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			1547
				Me	Me
			catalyst		(50)
	Me	Me
		Me Me
		(88)		Me	Me

2. Five-membered rings

Among the more recently used catalyst systems for the ROMP of cyclopentene the following are of particular interest.

(i) W(DCHCMe3)(DNAr)(OCMe3)2 (7W) (0.04 M in benzene) will polymerize 50 equiv of cyclopentene to give an equilibrium mixture containing about 5% monomer (0.1 M) at 60 °C and about 95% monomer (1.9 M) at 60 °C. The monomer may be stripped completely from the living polymer by continuous evacuation, reforming the original initiator. In order to make a polymer of narrow MWD (Mw/Mn D 1.08) with this system it is necessary to work at 40 °C and to terminate the reaction after 1 h so as to forestall the tendency towards a thermodynamic distribution341,342. Star polymers can be made by terminating such a living polymer with a polyfunctional aldehyde343.

(ii) Tetraphenylporphyrinatotungsten tetrachloride/tetraisobutylaluminooxane (1/2) polymerizes cyclopentene (4.8 M) in toluene to give a 20% cis polymer with a

surprisingly narrow MWD (Mw/Mn D 1.2)344. The polymer formed initially using (C17H35COO)2MoCl3/Et2AlCl as catalyst also has a narrow MWD345. These too may be living systems.

(iii) The ruthenium carbene complex 19, containing the electron-rich ligand PCy3, brings about the ROMP of cyclopentene, the propagating species being quite stable and

detectable by 1H NMR110. Certain other ruthenium complexes are effective in the presence of diazoesters61,346.

In poly(1-pentenylene) the chemical shifts of the ˛-carbons are about 0.5 ppm upfield from those in the polymers of the other cycloalkenes, an effect which is attributed to a higher proportion of gauche conformations about the CH2 CH2 bonds arising from the influence of the -olefinic carbons347.

The cyclopentene derivatives 89348, 92349 and 93350 do not appear to undergo ROMP, probably because their free energy of polymerization is positive. However, the fact that 1% of 89 can completely inhibit the polymerization of 90 and 91 indicates that it is likely to add preferentially to the active site forming the head carbene complex, [W](DCMeCH2CH2CH2CHDCHR), which is then unable to add any of these three monomers. It should be capable of copolymerization with norbornene.

Individual enantiomers of 90 have been prepared by RCM (Section VII.A.1) and polymerized by 8 (R D Ph) at 30 °C to give a 52% yield of a 74% trans polymer226.

Likewise 91, with the same initiator at		55 °	C, gives a		51% yield of 60% cis poly-
						13	C NMR spectrum of the
mer, with a blocky cis/trans distribution (rtrc			. D	6.3); the
hydrogenated polymer shows it to be atactic326

The ring-opened polymer of norbornadiene consists of 3,5-disubstituted cyclopentene units (94). When the concentration of these units is kept below 0.2 M the polymer remains soluble, but above this concentration, in the presence of WCl6/Me4Sn (1/2), it gels. This is caused by cross-linking, brought about by the ROMP of the enchained cyclopentene

1548		K. J. Ivin
Me
		Me	CHMe2
		Me
(89)	(90)	(91)	(92)
F2	F2	CH	CH
	F2
	(93)	(94)

rings. This process is reversed by dilution351. The opening of the second double bond, leading to cross-linking of the polymer, is thus thermodynamically allowed above a critical concentration, as for the ROMP of cyclopentene itself. From the slight variation of this critical concentration with temperature ( 40 to 18 °C) one obtains H° D 4.6 kJ mol 1

and S° D 2.9 J K 1 mol 1 (standard state 1 M) for the opening of the second double bond352.

