Polymerisation ring opening metathesis

The first ring-opening polymerisation of cycloolefin was reported shortly after discoveries by Ziegler and Natta Anderson and Merckling [40] claimed that the polymerisation of norbornene could be promoted by the TiCU—MgEtBr catalyst. The structure of the product, however, was not recognised immediately, but is now known to have been poly(l-vinylene-3-cyclopentylene)  

Subsequently, Eleuterio [41] filed a patent concerning the ring-opening polymerisation of cyclopentene leading to poly(l-pentenylene)  

Later on, Calderon et al. [42,43] recognised that the ring-opening polymerisation of cyclic olefins is a special case of the more general alkene metathesis reaction, e.g. as for propylene  

Confirmation that the ring-opening polymerisation of cycloolefins also proceeds by complete scission of the C=C bond (transalkylidenation) was provided soon after by Dall Asta and Motroni [44]. 

These early results, along with a vast number of other data [45], establish that ring-opening metathesis polymerisation proceeds via a chain process [scheme (4) in Chapter 2] in which the structures of the active species fluctuate between metal alkylidenes (carbenes) and four-membered metallacycles (metallacyclo-butane)s, a concept that was first introduced by Herisson and Chauvin [46]. 

The driving force for this reaction is the relief of ring strain in cyclic olefins (e.g., norhornene or cyclopentene). If more than one type of strained, unsaturated ring is present in the reaction medium, a copolymer is formed. This phenomenon is the first limitation of ROMP reactions the availability of this mandatory strained cyclic structure. Various hackhones can be created through monomer functionalisation, but such alterations can negatively affect ring strain (and hence the success of the corresponding ROMP). 

ROMP is not the most suitable method if fatty acids are used as feedstock because of the low ring strain of the ensuing unsaturated cyclic monomers. Thus, for vegetable oils to participate in ROMP, modification of fatty-acid chains is necessary, and this can be achieved by coupling fatty-acid derivatives with conventional unsaturated cyclic building blocks [18, 56]. 

Last but not least, Meier and Mutlu reported on the synthesis of various ester-functionalised norbornenes using fatty acids of different chain lengths (C6 to Cl 8) [62]. Their ROMP led to polymers with Mn values 165 kDa. Some properties of these materials were dependent upon the chain length of the fatty acid used to prepare them. For instance, the Tg decreased from 102 to -32 °C for polymers based on fatty acids with a chain length of 6-18 carbons, and the T decreased from 30 to 6 C with the same increase in chain length [16]. 

Despite being an interesting tool for polymerisation if associated with vegetable oils, ROMP is very limited in scope, which is why there are so few reports addressing this subject. 

4 Special Cases of Acetal Metathesis Polymerisation and Alternating 

Before the development of CRP methods, ionic (cationic and anionic) or metal-catalysed polymerisations were efficient techniques for the synthesis of glycopolymers with controlled architectures. However, the use of ionic polymerisations quickly became limited because of the harsh polymerisation conditions and the requirement for protected monomers. Anionic polymerisation was limited to vinyl monomers possessing electron-withdrawing groups (nitrile, carbonyl) and required aprotic solvents and low reaction temperatures [ 110-112]. With the development of metal-based catalysis, which was tolerant to a number of functional groups, ring-opening metathesis 

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Controlling the exact architectnre of polymers has always attracted attention in macromolecular chemistry. Snccessfnl synthesis of alternating copolymers nsing ring opening metathesis polymerisation is of great interest also from a mechanistic perspective. NHC ligands were fonnd to be ideal to tune the selectivity of the metathesis initiators. 

Since one of the substrates is a cyclic alkene there is now the possibility of ring-opening metathesis polymerisation (ROMP) occurring which would result in the formation of polymeric products 34 (n >1). Since polymer synthesis is outside the scope of this review, only alkene cross-metathesis reactions resulting in the formation of monomeric cross-coupled products (for example 30) will be discussed here. 

Fig. 6.3 Ring-opening metathesis polymerisation (ROMP) of norbornene (the two cyclohexyl groups on each of the P atoms are omitted for the sake of clarity according to Astruc et aI.)...

Up to third-generation ruthenium-carbene complexed dendrimers (Fig. 6.2) prepared by Astruc et al. contain a chelating diphosphane, which is sufficiently stable for construction of the dendritic architecture while also sufficiently reactive to permit synthesis of the dendrimer depicted in Fig. 6.3 by ring-opening metathesis polymerisation (ROMP) [3]. 

