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

1577

Me

(225)

(226)

(227)

R1

R2

(228)	(229)	R1	= R2	= Me
	(230)	R1	= Me, R2 = Ph
	(231)	R1	= R2	= Ph

			Me
	F	Me	F
CF3		F	F
CF3		F	F
	F		F
(232)	(233)		(234)

All the above polymers containing fused aromatic rings are susceptible to photoxidation but remain stable at low temperature in the absence of air and light. They can also be dehydrogenated to yield conjugated polymers.

16. Dicyclopentadienes

The ROMP of the common endo isomer 235 was first reported in the late sixties. Since then numerous other catalysts have been shown to be effective. More recently reported

catalysts include titanacyclobutane complexes432,434,561, IrCl3 and RuCl3516, ReCl5516,562, various WCln(OR)6 n compounds55,563 566, the robust complex Mo(DNC6H3-i-Pr2-

2,6)(DNCMe3)(CH2CMe3)2 in conjunction with a phenolic activator567, and various polymetallates54. Since the eighties the ROMP of 235 has been developed into a commercial process for the production of quite large objects (300 kg) by reaction injection moulding. The exo-isomer 236 also readily undergoes ROMP.

In many cases the polymer formed is only partially soluble in organic solvents, but sometimes it is fully soluble, particularly if [M]0 is not too high, or if a chain transfer agent has been used to restrict the MW. Insolubility of the polymer indicates a certain degree of cross-linking and the question arises as to how this comes about, in particular whether the double bonds in the rings of 235P and 236P can also open by metathesis. The opening of these double bonds, situated in a disubstituted cyclopentene ring, is thermodynamically much less favourable than the opening of the double bond in the norbornene ring system. However, there is good evidence that there is a critical concentration above which these

1578										K. J. Ivin
10										8								8
		5							7								7
4											9							9
		6
									6 •		• 1				[H]			• •1
				ROMP			4										6
												3							3
2			7												4
									•		•						•	•		]
3		1					[ CH					CH ]				[ CH			CH
									5		2				2		5	2	2
		9	8							10								10
(235)										(235P)								(235PH)
10												8
		5									7		9
4		6	7									6	1
					ROMP					4					3
			8									•	•
2										[ CH					CH ]
3		1	9									5	2
												10
		(236)										(236P)
									8			[H]
							7			9
			4				6			1	3
							•			•
				[ CH2							CH2]
							5			2
							10

(236PH)

double bonds do in fact undergo the metathesis reaction thereby giving rise to cross-links. The best documented case is that of the ROMP of 236 initiated by various tungsten or molybdenum carbene complexes, I. For [I]0 D 0.005 M in toluene at 70 °C and [M]0 up to 1.0 M the polymer is completely soluble and the polymerization living (Mw/Mn D 1.14); but with neat monomer (7 M) the product consists of about 50% insoluble polymer, 25% soluble polymer and 25% unreacted monomer. If the soluble polymer is isolated, dissolved in fresh initiator solution and gradually concentrated by evaporation, up to 35% insoluble material is obtained. On the other hand, if the insoluble polymer is treated with fresh initiator solution for 36 h, 70% goes into solution and this soluble product is identical with the original soluble fraction561. The opening and closing of the second double bond is thus reversible and the behaviour is parallel to that observed for the polymer of norbornadiene; see Section VIII.B.2. For the ROMP of 235 catalysed by ReCl5/Me4Sn in CCl4 at 50 °C the critical concentration for the opening of the second double bond appears to be close to that of the neat monomer as judged by the change of viscosity as the reaction proceeds; but at 60 °C the critical concentration appears to be not much greater than 0.3 M562. Cross-linking may also occur by cationic reactions through the second double bond, especially for those catalyst systems known to generate acidic species. Such cross-linking would not be subject to a critical concentration effect and would not be reversible.

The aforesaid ReCl5/Me4Sn-catalysed ROMP of 235 (3 M in CCl4) is notable in that within a few minutes the main part of the monomer is transformed into oligomers (presumably cyclic), while the formation of high polymer begins slowly and reaches an

24. Advances in the metathesis of olefins

1579

asymptotic value corresponding to 50% conversion of monomer after 4 h. The concentration of oligomers passes through a maximum and reaches an ultimate equilibrium value corresponding to 30% of the original monomer562.