Four disubstituted 1-silacyclopent-3-enes (95 98) have been studied; also 99. When

polymerized	in	bulk,		using	8 (R		D	Ph) as initiator, 96 and 97 give high			polymer
		247,353	,	as does				300	but	95 gives low	polymer
(Mn ca 40, 000)						99		(Mn ca 10, 000) ,
(Mn ca 4000)	not only			with	this		initiator but also with		the	catalyst systems	WCl6/i-

Bu3Al/Na2O2354, Re2O7/Al2O3/Bu4Sn355 and WCl6/Me4Sn247. WCl6/Ph4Sn is also an effective catalyst for the ROMP of 97 and 98 provided that a small amount of cyclopentene or cyclohexene is present to assist initiation356,357. WCl6/Me4Sn (and WCl6/Ph4Sn) gives a high-cis polymer of 97, whereas 8 gives a polymer with only 45% cis double bonds. The corresponding polymers of 96 contain 80% and 25% cis double bonds, respectively. The former has a 29Si NMR spectrum (in CDCl3) with three signals at υ 4.31 cc , 4.44 ct and 5.18 tt in the ratio 72:20:8, corresponding to a blocky cis/trans distribution (rtrc ca 5.5); the latter has a random distribution (rtrc ca 1). These polymers all have a broad MWD (Mw/Mn 2). The polymer of 98 initiated by WCl6/Ph4Sn has 84% cis

double bonds and both the 29Si and benzylic 13C NMR signals are sensitive to cc, ct, tt dyads357.

When 97 is sufficiently dilute its polymerization is no longer thermodynamically allowed, but, in the presence of 8, a mixture of cyclic dimers, cc (84%), ct (14%) and tt (2%), is readily formed, in equilibrium with the monomer. The cc dimer has been isolated and its crystal structure determined247. These cyclic dimers can also be made by metathesis degradation of the polymers or by ADMET of diallyldiphenylsilane (Section VII.A.6).

The ROMP of neat 99 is remarkable in that it proceeds to 96% conversion in 18 h at 25 °C to yield an all-HT, 97% trans polymer. Dilution of the living polymer with benzene causes neither reversion to monomer nor backbiting to form cyclic oligomers. The driving force for polymerization in this case derives from the relief of torsional strain in the monomer caused by interaction between eclipsed methyl groups on the adjacent silicon atoms300.

	24. Advances in the metathesis of olefins			1549
Me		Ph	Ph
Si		Si	Si	Si
Me		Me	Ph
(95)	(96)	(97)		(98)

SiMe2

SiMe2

(99)

3. Six-membered rings

It is well known that the failure of cyclohexene to form long-chain polymer by ROMP is due to the greater thermodynamic stability of the monomer. However, if a 5 M solution of cyclohexene in toluene is added to a WCl6/Me4Sn solution at 25 °C and then cooled to 77 °C, 12% of the cyclohexene is consumed; it is regenerated on warming to 25 °C. If cold wet acetone is added to the reaction mixture at 77 °C, the products are found by GC to consist of oligomers containing 2 6 monomer units. The number of peaks indicates that several cis/trans isomers of the various oligomers are present, with cis double bonds preferred. Exposure of the mixture of oligomers to fresh catalyst brings about reversion to cyclohexene.

If norbornene is added to a mixture of WCl6/Me4Sn/cyclohexene (1/2.4/5) in toluene at 25 °C, 5% of the cyclohexene is consumed and incorporated into the polymer of norbornene, as shown by the presence of 7.5% 1,6-hexanediol in the products obtained by ozonolysis of this polymer followed by reduction with LiAlH4. If the copolymer is allowed to stand overnight in the presence of the catalyst at 25 °C the cyclohexene units are split out, with 99% recovery of the original cyclohexene.

These two pieces of evidence show that cyclohexene can add to metal carbene complexes to some extent, but that at low temperature backbiting to give oligomers is preferred to propagation, while at room temperature the product of addition can be trapped, at least for a time, by reaction with norbornene358.

There is also some indirect evidence for the interaction of cyclohexene with catalyst systems. First, the presence of cyclohexene assists the formation of the initiating species in the WCl6/Ph4Sn-catalysed ROMP of 28356. Secondly, the presence of cyclohexene increases the rate of ROMP of cycloocta-1,5-diene catalysed by WCl6/Me4Sn at 25 °C by 30%, without itself being consumed359. However, there is no NMR spectroscopic evi-

dence that W[DC(CH2)3CH2](Br)2(OCH2CMe3)2/GaBr3 can interact with cyclohexene although it does so with cycloheptene and cyclooctene. Ring-strain relief is therefore of some importance in the formation of these cycloalkene adducts120.

