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								1557
	X	X
αn	[		]m
[Mo]				CMe3
βn
						X	X
X	X				X		X
(144) X = CF3 , m = n−1, n = 1, 2
		[Mo]	α′n	[				]m
									CMe3
							(145)
			β′n
α′′n
[Mo]							cis/trans
β′′n	[		]m	CMe3			isomerization
	(146)	X	X

by heating to effect the retro-Diels Alder reaction408. In a similar manner block copolymers, containing ring-opened units of 139, can be prepared and then heated to 220 °C to eliminate naphthalene, so yielding copolymers containing polyacetylene blocks407.

If it is desired to introduce a controlled proportion of cross-links during the ROMP, say, of cyclooctene, one may use a small proportion of syn-tricyclo[4.2.0.02,5]octa-3,7-diene, 133, as cross-linking agent, with catalysis by WCl6/Me4Sn. Both the double bonds in 133 react completely and, if sufficient 133 has been used, the cyclobutane rings which form the cross-links give rise to observable signals at 41.2 ppm in the 13C NMR spectrum and 3.5 ppm in the 1H NMR spectrum394. The physical properties of polyacetylenes prepared in these various ways have been closely studied402,403,406.

Monomers that contain (i) a cyclobutene ring substituted at the olefinic carbons by COOMe or CF3, and (ii) a norbornene ring system, undergo ROMP by preferential opening of the norbornene ring409.

2. Benzvalene and deltacyclene

The ROMP of benzvalene (147), equation 52, proceeds smoothly using tungsten carbene initiators, and films of the polymer (147P) can be cast directly from the reaction mixture410,411. The polymer has a tendency to cross-link and to decompose spontaneously once isolated in dry form, so is best handled in solution, especially as the decomposition can be explosive. The DSC thermogram of the polymer shows an exotherm at 153 °C, attributed mainly to isomerization to polyacetylene (148), and a second exotherm at 308 °C, of unknown origin. The polymer 147P made using 7W (R D Me) as initiator has a very simple three-line 13C NMR spectrum but it is not known whether the structure

1558	K. J. Ivin

is all-cis or all-trans. Polymers made with other initiators give spectra of greater complexity, probably due to partial isomerization to 148. For clean conversion of 147P to 148 it is best to treat freshly cast films with a 5% solution of HgCl2 in THF. The films turn red within seconds, to blue-green over the next 30 s, and finally to a black, silvery, shiny film within 2 3 min.

]

	ROMP	∆	[
	[	]
(147)	(147P)		(148)

(52)

The ROMP of deltacyclene (149), equation 53, leads to polymers (149P) of high MW, with a range of cis content depending on the catalyst: 27 35% for 7 (R D Ph), 60 70% for RuCl3/60 °C, 70% for WCl6/Ph4Sn and 100% for ReCl5412 414. The all-cis polymer can be fully epoxidized using dimethyldioxirane; with cis/trans polymers only the cis double bonds are epoxidized413.

ROMP	(53)
[	]
(149)	(149P)

3. Norbornene

The original discovery of the ring-opening polymerization of norbornene (equation 54) was made using TiCl4/LiAl(C7H15)4 as catalyst in an attempt to carry out Ziegler Natta polymerization (ZNP) of this monomer32. Since then the relationship between ZNP and ROMP has been a matter for considerable speculation415 418. In many cases the same catalyst system can induce ZNP of one olefin and ROMP of another. For example, WCl6/Bu4Sn (1/10) in hydrocarbon solvents initiates ZNP of ethene419 but ROMP of cyclopentene420. When these two olefins are reacted together with this catalyst only the two homopolymers are formed, indicating the presence of metal alkyl complexes propagating ZNP and metal carbene complexes independently propagating ROMP. Evidence for an equilibrium between a tungsten(IV) methyl complex and a tungsten(VI) methylene hydride complex (equation 55) was first provided by Cooper and Green421 423. The question then arises as to whether systems can be found where such species are in sufficiently rapid equilibrium for polymers to be formed containing both types of repeat unit in the same chain.

ROMP	[ CH	•	• CH ]	(54)

(150)

(150P)

24. Advances in the metathesis of olefins								1559
				H				(55)
[Mt]		CH2 P
				[Mt]			CHP

Consider, for example, the reaction of a metal alkyl complex [Mt] CH2P with norbornene (NBE) by the following sequence of events: (i) insertion of one molecule of NBE into the Mt C bond, (ii) migration of an ˛-hydrogen to the metal, (iii) addition of one molecule of NBE by metathesis with the MtDC bond, (iii) migration of the hydrogen atom on the metal back to the ˛-carbon so as to re-form the Mt C bond, (iv) insertion of a further molecule of NBE into the Mt C bond. The resulting structure 151 will contain two characteristic features at the junctions between the two types of unit, namely an olefinic carbon with no attached hydrogens (C-2) and a CH2 group between the rings (C-6).