With a titanacyclobutane complex as initiator the rate of polymerization is independent of [M] and therefore governed by the rate of opening of the titanacyclobutane chain carrier432. The comparative stability of the metallacyclobutane bearing an endo substituent is a feature which is also found in the ROMP of other substituted norbornenes; see Section VIII.C.12. In contrast, the rate of ROMP of 235 induced by ReCl5/Me4Sn (1/1.5) is first-order in both catalyst and monomer and here the addition of monomer to the metal carbene complex must be rate-determining562.

The cis content of the C3DC4 bonds in 235P can be varied from 19% using IrCl3 as catalyst to 100% using RuCl3 or ReCl5 as catalyst. The high cis content with RuCl3 is attributed to the ability of the monomer to provide a chelated spectator ligand in which both double bonds are coordinated to the metal centre, favouring the approach of the reacting monomer molecule which leads first to the formation of a cisoid metallacyclobutane complex and then to a cis double bond. This effect is not found with the exo-isomer because the two double bonds cannot coordinate simultaneously to the metal centre. In this case RuCl3 gives a 10% cis polymer 236P while ReCl5 still gives a 100% cis polymer. The 13C NMR spectra of 235P, 236P and their fully hydrogenated products 235PH and 236PH show that the repeating units are randomly oriented in the chains. Tacticity splittings (m/r) are observed for C-3 and C-4 in high-trans 235P, for C-2,5 in 235PH and for C-2,5, C-3,4 and C-10 in 236PH. Most of the polymers appear to be atactic except for those made with ReCl5516.

Compounds of the type 237 and 238, having two double bonds of equal reactivity, have been used to assist cross-linking during the ROMP of 235568. Star-block copolymers can also be made through the controlled use of 238126. For other polycyclic compounds containing two double bonds (but only one reactive) see elsewhere520,521.

(237)

(238)

17. Bicyclo[2.2.1] compounds containing heteroatoms in the ring system

7-Oxanorbornene and its derivatives, 239 266, have been intensively studied over the past few years, the polymers being potential complexing agents for metal ions. All are capable of ROMP if the catalyst is properly chosen: 239319; 240, 259, 260,318; 241318,569;

242318,319; 243, 255501; 244570; 245, 246, 263115; 247571,572; 248279,318,319,570 577; 249, 250, 257, 261, 262572; 251578; 252449; 25362,449,578 582; 254449; 256583; 258572,576; 264572,576,584,585; 265584; and 266586,587.

Most of the work has been done with RuCl3, OsCl3 or IrCl3 as catalysts at 50 80 °C in water, aqueous emulsion, an aromatic solvent, or mixtures of an alcohol and water. Tungsten or molybdenum carbene complexes in toluene are effective at 20 °C with monomers that do not contain hydroxyl groups. Thus 8W (R D Me) gives polymers of very high

1580	K. J. Ivin
O		O
(239)	X = H				(244) Y = CH2 OMe
(240)	X = Me
					(245) Y = OCOMe
(241)	X = CH2 OH
				Y (246) Y = OC(			CH2 )Me
(242)	X = endo-CH2 OMe
X (243)	X = exo-COO(CH2 CH2 O)2 Me
			Y

(239− 243)

O

(247) X = Y = CH2 OH X (248) X = Y = CH2 OMe

(249) X = Y = CH2 OCOMe

Y(250) X = Y = CH2 OSiMe3

(251) X = Y = COOH

(247− 258)	(252) X = Y = COOMe
O R	O
	CH2 OMe
	CH2 OMe

(244− 246)