The formation of polymers containing [DCH(CH2)4CHD], units is possible through the ROMP of an appropriate cyclic diene, such as cycloocta-1,3-diene, or by a double-bond shift reaction of a polymer such as poly(1-pentenylene). Such units can be eliminated as cyclohexene so long as metathesis activity is present in the system360. The ROMP of 2,3-dihydropyran, initiated by Mo(CO)6/CBr4/h, has been reported361.

4. Seven-membered rings

The ROMP of cycloheptene can lead mainly to dimer or to high polymer depending on the conditions. The equilibrium monomer concentration is temperature-dependent271.

1550	K. J. Ivin

When a 30% solution in hexane is refluxed in a Soxhlet apparatus through a fixed bed of Re2O7/Al2O3, preferably pre-treated with Me4Sn, a 68% yield of cyclic dimers is obtained, the proportions being close to the equilibrium values: tt 88%, tc 9%, cc 3%272,273. Metal carbene complexes give 20 50% cis polymers120,271,347.

5.Eight-membered rings

R

		R		R
(100)	R = H	(102)	R = H	(104)	R = Me
(101)	R = Me	(103)	R = Me	(105)	R = Ph

R

O

Ph

(118)(119) R = Me

(120) R = Ph

	X
(106)	X = Me, (107)	X = Cl,
(108)	X = Br, (109)	X = CO2 Me,
(110)	X = CO2 SiMe3 , (111) X = OH,

(112)X = OAc,

(113)X = O.CO.CMe( CH2 ),

(114)X = I, (115) X = SR′,

(116) X = BEt2 , (117) X = CN

Cis- and trans-cyclooctene, 100 and 102 respectively, and their derivatives 103 107, all undergo ROMP295; also 10862,362, 109 and 11062, 111 113362, 114363, 115364, 116365, 118362, 119 and 120366,367. Only 101295 and 117362 fail to polymerize, perhaps due to unfavourable choice of catalyst and conditions. The trans monomer 102 gives a 43% cis polymer very rapidly in the presence of MoCl2(PPh3)2(NO)2/EtAlCl2368 and is polymerizable by 18110. With a catalyst of type 10 secondary metathesis reactions of the double bonds in the polymer of 100 cause the cis content to fall from 75% to 25% as the reaction proceeds271.

The 5-substituted cyclooctenes (106 116, 118) generally give unbiased polymers, the substituent being too far away from the reaction site to influence the direction of addition of monomer. This is particularly clearly seen in the 13C NMR spectrum of the polymer of 118, made using 19 as initiator, the olefinic region consisting of two well-defined symmetrical quartets (1:1:1:1) attributable to cis and trans olefinic carbons within HH, HT, TH, TT structures362.

For the ROMP of the 5-alkylthiocyclooctenes (115), with R0 D Et, Bu, Hex, c-Hex, t-Bu, initiated by 12, the most reactive monomers are those with branched alkyl substituents on the sulphur atom; for R0 D t-Bu, reaction is 95% complete in about 10 min. The variations in rate are likely to be connected with the strength of coordination of the sulphur atoms in the monomer and/or the propagating species to the tungsten centre. Coordination of the monomer to the metal centre through the sulphur atom will be impeded when R0 is t-Bu or c-Hex, allowing a higher equilibrium concentration of the precursor complex that leads to addition of monomer. For R0 D Bu the rate of polymerization is proportional to both monomer and initiator concentrations189,364.

24. Advances in the metathesis of olefins

1551

	(121)			(122)		(123)			(124)	(125)
All the unsubstituted cyclooctapolyenes 121								125	undergo	ROMP with	the
usual	catalysts		but,	if	the	reactions of 121, 124 and 125 are carried out
in dilute solution, C6 ring						compounds are eliminated by backbiting reactions,
while	122	and	123	form		cyclic oligomers. Thus with 8W (see Table 2) in
dilute	solution		121	yields		cyclohexene. This suggests that the preferred mode
of addition		is	that	to	give [W]DCH(CH2)4CHDCHCHDCHCMe3 rather						than

[W]DCHCHDCH(CH2)4CHDCHCMe3 since the former can eliminate cyclohexene immediately. With the same catalyst in dilute solution 124 yields some cyclohexa-1,3- diene and 125 gives a 75% yield of benzene360; also see elsewhere369 371.

