Formation 2 Isotactic and Syndiotactic 1,2-Polydiene

The polymers obtained from unsymmetrically terminally disubstituted butadiene such as 4-methyl-1,3-pentadiene are made up of 1,2 monomeric units only, irrespective of the catalyst used this is due to the presence of two methyl substituents at the C4 atom in the monomer. Two stereoregular polymers have so far been obtained from 4-methyl-l,3-pentadiene, one with a 1,2-isotactic structure and one with a 1,2-syndiotactic structure. The isotactic polymer has been yielded by heterogeneous Ziegler-Natta catalysts, e.g. TiCU—AlEt3 and a — TiCl3— [AlEt3 [182]. The factors that determine the orientation of the coordinating monomer in this case are not, however, completely clear [41]. 

It is interesting that the polymerisation of 4-methyl-1,3-pentadiene proceeds faster at —20 °C than at 20 °C. It has been suggested [43] that at a temperature below 0°C this monomer coordinates as an s-trans-t]2 ligand. At a higher temperature it can probably coordinate either as an s-trans-tf ligand or as an s-cis-tf ligand, but monomeric units in the polymer are derived mainly from the 

One may note that the rearrangement of the anti form of an )j3-butenyl group occurs through a rotation around the C2-C3 bond in the intermediate 71 -allylic species and a reattachmnent of the double bond to the metal atom, with the other face, to yield a new (/ -butenyl group (syn form) the inversion of the configuration at the C3 atom from anti to syn is accompanied with the inversion of the chirality at the Q atom. 

In general, copolymerisation of conjugated dienes with coordination catalysts is difficult because of the strong selectivity of the catalysts towards monomers of the particular type. The use of suitable catalysts, however, makes it possible to obtain copolymers of conjugated dienes with various monomers. Obtained copolymers can be random or alternating, depending on the kind of comonomers and catalysts. 

---