[Mt]	6	5	4	H	2
	CH2			C

3

1

PCH2

(151)

Two catalysts have been found which, under limited conditions, give polymers containing both these features. With Mo(CO)5(py)/EtAlCl2/R4NCl as catalyst the polymer made at 26 °C has 100% ring-opened units, but when made at 110 °C, only 31% such units; and with ReCl(CO)5/EtAlCl2 as catalyst the polymer made at 100 °C contains 98.6% ring-opened units, falling to 5.1% when made at 110 °C and zero when made at 132 °C (i.e. 100% ZNP). These figures are based on the olefinic to aliphatic proton intensity ratio in the 1H NMR spectra which is not in itself proof that the two types of unit are present in the same chain, but the fact that the GPC of the polymer exhibits a single, relatively narrow peak is a strong indication that this is so. The conclusive evidence is as follows424. (i) The 13C NMR spectra show four resonances in the olefinic region (131.8, 132.3, 132.5 and 132.8 ppm) which disappear in the DEPT spectrum, showing that these carbons do not carry hydrogens and can therefore be assigned to C-2. The fine structure can be attributed to the four stereomers arising from the two possible configurations about C-3 (E and Z) and about C-1 (endo and exo). (ii) The product of ozonolysis of the polymer gives a resonance at 218.4 ppm consistent with the presence of an aliphatic ketone similar to that in norcamphor (216 ppm). (iii) When 2,3-dideuterionorbornene is used in place of NBE the hydrogens on C-6 in 151 are replaced by deuterium; C-6 can then be found in the 13C-f2Hg INEPT spectrum at 27 ppm.

Although most catalysts for the polymerization of NBE and other cycloalkenes give mainly ZNP or ROMP59, quite a number are known where the IR spectrum or the olefin/aliphatic proton ratio indicates a mixed product27. If subjected to examination along the above lines some of these might be found to have both types of unit in the same chain. Similar questions arise in the reactions of acyclic olefins on heterogeneous supported oxide and other catalysts, where olefin metathesis is often in competition with polymerization or homologation reactions. An IR study of the polymerization of ethene on a sulphatecontaining TiO2 (anatase) sample shows evidence of an alkylidene surface end-group of the polymer, and for substantial perturbation of CH2 groups of the polymer chain by interaction with the oxide surface425. However, for most Mo-based catalysts the evidence from isotopic labelling and selective poisoning experiments is that the different types of reaction proceed independently at distinct catalyst sites426 429.

1560 K. J. Ivin

In the rest of this section we report results on some of the more recently used catalysts for the ROMP of NBE, taken in Group order.

A number of titanacyclobutane complexes act as initiators of living ROMP of NBE321,430,431. Such initiators can be used to make block copolymers432 and adapted to the production of star-shaped polymers433. The living ends can be terminated with

benzophenone to yield a Ph2CD end-group9 or with terephthalaldehyde to yield a DCHC6H4-(CHO)-4 end-group434.

Tantalum carbene complexes such as 5 and Ta(DCHCMe3)(S C6H2-i-Pr3-2,4,6)3(py) are effective, provided the conditions are such as to allow the coordinated base (THF or py) to give way to monomer (M). In the first example the initially formed tantalacyclobutane complex has been isolated and shown to have a trigonal bipyramidal structure, and to polymerize NBE at a rate that is independent of [M]. In this case the rearrangement of the intermediate tantalacyclobutane complex, to form the tantalum carbene complex, controls the rate of polymerization. In contrast, in the second example the rate is first-order in monomer; here the reaction of the tantalum carbene complex with the monomer is the slower step. In both cases the polymer, after termination by reaction with benzaldehyde, is nearly monodisperse93,94.