(253)X = Y = COOCH2 Ph

(254)X = Y = COO(CH2 )6 H

(255)X = Y = COO(CH2 )16 H

(256)X = Y = OCH2 OMe

(257)X = CH2 OH, Y = CH2 OMe

(258)X = COOMe, Y = COOH

O

O

X

X

			Y		O
			(261− 263)	(264− 266)
(259)	R = Me	(261)	X = Y = CH2 OMe	(264)	X = O
(260)	R = Et	(262)	X = Y = CH2 OCOMe	(265)	X = NH
		(263)	X = Y = CN	(266)	X = NMe

cis content with 239, 242 and 248, while 7 (R D Me) gives a living polymer (60% cis) of 245. However, 7 (R D Ph) with 263 gives only a 2:1 mixture of the two initial transoid metallacyclobutane square-pyramidal complexes (in which the cyano group nearest to the metal is in either the endo or exo position). The more powerful initiator Mo(DCHCMe2Ph)(DNAr)(OCMe2CF3)2 brings about the formation of a living polymer of 263, having Tg D 193 °C and decomposing above 240 °C.

The high-cis polymer of 242 (see above), when made from enantiomeric monomer, has a mainly HH, TT structure and is therefore largely syndiotactic. On the other hand, the 96% trans polymer made from enantiomeric monomer with [RuCl( -Cl)( 3: 3-C10H16)]2 as catalyst (C10H16 D 2,7-dimethyloctadienediyl) has an HT structure and is therefore essentially isotactic. These tacticities are as predicted from the pseudo-octahedral model if the ligands are not labile and one site is available for coordination of monomer319; see Section VIII.A.5.

With RuCl3 as catalyst the cis content of the polymer is often markedly dependent on the solvent, for example with 248318, 252581 and 261572. Another feature of catalysis by RuCl3 is that polymerization is generally preceded by an induction period (IP) during which the concentration of propagating ruthenium carbene species is building up to a stationary state. The length of the IP is very dependent on the solvent. Thus with 247 in chlorobenzene/ethanol at 55 °C it is 2 3 days, but in water it is only about 30 min; and if the same solution is used to initiate the ROMP of further batches of monomer the IP drops to 10 s. There is good evidence that initiation proceeds through an Ru(II)-complex with the monomer89,571.

24. Advances in the metathesis of olefins

1581

Derivatives of acyclic olefins can be used as chain transfer agents in these polymerizations. The most effective are those with a terminal double bond. For example, in the ROMP of 248 catalysed by [Ru(H2O)6](OTs)2 the transfer constant (ktr/kp) for CH2DCHCH2CH2OH is 0.21. The size of the polymer particles produced by emulsion polymerization of 248, using RuCl3 with a non-ionic surfactant, is of the order of 0.03 m577.

In many cases side reactions are liable to occur during the ROMP of these monomers, for example esterification and trans-esterification when the ROMP of carboxylic acids, anhydrides or esters are carried out in solvents containing alcohols; or hydrolysis when carried out in water572,576,579. Retro-Diels-Alder reactions can also be a problem. Thus, although 266 is cleanly polymerized to high conversion by [Ru(H2O)6](OTs)2 under mild conditions (55 °C), its endo-isomer fails to polymerize because the retro-Diels-Alder reaction produces N-methylmaleimide which complexes with the catalyst and puts it out of action. The exo-N-phenyl analogue is likewise unable to polymerize587.

Much interest centres on whether these polyfuran derivatives can behave like crown ethers. The polymer of 248 does in fact coordinate alkali metal ions. The flexible binding cavities formed by this polymer also allow it to complex preferentially with large polyaromatic cationic dyes such as methylene blue and rhodamine 6G318. Analogues of 248, in which the methyl groups are replaced by (CH2)mCH3 (m D 9, 13, 15, 17, 19, 21), also undergo ROMP when heated with RuCl3 in ethanol. For m D 15 the polymer has much the same value of [ ] in toluene as in THF indicating similar hydrodynamic behaviour of the polymer molecules in the two solvents, and therefore most probably a coil rather than helical conformation, contrary to previous suggestions588.

