Norbomene polymerisation

More success has been had with Ir complexes incorporating permelhylcyclopentadiene and NHC ligands. Complexes 18-20 (Fig. 4.7) were evalnated for norbomene polymerisation following activation with MAO [22]. Complex 19 was the most active, giving a TOF of 12 220 h over 10 min, followed by 18 (TOF = 3 220 h" ), while 20 was inactive, indicating that a hemilabile pendant group seems essential. Analysis (NMR) of the polymers formed with 18 and 19 shows that polymerisation proceeds via an addition (coordination-insertion) mechanism. 

The remainder of the work on Ni(II) complexes involves the use of chelating ligands in which the carbene is functionalised with pendant heteroatom donor(s). The picolyl-functionalised NHC dicationic complex 29 (Fig. 4.11) was tested for ethylene polymerisation after treatment with MAO [34]. This complex was found to be highly active in a preliminary test (330 kg moF bar h" ), giving predominantly linear polyethylene. Unfortunately this work does not seem to have been followed up. The same system was active for norbomene polymerisation (TOF = 24 400 h" over 1 h). Maximum activity was achieved at 80°C whereafter thermal deactivation became significant, although the nature of this deactivation was not studied. The phenoxide-functionalised carbene complex 30 (Fig. 4.11) was also... 

Allyl-Pd(ll) complexes featuring a single NHC ligand have been explored for norbomene polymerisation. The first such disclosure appears in a patent of 2006 [39] in which complexes of the form 34 (Fig. 4.12) are activated with AgSbF and LiB(CgFj). The resulting cationic complexes catalyse the polymerisation of norbomene and polar-substituted norbomenes with very high activity. The best result... 

The Ticona materials are prepared by continuous polymerisation in solution using metallocene catalysts and a co-catalyst. The ethylene is dissolved in a solvent which may be the comonomer 2-norbomene itself or another hydrocarbon solvent. The comonomer ratio in the reactor is kept constant by continuous feeding of both monomers. After polymerisation the catalyst is deactivated and separated to give polymers of a low residual ash content and the filtration is followed by several degassing steps with monomers and solvents being recycled. 

The monomer, norbomene (or bicyclo[2.2.l]hept-2-ene), is produced by the Diels-Alder addition of ethylene to cyclopentadiene. The monomer is polymerised by a ring-opening mechanism to give a linear polymer with a repeat unit containing both an in-chain five-membered ring and a double bond. Both cis-and trans- structures are obtainable according to the choice of catalyst used ... 

Fig. 3.31 Steric control in alternating ROMP Tendencies of norbomene and cyclooctene to give productive olefin metathesis upon coordination are illustrated by a thick arrow (preferred monomer) or a thin arrow (less favoured monomer) (a) only minor steric hindrance SlMes greatly favours the polymerisation of the strained norbomene (b) the rotating phenylethyl-group induces a steiically more congested active site, leading to preferred incorporation of the smaller cyclooctene (c) the flexible and small cyclooctene derived polymer fragment permits the incorporation of the bulky norbomene...

As said in the introduction there are many more polymers than can be discussed within the limits of this chapter, but we want to add just one example of a group of high-value polymers that is made using the same principles of coordination polymerisation as shown above for the commodity polymers. We mentioned metallocene catalysts that can be used to copolymerise ethene and norbomene to give Topas type products. 

Scheme 7.5 Ring-opening polymerisation metathesis of norbomene...

Remarkably, the heterogenisadon process can confer dramatically enhanced activity on the metal complex. Thus, the dimolybdenum complex [Mo2(MeCN)8][BF4]-Si02 prepared by direct reaction of silica with the metal complex is unusual in its ability to catalyse the polymerisation of norbomene in the absence of an aluminium co-catalyst and at moderate temperatures.165 Sol-gel processing of the ruthenium complex m-Cl(H)Ru(CO)(P)3 (where P is a coordinated ether-phosphine) with tetraethoxysilane and [Al(0-i-Pr)3] gives a stable material with moderate surface area which is active in the hydrogenation of fraras-crotylaldehyde with reasonable chemoselectivity to the carbonyl reduced products, cis- and trans-crotyl alcohol.166... 

In a later study, bis-NHC ruthenium-alkylidene complex was activated under compressive strain [87] (Fig. 16). In order to initiate Ru-mediated polymerisation of norbomene in solid state, polymer catalyst (34 kg mol ) and a norbomene monomer were incorporated in a high molecular weight poly(tetrahydrofuran) (pTHF) matrix (Mn=170 kDa, PDI=1.3) which provided the physical cross-linking through the crystalline domains and allowed macroscopic forces to be transferred to the metal-ligand bonds. Consecutive compressions showed that up to 25% of norbomene monomer was polymerised after five loading cycles. 

Fig. 16 Mechanoactivation of latent ruthenium catalyst in the solid state, initiating in situ polymerisation of norbomene monomer in response to stress. Reprinted with permission from [87] (Copyright American Chemical Society 2013)...

It should also be noted, however, that in a 5-substituted norbomene derivative the whole molecule is chiral. Normally, samples of such molecules are prepared as racemic mixtures, but if an optically resolved sample is prepared and polymerised using ROMP, then the placement of m/r dyads (tacticity) is necessarily related to the head/tail placement of repeat units (see Figure 10). 

Reversely, poly(e-caprolactone) (PCL) macromonomers capped by a norbomene end-group were synthesised and (co)polymerised by ROMP with the same catalytic system (Scheme 4) [17]. 

The complexes generated in such a way were shown to be catalytically inactive in the metathesis polymerisation of norbomene at room temperature [13b,28]. However, we recently observed that they were moderately active at 85 °C -the temperature generally... 

The controlled synthesis of poly(norbomene-g-/i-butyl acrylate) started from the same poly(NB-co-NB-Br) macroinitiator and took advantage of the high efficiency displayed by Ru(Cp )Cl(PPh3)2 (Cp = pentamethylcyclopentadienyl) (5) in the polymerisation of acrylates. The final graft copolymers were produced with narrow molecular weight distributions (MJM were around 1.2-1.3). 

TABLE 1. Experimental details for the polymerisation of xo-n-alkyl norbomene dicarboxyimides using classical initiators 1 and II, and thermal characterisation data. 

These complexes spontaneously not only promote the ROMP of norbomene derivatives to high molecular weight polynorbomenes, but also they are effective at polymerising... 

Conversion (%) versus time (min) for the polymerisation of norbomene with the heterogeneous salicylaldimine catalyst... 

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