There are numerous studies of the ROMP of 123 of which the most recent have involved the use of molybdenum carbene347 and ruthenium carbene catalysts60,110. Most catalysts give initially a polymer of at least 80% cis content, since one of the cis double bonds is pre-formed.

The ROMP of cyclooctatetraene (COT, 125) to give polyacetylene was first reported in 1985372,373 with W[OCH(CH2Cl)2]nCl6 n/Et2AlCl (n D 2 or 3) as catalyst at 20 °C. When the reaction is conducted in toluene the yield of black insoluble polymer is only 6%, but if the monomer is condensed onto a solid layer of catalyst a yield of up to 40% polymer is obtained. In the former case there is a much greater tendency towards formation of oligomers, amongst which the cyclic products of backbiting reactions, (CHDCH)n, n D 5 8, can be identified by MS. The nature of the polyacetylene formed by the second method depends on the initial Al/W ratio. When Al/W is 1 the polymer is formed as a blue-black film containing 84% cis double bonds, but when Al/W is 2 the film is golden and contains only 39% cis double bonds. A 50% yield of polymer can also be obtained from neat monomer with WCl6/BuC CH as catalyst348. Better control over this reaction can be achieved using 8W as catalyst360. Dissolution of catalyst in neat 125 produces, within a few seconds at room temperature, a high-quality lustrous silver film with smooth surface morphology. When first prepared, its CP-MAS 13C NMR spectrum shows two olefinic peaks: a stronger peak at 126.4 ppm (cis) and a weaker peak at 132.2 ppm (trans). On heating the sample the spectrum changes, giving a main peak at 135.9 ppm (trans) and an upfield shoulder (cis). The chemical shift of the trans olefinic carbons are known to be sensitive to the configuration of the surrounding double bonds. Heating thus induces cis/trans isomerization to produce long segments of trans transoid structure within the polymer chains. When doped by exposure to iodine the polymer acquires a conductivity greater than 100 ohm 1 cm 1. Linear copolymers of varying conjugation length can be produced by the inclusion of a second monomer such as 123 during the preparation of the film, allowing a wide range of conductivities in the doped copolymers374.

Ring-opened polymers have been made from monosubstituted cyclooctatetraenes (RCOT) with the following substituents R: Br360, Me, i-Pr, Bu, s-Bu, t-Bu, neopentyl, 2-ethylhexyl, octyl, octadecyl, cyclopropyl, cyclopentyl, phenyl, methoxy

and t-butoxy375 377, SiMe3378,379 and the chiral substituents CHMeCHMe(OMe), CHMeCHMe(OSiMe2CMe3) and CH2CHMe(OSiMe2CMe3)380. The initiators 8W and 13 are both very effective, but even with pure monomer some backbiting reaction occurs in competition with propagation, giving rise to 7 16% C6H5R. No benzene is detected, showing that ring-opening does not occur at the substituted double bond. Ring-opening

1552	K. J. Ivin

may, however, occur at any of the other three double bonds, so that the product is a polyacetylene having substituents placed on average on every fourth or fifth double bond (elimination of C6H5R will tend to decrease the frequency), but never on adjacent double bonds, unlike the polymers of substituted acetylenes; see Section X.C.

The initially formed polymers have a high cis content and, except when R is Me, are generally soluble in tetrahydrofuran, chloroform and benzene (Mn ca 104 106). On standing at room temperature the solutions slowly change colour, and a new absorption maximum appears at longer wavelengths. This change is fastest for the polymers with straight-chain substituents and slowest for those with branched substituents. It is accelerated by exposure to light and is due to cis/trans isomerization; see Figure 9. Long sequences of trans double bonds allow a much greater degree of conjugation, giving rise to a low-energy ! Ł electronic absorption. For monomers with straight-chain or alkoxide substituents the predominantly trans polymer comes out of solution as it is formed. In contrast, polymers containing a secondary or tertiary substituent adjacent to the backbone remain soluble in the mainly-trans form. Effective conjugation lengths of up to 30 double bonds have been observed for these soluble polymers.