The synthesis of an agglutination inhibitor by aqueous ROMP has been successfully accomplished. Cell-surface oligosaccharides have been implicated as key participants in many intercellular recognition events, and the ROMP of suitable monomers with carbohydrate substituents offers a means of obtaining polymers of potential recognition capacity. A diester of 251 has been prepared in which the ester groups are COO(CH2)2(glu), where (glu) is an ˛-glucose substituent attached via a C glycoside linkage. The polymer, made using RuCl3 in water as catalyst, is soluble in water and is 2000 times as effective as the monomer in inhibiting erythrocyte agglutination by concanavalin A (Con A, a carbohydrate-binding protein). The application of ROMP to the synthesis of these polyglycomers offers new opportunities for the design of materials for modulation of cell adhesion, immobilization of particular cell types and study of multivalency in extracellular

interactions589.					derivatives 267		272 has been reported:
The ROMP of the 7-oxanorbornadiene
267115,584,590			592; 268, 269591; 270, 271115,449; 272449.
O				O		O
			COOR		CF3
							R
			COOR		CF3
(267)	R = Me			(270)			(271) R = H
(268)	R = Et						(272) R = Me
(269)	R = Pr

1582	K. J. Ivin
RuCl3	in PhCl/EtOH at 100 °C gives a 93% trans polymer of 267, while

MoCl5/Me4Sn/Et2O gives a 90% cis polymer. Only the unsubstituted double bond is broken, not the 1,4-epoxide ring. The high-trans polymer 267P is readily dehydrogenated by refluxing in benzene with a stoichiometric amount of dichlorodicyanobenzoquinone (DDQ) to afford the soluble, purple-red ( max D 460 nm), conjugated polyene 273, having an estimated conjugation length of 10 double bonds (equation 58). This material is moderately paramagnetic and gives a symmetrical ESR signal with g D 2.0027. The high-cis polymer is less readily dehydrogenated and gives a red product solution, but UVirradiation of this solution causes the colour to change quickly to purple-red as a result of cis ! trans isomerization. These polymers are readily hydrolysed to the corresponding polymers of the disodium carboxylate, the UV/vis spectra of which are dependent on the cis content, pH, state (solution or film) and sample history. This is to be attributed to the dependence of the conjugation length on these variables592. The dehydrogenation reaction succeeds only for polymers in which the enchained 5-membered rings are already unsaturated; it does not work for the analogous polymer of 252.

MeOOC	COOMe		MeOOC	COOMe
		DDQ				(58)
[ CH	CH ]		[ CH	CH	]
	O			O
	(267P)			(273)

Unlike the ROMP of 252, that of 267 catalysed by RuCl3 in PhCl proceeds without an induction period. In the series of esters, 267 269, the rate declines with increasing bulk of the ester group while the cis content of the polymer goes up from 15 to 22 to 26%. The presence of the 7-oxa group enhances the reactivity591.

The monomers 267, 270 and 271 react with 7 (R D Me) to give the relatively stable transoid metallacyclobutanes. That derived from 270 is the most stable. X-ray studies show that it is square-pyramidal with the 7-oxa atom 3.33 A˚ from the Mo centre, too great a distance for bonding. The comparative stability of the metallacycles formed from these monomers is therefore ascribed mainly to inductive effects. The metallacycle formed from 270 opens slowly (k D 4.2 ð 10 5 s 1 at 35 °C in benzene) to yield two distinct metal carbene complexes, in a constant 9:1 ratio, with carbene proton resonances at υ 11.233 and 11.072, assigned to the syn and anti rotamers respectively, at equilibrium. For the reaction of 271 initiated by 7 (R D Me), the molybdenum carbene complex formed by the rapid rearrangement of the initial metallacyclobutane (60% conversion in 1 h at 0 °C) adds further monomer to yield long-chain living polymer (Mw/Mn D 1.06). The ROMPs of both 267 and 270 can be better achieved by initiation with the more active Mo(DCHCMe2Ph)(DNAr)(OCMe2CF3)2 in CH2Cl2; kp/ki D 2.4 for 270115.

