Diolefins cyclopolymerisation

Coordination polymerisation via re complexes comprises polymerisation and copolymerisation processes with transition metal-based catalysts of unsaturated hydrocarbon monomers such as olefins [11-19], vinylaromatic monomers such as styrene [13, 20, 21], conjugated dienes [22-29], cycloolefins [30-39] and alkynes [39-45]. The coordination polymerisation of olefins concerns mostly ethylene, propylene and higher a-olefins [46], although polymerisation of cumulated diolefins (allenes) [47, 48], isomerisation 2, co-polymerisation of a-olefins [49], isomerisation 1,2-polymerisation of /i-olcfins [50, 51] and cyclopolymerisation of non-conjugated a, eo-diolefins [52, 53] are also included among coordination polymerisations involving re complex formation. 

Cyclopolymerisation leading to polymers with monocyclic units in the main chain proceeds in two steps the first step involves a 1,2-insertion of the coordinated a, ffl-diolefin via one olefinic bond, and the second step, which is a ringclosing reaction, involves an intramolecular insertion of the other olefinic bond undergoing coordination scheme (89) presents both steps for 1,5-hexadiene cyclopolymerisation leading to a cycloaliphatic polymer with poly(methylene-1,3-cyclopentane) structural units [30,450,497] ... 

In view of the observed inactivity of a, a-disubstituted olefins towards polymerisation with Ziegler-Natta catalysts, it is interesting to note that a, co-diolefins substituted at unsaturated carbon atoms, such as e.g. 2,5-dimethyl-l,5-hexa-diene, also undergo cyclopolymerisation, analogously to unsubstituted parent monomers [2,446], This can be interpreted in terms of a reaction pathway analogous to that shown in scheme (89). The insertions in the cyclopolymerisation appear to be facilitated by the nature of such a process. 

It has been reported [497] for cyclopolymerisations with metallocene-based catalysts that the enantioface selectivity of the insertion of the first olefinic bond in the a, co-diolefin determines the relative stereochemistry between the rings (the occurrence of m or r diads), i.e. the tacticity of the cyclopolymer the diastereoselectivity of the subsequent cyclisation involving the remaining olefinic bond determines, on the other hand, the relative stereochemistry in the rings (the occurrence of M or R diads), i.e. the cis-trans geometrical isomerism of the rings. 

The cyclopolymerisation of unsymmetrical a, co-diolefins such as 2-methyl-1,5-hexadiene in the presence of catalysts such as Cp2 ZrMc2 M(Me)0 x, [Cp2 , ZrMe]+ [B(C6F5)4] or [Cp2 ZrMe]4 [McB(C6 F5)2] yields highly regiore-gular cyclopolymers [501]. The perfectly head-to-tail linked monomeric units in the formed poly[methylene-l,3-(l-methylcyclopentane)] arises from the chemo-selective insertion of the less hindered terminus of 2-methyl-1,5-hexadiene into the active Mt—P bond, followed by cyclisation involving the insertion of the disubstituted olefinic bond (Figure 3.50) [497]. The insertion of the disubsti-tuted olefinic bond is made easier by its intramolecular nature. 

The symmetry properties of cycloaliphatic polymers are such that polymers with certain microstructures, e.g. tram-isotactic poly (methylene-1,3-cyclopen-tane), are chiral therefore, the cyclopolymerisation of a, trans selective catalysts of C2 symmetry, such as methylaluminoxane-activated resolved (li )-(Thind CH2)2Zr l,l -bi-2-naphtholate, yielded optically active tram-isotactic poly(-methylene-1,3-cyclopentane). The cyclopolymerisation with the (15) enantiomer of the catalyst gave an enantiomeric polymer [505], On the basis of analysis of 13C NMR spectra, the degree of enantioface selectivity for this cyclopolymerisation was estimated to be of 91% [503,505]. 

Figure 3.51 Enantiomorphic site control versus conformational control in the cyclisation step during cyclopolymerisation of a, co-diolefins 1,5-hexadiene (x=l), 1,6-heptadiene (x—2) and 1,7-octadiene (x=3) in the presence of a metallocene-based catalyst of C2 symmetry...

The polymerisation of monocyclic non-conjugated diolefins, which takes place by the double bond opening with simultaneous transannular migration, leads to cyclopolymers having bicyclic repeating units present in the main chain. A typical example of such cyclopolymerisation is the polymerisation of 1,5-cyclooctadiene [31,32] ... 

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