For the polymer of s-BuCOT in benzene cis/trans isomerization can be monitored by the change of absorbance at 560 nm. The reaction is first-order, with a half-life of about 27 min at 65 °C and an activation energy of 89 kJ mol 1. However, the isomerization of the trisubstituted cis double bond proceeds only part way. Its reaction can be followed separately by means of 1H NMR spectra, the methine signal of the side groups being sensitive not only to the configuration of the nearest double bond but also that of the next-nearest double bond: tt 2.75, tc 2.51, cc 2.12 ppm. At equilibrium the preference for the cis configuration means that the proportion of long trans sequences is small. During the isomerization process the absorption spectra exhibit an isosbestic point at 400 nm, which is consistent with a mechanism involving multiple isomerization of the cis double bonds in one chain by a cooperative motion.

When R D t-Bu the polymer is freely soluble and yellow-orange in colour ( max D 432 nm after isomerization); it also remains an insulator in the presence of iodine.

FIGURE 9. Absorption spectra of polymer of trimethylsilylcyclooctatetraene in CCl4 (10 6 M) obtained between eight periods of photolysis (10 s each). Reprinted with permission from Ref. 378. Copyright (1989) American Chemical Society

24. Advances in the metathesis of olefins	1553
This indicates a very low effective conjugation length even after	isomerization. This

is attributed to a twist in the polymer chains, caused by the bulky t-Bu groups. This effect is much less for the polymers with other R groups and their films can be made conducting by doping with iodine375. For the polymers bearing chiral substituents, the backbone! Ł transition shows substantial circular dichroism, the magnitude of which is characteristic of a disymmetric chromophore. The chiral side groups thus twist the main chain predominantly in one sense rather than just perturbing that chromophore electronically380.

The possible application of these materials to form surface barrier solar cells, and to make conductor/insulator/conductor sandwiches by sequential polymerization of different monomers has been explored376,379,381.

6. Larger rings

The ROMP of cis-cyclodecene and cyclododecene by metal carbene complexes gives high-trans polymers347. Methyl 3,7-cyclodecadienecarboxylate has been successfully polymerized by ring-opening using WCl4(OC6H3-Ph2-2,6)2/Et4Pb as catalyst at 60 °C. 13C NMR spectra show that the monomer consists of either the cis,trans or the trans,cis isomer; also that the polymer has a very regular structure382.

9-Phenyl-1,5-cyclododecadiene undergoes ROMP in the presence of WCl4(OC6H3-Cl2- 2,6)2/Et4Pb in PhCl at 80 °C; 98% yield in 2 h383.

[2.2]Paracyclophan-1-ene (126) can be regarded as a 12-membered ring but its rigid structure is akin to that of cyclobutene. The ROMP of 126 is initiated by 8 (R D Ph) in toluene, giving a living polymer with a carbene proton singlet at 12.84 ppm, the intensity of which increases as the reaction proceeds slowly to completion (18 h). The polymer (127) has 98% cis double bonds, but on irradiation or exposure to catalytic amounts of iodine the double bonds undergo cis/trans isomerization and the polymer comes out of solution. Many other Moand W-based catalysts are also effective but give insoluble polymer, presumably because of a high trans content. A solution of the high-cis polymer fluoresces when excited by irradiation at 330 nm. The initial spectrum displays a weak emission at about 370 nm attributed to 2% of trans-stilbene segments originally present in the chains. With continued irradiation (2 min) there is an increase in the emission from this band as more trans double bonds are generated, and a new emission appears at 445 500 nm. After further irradiation, this intense red-shifted luminescence becomes the predominant feature before the polymer precipitates. In a statistical copolymer of 9% of 126 and 91% of norbornene, the units of 127 are isolated between norbornene units, and the fluorescence spectrum is confined to the shorter-wavelength region with a maximum around 360 nm384.