Competition experiments show that 270 reacts more than 50 times faster with 7 (R D Ph) than does its norbornadiene analogue (219). The oxygen in the 7-position enhances the activity presumably by assisting the initial coordination of the monomer to the molybdenum centre, although it is released once the metallacycle has formed. Further competition experiments have shown that 270 also reacts about 30 times faster than norbornene with 7 (R D Me), but that when norbornene does react, the resulting metal carbene adds norbornene much more readily than 270. When a molecule of 270 does manage to add to the growing chain to form the metallacycle, further addition of monomer is effectively halted115.

24. Advances in the metathesis of olefins

1583

The ROMP of 274 is catalysed by WCl6/Et3Al (1/4) in PhCl at 60 °C (34% yield). The 13C NMR spectrum of the polymer in CDCl3 shows only one line for each carbon, which indicates that the polymer is certainly all-HT with one dominant double-bond configuration593. The ROMP of 275 using a W-based catalyst at 70 °C gives an 11% yield of soluble polymer, Mn D 5240, but if the benzyl group is replaced by methyl it fails to polymerize499.

	O
NH	NCH2 Ph
(274)	(275)

18. Miscellaneous

The ROMP of bicyclo[2.2.2]oct-2-ene 276 (equation 59) requires attack on the steric equivalent of the endo face of norbornene. ROMP can in fact be brought about in chlorobenzene at 20 °C by W(CO)3(mesitylene)/EtAlCl2/2,3-epoxynorbornane containing a trace of norbornene and Me4Sn. The 13C NMR spectrum of the polymer is entirely consistent with the ring-opened structure 276P. The double bonds are 34% cis and the c/t distribution is somewhat blocky. The main chain bonds attached to the rings must necessarily have a formal cis relationship, as in the monomer. For a chair configuration this corresponds to an axial/equatorial relationship. The absence of a/e fine structure in the 13C NMR spectrum shows that either there is rapid ae $ ea interconversion or that the rings exist in an intermediate twist-boat conformation594.

ROMP

(276)

[ CH •	•	(59)
		CH ]
	(276P)

The ROMP of 277 proceeds readily at 25 °C in CD2Cl2 when initiated by Mo(DCHCMe2Ph)(DNC6H3-i-Pr2-2,6)(OCMe2CF3)2, giving a living polymer which can be terminated by capping with benzaldehyde (equation 60). The carbene proton of the propagating species exhibits a doublet at υ 12.69. The reaction proceeds much more slowly in THF. In both solvents propagation is somewhat faster than initiation. Approximately equal proportions of cis and trans bonds occur between the rings in 277P. Reaction is presumed to occur on the face of the monomer that does not bear substituents, but whether the opening of one double bond is preferred over the other is not clear. On heating a film of 277P to 280 °C under a flow of argon it loses carbon dioxide and methanol to afford a yellow film of poly(1,4-phenylenevinylene) (278). The pyrolysis temperature can be reduced to 80 °C by the addition of catalytic amounts of tri-n- octylamine595.

1584	K. J. Ivin
		MeO(CO)O		O(CO)OMe
O(CO)OMe	ROMP	[ CH •		• CH ]
O(CO)OMe
(277)			(277P)	(60)
		− 2 CO2
		− 2 MeOH	∆
[ CH		CH ]

(278)

The bicyclo[3.2.1] compounds 279 283 have a structure derived from cyclopentene and contain a second double bond in the three-atom bridge. Only 279 281 undergo ROMP, catalysed by WCl6/Me4Sn or MoCl5/Me4Sn. The failure of 282 and 283 to polymerize is not due to any poisoning of the catalyst since it remains capable of polymerizing norbornene. Rather it is to be attributed to the adverse effect of the endo chlorine substituent514.

	Me
	Me
	Cl
Cl		Cl	Cl
			Cl
(279)	(280)		(281) (exo)
		Cl
Cl			Cl
Cl	Cl	Cl
(282) (endo)	(283)

IX. COPOLYMERIZATION

There are several ways of using the olefin metathesis reaction to generate copolymers. Occasional reference has been made earlier to the formation of copolymers. Here we give further illustrations. For the ADMET copolymerization of linear dienes, see Section VII.C.