Molybdenum carbene complexes like 7 are very effective for the ROMP of NBE341 and can be manipulated (i) with chain transfer agents such as penta-1,3-diene and styrene to reduce the MW without sacrificing the narrow distribution283, (ii) with norbornadiene dimer to produce star polymers and star-block copolymers126 and (iii) with substituted benzaldehydes as terminating agents, to give polymers with a variety of functionalized end-groups435; also to make other block copolymers407. The ROMP of NBE in THF initiated by 7 (R D Ph) is first-order in both monomer and initiator; kp D 4.3 M 1 s 1 at 20 °C, H‡p D 43 kJ mol 1, Sp‡ D 84 J K 1 mol 1436. With the same initiator

in toluene at 22 °C, and using Me3CCH						D	CH2 as transfer agent, kp		D	17 M 1 s 1	and
ktr		3		129
	D		ð	10 5 M 1 s 1	. A molybdenum carbene complex with a tridentate ligand
							437	.
[tris(pyrazolyl)borate] is effective in the presence of AlCl3

An interesting heterogeneous catalyst has been obtained by adding one drop of 1.8 M EtAlCl2 in toluene to a crystal (20 mg) of (Bu4N)2(Mo6O19); its surface colour changes from yellow to dark brown, corresponding to Mo(VI) ! Mo(V) ! Mo(IV). If a solution of NBE in toluene is added after 30 s there is instant polymerization to a gel, which is partially soluble in chloroform and contains 33% cis double bonds. On removing the crystal from the polymer with forceps its surface colour is restored by atmospheric oxidation and it can be used to repeat the process without loss of activity. Similar results are obtained with related tungstates438,439.

A variety of tungsten carbene catalysts has been used having (i) all monodentate ligands, of which two may be alkoxy or aryloxy100,102,112 114,307,341,440 442, or (ii) one bidentate

ligand101,103,104,443,444 or (iii) one tridentate ligand445. With W[DC(OMe)Ph](CO)5 there is evidence that one permanent ligand in the propagating species is derived from the original carbene ligand446.

Re2O7/Al2O3 gives an all-cis polymer, but when pretreated with Me4Sn gives a polymer with comparable proportions of cis and trans double bonds showing that a different active species must then be involved45. One rhenium carbene complex is reported to be active105.

The hydrate of Ru(OTs)2 is effective at 50 °C both in protic solvents giving high- trans polymers61,447, and in supercritical carbon dioxide to give a high-cis polymer448.

Arene complexes	of this salt are very	active when exposed to UV radiation449.
RuCl2(PPh3)2(py)2	and related complexes	are active even at 20 °C in the presence of

oxygen450. The monomer first undergoes catalytic epoxidation, followed by formation

24. Advances in the metathesis of olefins

1561

of an oxaruthenacyclobutane complex which can then generate the propagating carbene complex. Catalysis by OsO4 at 60 °C probably operates in a similar fashion451. Ethyl diazoacetate enhances the activity of ruthenium compounds for the ROMP of NBE346. The complex 19 polymerizes 142 equiv of NBE in CD2Cl2/C6D6 (1/4) at room temperature in less than a minute, giving a polymer with 86% trans double bonds, but very little of the initiator is used and the polymer has a broad MWD110. The complex 18 is somewhat less active and gives a living system; the carbene proton in the propagating species appears at

17.79 ppm in the 1H NMR spectrum108. These complexes are also active when supported on polystyrene452. The complexes Ru(DCHR)(Cl)2(PPh3)2 (R D Me, Et, Ph) are much more efficient (ki/kp D 9 when R D Ph compared with 0.006 for 18 and give polymers with very narrow MWD Mw/Mn D 1.04 60,91.

Dienes can have a significant effect on the course of reaction by coordinating to the

metal centre. Thus isoprene (6 ð 10 4 M) can completely suppress the formation of cyclic oligomers during the ROMP of NBE catalysed by WCl6/Me4Sn453. In the absence of isoprene, 40% of the product consists of oligomers containing from 2 to 14 monomer

units, as detected by GPC454. Again, RuCl3 which has been pretreated with DCPD gives a 95% cis polymer of NBE289, whereas RuCl3 alone gives a 5% cis polymer. For OsCl3, the effect is somewhat smaller, 85% cis and 29% cis respectively, while with IrCl3, the difference is very small, 43% and 36% cis respectively285,289. The ability of the doubly coordinated diene molecule to influence the relative ease of approach of the monomer to form cis and trans double bonds is thus a sensitive function of the nature of the metal. A similar effect is observed with certain bis(allyl)ruthenium(IV) complexes as catalysts which give polymers of NBE containing 30 90% cis double bonds455,456.

NMR spectra of the polymers of NBE are insensitive to ring tacticity but their fully hydrogenated derivatives show fine structure when the spectra are run under the most favourable conditions457.