HC	CH2	ROMP	[ CH	CH2 CH2	CH ]
HC	CH2
(126)				(127)

Similar results have been obtained for a derivative of 126 in which one of the CH2 hydrogens is substituted by OSiMe2CMe3. The polymer is unbiased but differs from that made from 126 in that after isomerization to a high-trans polymer it remains soluble in organic solvents. The silyl group in the polymer can be removed by treatment with Bu4NF to give the hydroxy analogue, which can then be dehydrated thermally at 105 °C or catalytically (HCl) at 25 °C to give poly(p-phenylenevinylene)385,386.

1554	K. J. Ivin

The ROMP of [2.2]paracyclophane-1,9-diene (128) yields poly(p-phenylenevinylene) (129) as an insoluble yellow fluorescent powder. Soluble copolymers can be made by the ROMP of 128 in the presence of an excess of cyclopentene387, cycloocta-1,5-diene388 or cyclooctene389. The UV/vis absorption spectra of the copolymers with cyclooctene show separate peaks for sequences of one, two and three p-phenylene-vinylene units at 290, 345 and about 390 nm respectively, with a Bernoullian distribution. The formation of the odd members of this series must involve dissection of the two halves of the original monomer units by secondary metathesis reactions.

HC	CH
	ROMP	[ CH	CH CH	CH ]
HC	CH
(128)			(129)

Attempts to bring about the ROMP of ferrocenophanes, in which the two cyclopentadienyl rings are linked by a divinylene group, have met with limited success. Soluble oligomers (Mn D 1700) are obtained using a tungsten carbene complex as catalyst. Soluble polymers, probably of higher MW, can be obtained by placing a methoxy group on the carbon adjacent to a Cp ring or by copolymerizing with s-butylcyclooctatetraene390.

C. Polycyclic Alkenes

1. Monomers containing a fused cyclobutene ring

The ROMP of the following compounds has been reported: 130109,391, 131392,

132392,393, 133394, 134395,396, 135396,397, 136341,395,396,398 404, 137397, 138405,406,

139395,407, 140395, 141 143337. In all cases the reactive double bond is that in the C4 ring. The repeat units in the polymer have an erythro structure corresponding to the cis relationship of the bonds which attach the cyclobutene ring to the rest of the ring system in the monomer.

				X
				X	X = H
(130)	(131)	(132)	(133)	(134)
				(135)	X = COOMe
				(136)	X = CF3
		F3 C
MeOOC
MeOOC	(137)	F3 C		(139)
		(138)

When 18 in CH2Cl2 is added to 20 equiv of 130 at 40 °C all the initiator I is consumed and ROMP proceeds at a rate proportional to both [I] and [M], with an apparent kp of

24. Advances in the metathesis of olefins

1555

0.183 M 1 min 1. The 1H NMR spectrum shows the presence of three propagating species in equilibrium, of which one is probably the main propagating species. One has two PPh3 ligands, while the other two are presumed to have one such ligand; the 31P NMR spectrum shows two signals. This system exhibits all the characteristics of a living polymerization and gives a polymer with 58% cis double bonds. On adding 8 to a mixture of norbornene and 130, all the 130 reacts first and then the norbornene so that a block copolymer is formed. The same result may be achieved by adding norbornene to the system after all the 130 has been consumed109.

CH2 Ph

O O N

O O O O

(140)	(141)	(142)	(143)

The ring-opened polymers of 134, 139, 140, 135, 136 (and 138) undergo a retro-Diels- Alder reaction, either at room temperature or on heating to 120 °C, with the elimination of benzene, naphthalene, anthracene, dimethyl phthalate and 1,2-bis(trifluoromethyl)benzene respectively, and the formation of polyacetylene, as illustrated for 136 in sequence 51. The elimination reaction is accompanied by a change of colour from yellow to deep red as longer polyenes are formed. The polymer 136P is moderately stable at 20 °C with a half-life of 20 h395. This is long enough for films to be made and stressed uniaxially, and

then

converted

to highly-oriented non-fibrous crystalline films

of polyacetylene398.