A. Direct Metathesis Copolymerization

In the 13C NMR spectrum of a statistical copolymer of monomers M1 and M2 four groups of olefinic peaks may be seen, corresponding to M1M1 and M2M2 dyads, and

24. Advances in the metathesis of olefins

1585

the two types of olefinic carbon in an M1M2 dyad, designated M1M2 and M2M1 and necessarily equal in number. Provided that the copolymer is made at low conversion the reactivity ratios r1 and r2 can be determined directly from such a spectrum, knowing the feed composition. Experimentally no distinction can be made between M1M2 and M2M1 dyads formed in the cross-propagation reactions P1 C M2 ! P2 and P2 C M1 ! P1 respectively. Detailed studies have shown that the copolymer composition formed from a given feed, and hence r1 and r2 for a given pair of monomers, are markedly dependent not only on the catalyst system but sometimes on the method of mixing. Thus, for cyclopentene (M1)/norbornene (M2) with various catalysts, r1 values vary from 0.07 to 0.62 and r2 values from 2.6 to 70. The copolymers are often compositionally blocky (e.g. r1r2 D 3 for WCl6/Bu4Sn catalysis), but sometimes the distribution of monomer units is close to ideal (Bernoullian), with (M1M1) : (M1M2) : M2M2 D x2 : 2x 1 x : 1 x 2, where x is the fraction of M1 units in the copolymer. This is the case for WCl6/Ph4Sn catalysis596. Statistical copolymers of cyclopentene and norbornene can also be obtained using Ru(OTs)2(OH2)6 as catalyst but only homopolymer of M2 is obtained with 18 as catalyst (Table 2). However, with 19 as catalyst a copolymer is again obtained597.

There are many factors determining the overall composition of copolymers made in this way and the experimental reactivity ratios can only be regarded as overall average values. To represent the propagating species as P1 and P2, according to whether the last unit added was M1 or M2, may clearly be an oversimplification. Apart from the fact that any given catalyst system may generate more than one type of initiating species, we have the possibility that each of these may exist in more than one conformation and/or configuration in which the previously formed double bond may or may not be coordinated to the metal site at the moment of reaction with the next monomer molecule. Penultimate unit effects, including the configuration of the previous double bond, may also influence the reactivity, and the reversible nature of the propagation reaction may also sometimes have to be taken into account, particularly for the addition of cyclopentene (M1) to the propagating species P1 above 0 °C. Some of these factors have been discussed by Efimov598,599.

Some apparent reactivity ratios for various pairs of monomers have been summarized27. Broadly speaking, reactivities of monomers towards a given propagating species lie in the order: cyclobutene derivatives > norbornene and its 5-substituted derivatives > cyclopentene and larger rings. Thus, for the reaction of 5-norbornen-2-yl acetate (M1) with cyclooctene (M2), catalysed by WCl6/Me4Sn, r1 D 1/r2 D 132, whereas for its reaction with norbornene (M2) or with dicyclopentadiene (M2), r1 ³ r2 ³ 1479,600. Likewise the reactivity ratios for the copolymerization of norbornadiene and norbornene, catalysed by OsCl3, are close to unity278, as also are those for the copolymerization of cyclooctatetraene and cycloocta-1,5-diene, catalysed by 8W (R D Me)360. Deltacyclene (149) is 2.4 times as reactive as norbornene when catalysed by RuCl3601. Alternating copolymers, corresponding to r1 D r2 D 0, have not been found in ROMP except in the special case of the alternating copolymerization of the enantiomers of 1-methylnorbornene, catalysed by ReCl5 to give an all-cis, all-HT polymer; see Section VIII.A.5.

In a living system, if M1 is much more reactive than M2 and polymerization is allowed to proceed to completion, the end-product is a tapered block copolymer, in which only the middle section contains units of both monomers, e.g. with anti-7- methylnorbornene (M1)/syn-7-methylnorbornene (M2), see Section VIII.C.4; also with norbornene (M1)/cyclooctatetraene (M2), catalysed by 8W (R D Me)360. In the extreme case the cross-propagation reactions may be so slight that the product is indistinguishable from a perfect block copolymer, e.g. with bicyclo[3.2.0]hept-2-ene (M1)/norbornene (M2) catalysed by 18109,597, or with anti-7-methylnorbornene (M1)/syn-7-methylnorbornene (M2), catalysed by 7 (R D Me)128. The successive polymerization of the two monomers can be readily followed by 1H NMR.

