215 (R D Me, Et, i-Pr, t-Bu), initiated by 8 (R D Ph) in toluene, have an isotactic bias (78, 84, 81, 97% m dyads, respectively), as judged by the C-7 fine structure, assuming that the line order is the same in each case556. A remarkable observation has been made for the system 72/223. The GPC of the polymer (Mn D 28, 200) shows two sharp peaks each of which has Mw/Mn ca 1.06, and taken together, 1.13. This has been attributed to the fact that the initiator is a 1:1 mixture of non-interconvertible enantiomers and that highly regular all-cis isotactic chains grow separately from each enantiomeric metal site. The chirality at the metal site, interacting with the chiral monomer, controls the rate of propagation, while the chirality of the chain ‘end’, i.e. the previously added monomer unit, independently controls the stereochemistry of the next monomer addition. This effect is not observed with 71 as monomer, where the rates of propagation at the two types of site are presumably not sufficiently different313.

The ROMP of the dicyano analogue of 219, also the tricyclic monomer having [CH2C(CN)2C(CN)2CH2] attached to the 5,6-positions, can be initiated by 7 (R D Me), the latter giving a 97% trans polymer515.

The ROMP of 220 proceeds readily in the presence of WCl6/Ph4Sn at 70 °C to give an 80% trans polymer539. The ROMP of the bicyclofulvene derivatives 217, 218, 221 and 222 is catalysed by MoCl5/Ph4Sn at 70 °C to give high-molecular-weight products551. Surprisingly 216 is not polymerized by 7 (R D Me) although it adds one molecule slowly at 25 °C to give the molybdenum carbene adduct. Although the adduct will not add 216 it will add norbornene (kp/ki D 270). Both 1H and 13C NMR spectra indicate that only one rotamer is present in the solution of the adduct. An X-ray study shows that in the crystal the molecule is in the form of the syn rotamer and this is presumably also the dominant form in solution. The rate of reaction of 7 (R D Me) with 216 (k D 1.3 ð 10 3 M 1 s 1 at 22 °C) is some 500 times smaller than its rate of reaction with 215 as is to be expected for reaction at the exo face10.

The norbornadiene derivatives that have fused aromatic rings (225 234) all undergo ROMP readily at 20 °C in chlorobenzene by opening of the unsubstituted double bond of the norbornadiene ring system: 22510,432,548,558, 226432, 227 228559, 232 234560, 229 231306,508. This was first demonstrated for 225 using WCl6/Ph4Sn (1/2) as catalyst and later for 227, 228, 232 234 using WCl6/Me4Sn. The cis contents are mostly 40 50% and are somewhat lower with MoCl5/Me4Sn as catalyst, becoming as low as 4% for 234. The polymer of 228 is insoluble and has not been characterized. The complex 7 (R D Me) effects smooth and rapid ROMP of 100 equiv of 225 in toluene (kp/ki D 7) to give a

24% cis polymer with Mw/Mn D 1.0510. With Cp2 TiCH2CMe2CH2 as initiator for the ROMP of 225 and 226 in toluene the polymer comes out of solution after the addition of only 9 units of monomer432.

The most remarkable results for this group of monomers are those obtained with the fulvene derivatives 229 231. Their ROMP is initiated by both 7 (R D Me) and 8 (R D Me) in toluene at 20 °C, the latter giving the faster reaction. For each initiator the rate decreases in going from 229 to 230 to 231 i.e. as the bulk of the substituents becomes larger. Surprisingly the cis content of each polymer is independent of the catalyst: 20% for 229, 0% for 230 and 100% for 231. The NMR spectra of the polymers of 230 are in keeping with an all-HT structure306.