4. Monosubstituted alkylnorbornenes

Me

Me

			Me
(152)	(153)	(154)	(155)	Me
Me	Me

(156)(157)

The ROMP of (š)-2-methylnorbornene (2-MNBE). (152) gives an all-HT polymer (Table 6). The HT bias in polymers of (š)-1-MNBE (153) depends markedly on the catalyst, probably reflecting the polarity of the MtDC bond101,302 304,308,321,441,442,458 ,

1562	K. J. Ivin

whereas (š)-exo-5-MNBE (154) and (š)-endo-5-MNBE (155) give virtually unbiased polymers310,459. As normally made 7-MNBE is a 50 : 50 mixture of the anti- and syn- isomers, 156 and 157 respectively. When this mixture is treated with WCl6/Me4Sn the anti-isomer is selectively polymerized and the syn-isomer can be recovered from the final reaction mixture in good yield320,460. The syn-methyl group, being close to the double bond, hinders the approach of the exo face of the monomer. More active initiators, such as 7, 10 and W(CO)3(mesitylene)/EtAlCl2/epoxide, are less discriminating, and although the anti-isomer still reacts preferentially at the beginning of reaction, the syn-isomer reacts

later to give its own distinctive propagating species, detectable by 1H NMR; the product is then a block or tapered-block copolymer of the two isomers128,307,320,322.

5. Norbornenes with a 5-(Si-containing) substituent

The ROMP of norbornenes with the following substituents (endo/exo mixtures) has been reported91,461 465: SiMe3, SiMe2(CH2SiMe3), SiMe2[(CH2)3-9-carbazolyl], CH2Si(Me)(CH2CH2CH2), SiMeCl2, SiCl3, Si(OMe)3 and Si(OEt)3. For the ROMP of the Si(OR)3 monomers it is best to use Lewis-acid-free catalysts such as RuCl2(PPh3)3 at 60 °C464 or a molybdenum carbene complex95. The SiMe3-containing polymer has considerably enhanced permeability and diffusion coefficients for light gases, compared with polynorbornene463. The polymer containing carbazolyl groups can form charge transfer complexes with acceptors such as 2,4,7-trinitro-9-fluorenone464.

6. Norbornenes with a 5-(COOR) substituent

Monomers in this category fall into three groups: (i) those with small substituents, giving polymers which may be thermoplastic466; (ii) those with substituents of intermediate size and of such a character that the polymers can have liquid-crystalline or other interesting properties; and (iii) macromonomers in which a polymer is terminated by a norbornenyl group and which can be used to make graft copolymers by ROMP. Except where stated the monomers are endo/exo mixtures.

The first group of monomers includes those with the following substituents: endo- COOMe (158), endo-COOEt, exo-COOMe (159), exo-COOEt and COOC6H2Br3. All types of metathesis catalyst are effective467, including metal carbene complexes127 and RuCl2(PR3)(p-cymene)/Me3SiCH2N262. The 13C NMR spectra indicate that the substituents are randomly oriented in the polymers. The ethyl ester polymers can also be produced by heating the carboxylic acid monomer with IrCl3 in ethanol467.

The reaction of 158 with a tungsten cyclopentylidene initiator has been followed in CD2Cl2 at 27 °C by 1H NMR. The initiator is all consumed in 15 min and the monomer (3 equiv) in about an hour. The head and tail propagating species give separate carbene proton signals, and the head species formed by addition of one unit of monomer may be distinguished from the head species containing more than one monomer unit. The final concentration of head species is nearly twice that of the tail species, indicating a somewhat lower overall reactivity of the head species in the propagation steps. With 159 as monomer the spectra of the head and tail propagating species are indistiguishable, although the first addition product gives a distinct spectrum127.

In the second group some monomers (160 162) have been polymerized by RuCl3468, and others in a more controlled fashion by 7 or related complexes: 163469,470, 164471, 165 167472,473. Both 160 and its polymer form monolayers, but the polymer exhibits a higher collapse pressure and reduced collapse area compared with the monomer. The

24. Advances in the metathesis of olefins

1563

	COO(CH2 )11O	NO2
	COO(CH2 )16 H
(160)	(161)

COO(CH2 )nO

X

(162)X = NO2 , n = 11

(163)X = OMe, n = 2−12

(164)X = CN, n = 3−12

	OOC	O(CH2 )nH
COOCH2
(165)	n = 1−6
COO	OOC	OMe
CH2 OOC		OMe
(166)
	OOC	OMe
COO
CH2 OOC		OMe

(167)

1564	K. J. Ivin

hexadecyl chains must evidently be able to pack more closely in the monolayer of polymer. The polymers of 161 and 162 do not form well-defined monolayers.