1586	K. J. Ivin

For 5- or 5,6-substituted norbornenes the exo-isomer is usually rather more reactive than the endo-isomer. An extreme case is provided by the exo- and endo-isomers of 190 where the exo-isomer polymerizes first, followed much more slowly by the endo- isomer; see Section VIII.C.12. In other cases the endo-isomer will not polymerize but will copolymerize to some extent with its exo-isomer, as with the isomers of 197531. Other examples of this kind, where M2 will not homopolymerize using a particular catalyst, are the copolymerization of norbornene (M1), (i) with cyclopentene (M2), catalysed by Ru(OTs)2(OH2)6597, (ii) with 192, catalysed by WCl6/Me4Sn526 and (iii) with cyclo-

hexene (M2), catalysed by WCl6/Me4Sn358,359.
Some other	recently	reported metathesis	copolymerizations	are	the following.
(1) Norbornene	(M1) has	been copolymerized	with a number of	its	derivatives and

with 7-oxanorbornene derivatives (M2), using RuCl3 and other catalysts. The monomers are of comparable reactivity (r1 D 0.5 2.6) and the behaviour close to ideal (r1r2 D 0.9 2.2)602,603. (2) In the ROMP of cyclooctadiene (M1) with 4,7-dihydro-2-phenyl-1,3- dioxepin (M2) catalysed by Ru(DCHR)(Cl)2(PCy3)2, where R is CHDCPh2 or Ph, M2 is about half as reactive as M1. By using a small proportion of M2 one can produce a polymer of M1 containing occasional M2 units which may be broken by hydrolysis to yield 1,4-hydroxytelechelic polybutadiene (Mw/Mn ca 1.2)604. (3) The metathesis copolymerization of the cyclobutene derivative, 136, with norbornene or cyclopentene (M2) gives copolymers that are readily converted into acetylene copolymers by elimination of 1,2-bis(trifluoro)benzene from the M1 units, but the compositional sequence distribution in these copolymers is difficult to establish605.

Secondary metathesis reactions are sometimes encountered during metathesis copolymerization, leading to a reshuffling of the units in the chain and eventually to a random distribution; for example in the copolymerization of 248 and 258 using RuCl3 as catalyst, statistical copolymers are produced no matter whether the monomers are mixed initially or added sequentially576. See also the copolymers of 128; Section VIII.B.6.

B. Block Copolymers by Sequential Addition of Monomers to Living Systems

Early attempts at making block copolymers using first-generation catalysts met with limited success372. The big advance in this area came with the development of the welldefined metal carbene initiators. The first indication that these would provide a practical method of making long-chain block copolymers was the observation that the propagating tungsten carbene species P1 derived from monomer M1 (norbornene) could be readily converted, by dosing with a second monomer M2 (endo-5-methylnorbornene), to the propagating species P2, and then back again to P1 by a further addition of M1307. This was followed by qualitative observations of the appropriate increases in MW after each addition of monomer606 and by more quantitative studies that showed that block copolymers could be made with a narrow MWD (PDI < 1.1), even from norbornene

derivatives containing ester groups500. These observations have since been extended to a host of other systems, mostly involving norbornene334,384,407,432,500,510,513,601,606 614,

or methyltetracyclododecene483,510 512,615,616 as one of the monomers. The latter has the advantage of producing block copolymers that are more easily microtomed to a thickness of 300 400 A˚ for study by transmission electron microscopy (TEM)615.

In the formation of block copolymers by sequential addition of monomers it generally does not matter which monomer is polymerized first, and diblock or multiblock copolymers of narrow MWD and of any desired sequence length are readily prepared. Termination is usually effected by reaction of the living ends with aldehydes; ketones can be used for terminating titanacyclobutane ends, while unsaturated ethers are used for terminating ruthenium carbene complexes.