Another type of metal-carbon bond, the metal carbene bond (with carbene of an electrophilic or nucleophilic character), appears to be the active bond in transition metal-based catalysts for the ring-opening metathesis polymerisation of cycloolefins. Such a bond, which is co-originated with metal by the sp2-hybridized carbon atom, possesses a a, n double bond character (Mt = C) [34,35], The enchainment of the coordinating cycloolefin at the active site... 

A characteristic feature of cycloolefin ring-opening metathesis polymerisation is alteration of the metal-carbon active bonds from the metal carbene a, n bond into metallacycle a bonds, and vice versa, as polymerisation progresses. It is worth mentioning, in this connection, that metallacyclobutanes can be successfully used as catalysts for this polymerisation [36,37]. 

Note that the transformation of metal carbene vice versa, during polymerisation occurs as in the case of cycloalk-ene ring-opening metathesis polymerisation. Considering the mechanism of metathesis polymerisation of acetylenic monomers, it is worth noting that catalysts containing a transition metal carbyne bond (Mt=C) can induce polymerisation only when this bond is transformed into the respective metal carbene bond (Mt=C) [39],... 

In common with the polymerisation of acyclic olefins (oc-olefins) by Ziegler Natta catalysts, the ring-opening metathesis polymerisation of monocyclic and bicyclic olefins is promoted by alkylmetal-activated transition metal halides, and only a relatively small proportion of the transition metal atoms introduced into the system is converted into the active sites for the polymerisation. Also, as in the polymerisation of ethylene by Phillips catalysts, the metathesis polymer-... 

The above examples show that the ring-opening metathesis polymerisation of cycloolefins, even simple substituted bicyclic olefins, gives rise, in principle, to polymers with a very wide range of microstructures defined by the frequency and distribution of cis and trans vinylene units, m and r diads and h-h, t-t or h-t arrangements of cycloaliphatic units. 

There is a wide variety of transition metal compounds, ranging from group 4 (Ti) to group 8 metals (Ir), that can be applied as catalysts or catalyst precursors for the ring-opening metathesis polymerisation of cycloolefins. However, the most commonly used are W, Mo, Re and Ru compounds tungsten-based catalysts appeared to be the most effective. Other transition metal compounds such as Nb and Ta compounds have also often been used as catalysts, but especially for mechanistic studies [45]. 

Although this view is oversimplified and borderline metal carbene complexes have been isolated, this approach is convenient for discussing the activity of metal carbene species in the ring-opening metathesis polymerisation of cycloolefins. Calculations have predicted [81,82] and recent results have shown [83] that, in some systems, metal alkylidene reactivity is competitive with metal carbene reactivity, i.e. olefin metathesis is competitive with olefin cyclopropanation. 

It is interesting that, to promote the ring-opening metathesis polymerisation of cycloolefins, metal carbyne complexes can also be used in such a case, the carbyne complex is rearranged to form the actual metal carbene complex [scheme (9)] capable of initiating the polymerisation [95] ... 

When heated at 65 °C in the presence of excess monomer, the formed metallacycle promotes ring-opening metathesis polymerisation ... 

If one considers a general scheme of the ring-opening metathesis polymerisation of cycloolefins [scheme (22)], it involves complexation of the monomer molecule at the free coordination site, followed by an attack of the carbene carbon atom on the complexed monomer [46] ... 

An interesting instance of the ring-opening metathesis polymerisation of cyclic trienes is the polymerisation of the novel conjugated diene 3,4-diisopropylidene-cyclobutane in the presence of a bis(cyclopentadienyl)titanacyclobutane derivative as a catalyst, which affords a linear cross-conjugated polymer [156] ... 

The products of the ring-opening metathesis polymerisation of cycloolefins, poly(l-alkenylene)s, are known as polyalkenamers according to the nomenclat-... 

Commercially available polymers, produced from cycloolefins by ring-retaining polymerisation and ring-opening metathesis polymerisation of cycloolefins, and their typical uses are listed in Table 6.1 [12,14,144-147, 150,167-171,177-186],... 

Figure 6.5 Flow scheme of polydicyclopentadiene production via ring-opening metathesis polymerisation in the reaction injection moulding process...

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