With 163 and 164 not only has the spacer length n been varied, but a series of polymers of narrow MWD (Mw/Mn D 1.05 1.28, DP D 5 100) has been prepared in high yield through the use of living systems. The polymers of 163 (n D 2 8) exhibit an enantiotropic nematic mesophase. The glass transition and isotropization temperatures increase with increasing MW and reach a limit at about 30 50 repeat units (30 °C and 90 °C respectively for n D 8). With longer spacers (n D 9 12) some side-chain crystallization is observed in the lower-molecular-weight materials (DP D 10 20), along with a nematic or smectic mesophase, but this is suppressed at higher MW (DP D 50 100). The polymer of 164 (n D 3) is amorphous whereas with longer spacers (n D 4 12) the polymers display enantiotropic nematic mesophases that are independent of MW, and no side-chain crystallization is observed. Similar results have been obtained with the polymers of 165 167. The phase behaviour becomes independent of MW at about 25 repeat units and the transition temperatures decrease with increasing n in the polymers of 165. The polymer backbone in fact has little effect on the transition temperatures of laterally attached side-chain liquid-crystalline polymers displaying nematic mesophases, even when the chemical structures of the backbones are substantially different, as with polynorbornenes vs polyacrylates. This result is consistent with the proposal that mesogens jacket the extended polymer chain472. Monomers containing a long 5-substituent and bearing a methacrylate end-group have also been prepared and polymerized to give products with potential electro-optic applications474.

The third group in this category is exemplified by the macromonomer 168 which can be made by first initiating the anionic polymerization of styrene with s-BuLi, endcapping with ethylene oxide and then reacting with norborn-2-ene-5-carbonyl chloride475. It can then be copolymerized with norbornene using WCl6/Me4Sn as catalyst to give poly(norbornene-graft-styrene) copolymers. The ROMP of the macromonomer itself (Mn D 2700 11,000) proceeds to high conversion only when initiated by a molybdenum carbene complex, yielding comb-like polymers of high MW. The solution behaviour of these polymacromonomers is very dependent on both the MW of the monomer, which governs the length of the polystyrene side-chains, and that of the final polymer476.

COOCH2 CH2 [CH(C6 H5)CH2 ]nCHEtMe

(168)

7. Norbornenes with a 5-(OCOR) substituent

Monomers of this kind, with R D Me (169), Pr (170), Ph (171), readily undergo ROMP with all types of catalyst83,316,317,477 479. W(DCHCMe3)Cl(CH2CMe3)(OAr)2(O-i-Pr2)

induces complete polymerization of (š)-endo-169 in 10 s at 25 °C83. With ( )-endo-169 initiated by 7 (R D Ph) a 30% cis polymer is formed which is nearly atactic with respect to both cis- and trans-centred ring dyads, but when initiated by 8 (R D Ph) the polymer has a much higher cis content (87%) and the cis-centred dyads are biased towards isotactic, as shown by the inequality, TH > TT, of the olefinic signals in the 13C NMR spectrum316. The 87% cis polymer has nearly twice the specific rotation of the 30% cis polymer. The enantiomers of endo-170 and endo-171 likewise give optically active polymers317.

			24. Advances in the metathesis of olefins			1565
8. Norbornenes with 5-substituents containing OH or OR
	Norbornenes with the following 5-substituents (X) undergo ROMP: X				D	HOCH 480;
X	D MeO	481	315			2
			; X D MeOCH2	and a derivative in which the methyl group is replaced

by 6-N-carbazoylhexyl (the carbazole units in the side chains of the polymer can be oxidatively coupled to form dicarbazyls)482; X D endo-MeO(CH2)2, which has been used to make block copolymers offering potential for binding zinc and cadmium compounds through the oxygen donors483; X D MeOC6H4C6H4O CH2 5, yielding polymers (Mn D 31 130 ð 103) that show an isotropic/smectic transition at about 70 °C on cooling484.

9. Norbornenes with 5-substituents containing CN or halogen

Norbornenes with the following 5-substituents (X) undergo ROMP: X D

CN83,126,477,485 487 (optical discs can be made from the polymer, having a heat distortion temperature of 145 °C)466,488,489; X D CF3 (171a) giving polymer with Mw/Mn D 1.09490; X D BrCF2CF2485; X D perfluoroalkyl491; X D Cl465; X D ClCH2465,492.