A

A

A

A

[

]

[

]

B

•

•

•

•

+

metathesis

50 − 120 °C

catalyst

F3 C

CF3

F3 C

CF3

F3 C

CF3 (51)

(136)

(136P)

∆

[

]

The polymer of 138 is more stable than that of 136 but its conversion to polyacetylene is much more exothermic and not so easily controlled406. The double bonds B formed in the retro-Diels-Alder reaction are initially cis, as may be some of the double bonds A formed in the metathesis reaction, but these quickly isomerize to trans above 100 °C. In a DSC

1556 K. J. Ivin

trace of 136P the exotherm associated with this isomerization process can be detected at a higher temperature (110 °C) than that of the retro-Diels-Alder reaction (70 °C), but not in the case of the polymer of 138 where the former is merged with the latter (110 °C). The order of stability of the polymers is 134P < 136P < 135P < 138P ³ 139P < 140P. The last two are quite stable at room temperature and must be heated to 100 200 °C to effect their conversion to polyacetylene.

A closer insight into these reactions, particularly with monomer 136, has been gained through the use of 7 as initiator396, and its W analogue 7W401, the chains being terminated by reaction with Me3CCHO, Me3CCHDCHCHO or C7H15CHO, either before or after the retro-Diels-Alder reaction has occurred. When polyacetylene made in this way is passed through a column of silica gel under nitrogen at 40 °C one can isolate homologues containing up to 13 double bonds. Reaction of the tungsten carbene initiator with 136 gives a trans double bond, as also does the reaction of Me3CCHO with the living end, while the propagation reaction gives 75% trans double bonds. On the other hand, the initially formed double bond from the retro-Diels-Alder reaction is always cis. Such a procedure therefore yields a series of polyenes with (2n C 1) double bonds in which initially the most abundant components are those having alternate trans and cis double bonds: tc nt. If instead the living ends are terminated by reaction with Me3CCHDCHCHO a series of polyenes with an even number of double bonds is obtained, but the termination reaction is not stereospecific. Termination with 0.5 equiv of an unsaturated dialdehyde such as OHCCHDCHCHDCHCHDCHCHO can also be used as a means of joining two chains together and extending the polyene sequence, but solubility and stability problems limit the usefulness of this procedure396.

With 7 (R D Me) as initiator the various stages of reaction may be followed very closely by 1H NMR. The spectrum taken 20 min after mixing equivalent proportions of monomer and initiator shows two main carbene proton doublets (J˛ˇ D 6 Hz) at 11.21 and 11.09 ppm as well as the singlet from residual initiator at 11.24 ppm. The doublets are assigned to the protons ˛n in the products (144) of addition of one and two molecules of 136 respectively. Already in this spectrum may be seen small doublets (J˛ˇ D 11 Hz) at 12.63 and 12.48 ppm assigned to the corresponding protons ˛0n in the products of the retro-Diels-Alder reaction, which occurs most readily at the unit adjacent to molybdenum (145). As the reaction proceeds these signals strengthen at the expense of the ˛n protons, as also do those around 8 ppm (ˇn0 in 145). In 145 the newly formed double bond is cis. On heating to 50 °C for 90 min the cis double bonds are largely converted to trans giving rise to new carbene proton doublets at 11.96 and 11.85 ppm (˛00n in 146) and a new signal for the ˇn00 protons.

In 144 146 the alkylidene ligand lies in the N/Mo/C plane and may be oriented so that the growing chain points either toward the nitrogen atom in the imido ligand (syn rotamer) or away from the nitrogen atom in the imido ligand (anti rotamer). The syn rotamer predominates in each case but the anti rotamers are also detectable for 145 and 146. The carbene proton signals from the species corresponding to n 3 are also

resolved396. The rate constant for 144						!	145 at 25 °C is 1.06	ð	10 2	min 1	while that
for 145	! 146 is 1.80	ð 10	3	min	1
						, some six times smaller. These rate constants are

independent of solvent (C6D6 and THF-d8). When reaction is initiated by the tungsten complex the rate constants are somewhat smaller; also the rate of the retro-Diels-Alder reaction for the unit adjacent to the tungsten centre in the analogue of 144 is about ten times faster than for the unit that is further away401.

The ROMP of 136 may be used as the first stage in the preparation of polyacetylene molecules with mesogenic (liquid-crystalline) functional groups at the chain ends: the ROMP of 136 is initiated by a molybdenum carbene complex and the living ends terminated by reaction with a substituted benzaldehyde bearing a mesogenic group, followed