In ROMP it is sometimes found that the GPC of the product shows two peaks: a main peak and a much smaller peak at twice the MW of the main peak; for example, in the ROMP of exo-dicyclopentadiene432. The reason for this was first elucidated in the living polymerization of endo/exo-171a initiated by 7 (R D Me). If, after the monomer has polymerized, a trace of oxygen is present there is premature termination of some of the living ends to form an aldeyde end-group, which in turn terminates a second chain, to which it becomes attached. If benzaldehyde is then added, the main portion of the living ends react to give a benzylidene end-group. The GPC of the product shows the aforementioned two peaks if an RI detector is used, but only the main peak if a UV detector, set to detect absorption by the aromatic ring, is used493.

10. Other 5-substituted norbornenes

A series of glycomonomers represented as NBE-exo-5-CONH(glu)R4 has been prepared by the reaction of norborn-2-ene-exo-5-carbonyl chloride with glucosamine hydrochloride; R D H or a protecting group (COMe, COPh or SiEt3). These monomers all undergo ROMP when initiated by 19 in benzene at 50 °C. The reaction of the unprotected monomer proceeds quantitatively when carried out in aqueous emulsion but the polymer is rather insoluble in all solvents, unlike the polymers of the protected monomers. In principle this range of monomers can be extended to include those attached to biologically relevant carbohydrates via flexible extenders494.

The monomer 172 can be made by hydroboration of norborna-2,5-diene with 9- borabicyclononane (9-BBN) using excess of diene. It undergoes ROMP to yield a polymer 173 which can be readily oxidized to the hydroxy derivative 174. 174 is insoluble in water and common organic solvents, but soluble in mixed solvents such as CDCl3/CD3OD. The cis content varies with the catalyst: 93% with 8W (R D Me), 48% with WCl6/Me4Sn and 26% with 7W. The high-cis polymer has a relatively simple spectrum, with two main pairs of equally intense olefinic lines: υ 135.36 (TH), 134.00 (TT), 133.24 (HH), 131.88 (HT), indicating a random orientation of the substituents. The corresponding signals for the trans-centred dyads may be seen in the spectra of the polymers obtained with the other two catalysts, but with fine structure caused by tt/tc splitting: υ 134.42 (TH), 132.90 (TT), 132.05 (HH), 130.78 (HT)495.

The hydrophilic (OH) groups in these otherwise hydrophobic polymers allow the formation of stable monolayers at an air/water interface. The high-cis polymer occupies 38 A˚ 2/monomer unit which is much larger than the 9 A˚ 2/monomer unit occupied by the

1566	K. J. Ivin
			B
	ROMP	•	•
B	[ CH			CH ]
(172)		(173)
		H2 O2	NaOH
			OH
	[ CH	•	•	CH ]
		(174)

74% trans polymer, suggesting that the cis polymer is rather rigid and lies stretched on the water surface with most of its OH groups at the interface. The Tg values for these polymers are in the range 118 140 °C, much higher than those for polymers of norbornene

(5 °C)495,496.

Hydrogenation of polymers of substituted norbornenes is frequently a valuable aid to the determination of structural detail and is best carried out using diimide (NHDNH) generated in situ by the decomposition of p-toluenesulphonohydrazide in xylene at 120 °C. This procedure works well and selectively even in the presence of groups such as COOMe and PPh2497.

11. 5,5-Disubstituted norbornenes

Some systems which have been recently studied are summarized in Table 8. Polymers of the racemic monomers show little sign of HT bias for any value of the cis double-bond

TABLE 8. ROMP of some 5,5-disubstituted norbornenes (substituents exo-

X, endo-Y)

X	Y	Catalyst system		Reference
CH3	CH3	a		307
		[W](DCHCMe3)b
		[W](DCHCMe3)	c	442
		W(DZ)(Cl)2(OArO)(THF) c		498
		W(DZ)(Cl)3(OArO)(OEt2)		327
COOCH3	CH3	Various		467
CH2Cl	CH2Cl	Various		492
Spiro compoundsd		W-based		499

aW(DCHCMe3)(Br)2(OCH2CMe3)2/GaBr3; the head propagating species is present in higher concentration than the tail species and is thus the less reactive.

bSeven different complexes.

c X D O or NC6H3-Me2-2,6; cocatalyst Et2AlCl. The cis content of the polymer varies from 41 to 100% depending on the nature of the chelating diolate ligand.

dNorborn-2-ene-5-spiro-30 -exo-succinic anhydride, and norborn-2-ene-5-spiro-30 -exo-N- phenylsuccinimide and derivatives.