Cisoid monomers

The polymerization of butadiene (BD) on this site proceeds to yield a cis-1,4-polybutadiene through the addition of a cisoid monomer in the form of the cobalt complex. 

A conjugated diene can coordinate to a transition metal by only one double bond, as an s-trans-r 2 ligand, or with the two double bonds, as an s-cis-tf or as an s-trans-rf ligand [188]. A coordinated transoid monomer (as an s-trans-rj2 or an s-trans-rf ligand) is inserted into the metal-carbon bond, acquiring the syn-/ 3-allylic structure of the growing chain end. On the other hand, when a cisoid monomer coordinates to a metal (as an s-cis-rj4 ligand), an anti-t]2-allylic structure is formed. 

The anti and syn forms of the rc-allylic ligand are in equilibrium. If no bulky substituent is present at the C2 atom of the butenyl group, the equilibrium at ambient temperature is completely shifted towards the syn form which is thermodynamically much more stable than the anti form [148,189]. Therefore, trans-1,4 monomeric units can be generated, either involving a coordinated transoid monomer [pathways (a)-(b) and (a )-(b), scheme (10)] or involving a coordinated cisoid monomer [pathway (c)-(e)-(b), scheme (10)], if the rate of anti —> syn isomerisation [pathway (e), scheme (10)] is greater than that of insertion. When the rate of this isomerisation is lower that that of insertion, cis-... 

Catalyst complexation with a Lewis base or other electron donor may affect the polymer microstructure in different ways. If the added component occupies one coordination site, a monomer coordinates to another site of the active species with one double bond, i.e. as an s-trans-rf ligand, which gives rise to the formation of trans-1,4 monomeric units via the pathway (a)-(b) [scheme (10)]. Depending on the lifetimes of metal species complexed with the monomer and with the Lewis base or the other donor [scheme (11)], mixed cis-1,4/trans- 1,4-polybutadienes or an eb-czs-1, 1 A trans-1,4-polymer can be formed. One should mention in this connection that equibinary cis-l,A/trans- 1,4-butadiene polymers can also be formed in systems without the addition of a Lewis base or other electron donor in such cases, the equilibrium of the anti-syn isomerisation is not shifted and there are equal probabilities for the reaction pathways involving coordination of a transoid monomer and a cisoid monomer [7]. 

Alternatively, liquid phase polymerization (in bulk monomer at a temperature of 20° C) furnishes an isomer (II) characterized by a cis-transoid (or trans-cisoid) configuration of the main chain, with carboxyl groups located on both sides of it. These isomers will be shown later to differ in chemical and physicochemical properties. 

The calculation of the structure of complexes formed by the active centre with monomer showed that, for all lanthanides, the complexes which include cisoid conformers of dienes are energetically more favourable. In the lanthanide series, the preference for complexes with cisoid conformers with respect to similar complexes with transoid conformers changes from 4 to 7 kj/mol. The preference is measured in terms of the difference in the total energy of formation of corresponding complexes. This means that, for cisoid conformers, the energy of complexation is from 19 to 23 kJ/mol larger than for transoid conformers. 

Docking studies by Veld et al. suggest that for the cisoid lactones, competitive inhibition by wrongly bound substrate is responsible for the low experimentally observed reactivity (Fig. 2b) [38]. This finding indicates that in ROP of lactones to polyesters where a monomer activated mechanism is operational, the polyester product can be more reactive in the activation step than the lactone substrate. This has important implications for the ability to control the polydispersity, the molecular weight and the end-group fidelity in lipase-catalyzed ROP. 

However, the possibility that the type of conformation of the monomer which can be coordinated on the catalytic complex in a cisoid or transoid conformation plays a role, cannot be excluded. This role is decisive in Cossee s mechanism (70), according to which a monodentate-transoid or bidentate-cisoid coordination of the diolefin is responsible for the formation of either trans or cis 1,4-units respectively in the polymer (Scheme 14). As for the mode of the addition of metal and growing chain to the entering unit, the results obtained by Porri and Aglietto (71) in the study of the stereospecific polymerization of cis,cis-l,4-di-... 

Anionic polymerization in a non-polar solvent using Li as the counterions leads to the formation of polymers with high proportions of cw-1,4 repeat units. Under these conditions the monomer is held in the cisoid conformation by strong coordination to the small Li counter-ion as it adds to the growing chain... 

C leads to the anti-cisoid dimers (10,14,15) as major products in yields of 92,35, and 40%, respectively. Compounds 10 and 15 were hydrolyzed to 14, and final confirmation of the structure of 10 came from x-ray crystallographic analysis. The dipole moments of 10 and 11 were determined to be 5.90 and 3.65 D, respectively, in benzene at 25°C. On the other hand, irradiation of 13 in both the crystalline and the molten state gave the syn-cisoid-dimer (16) as the sole product in a yield of 65%. X-ray crystallographic analysis of the monomer 13 demonstrated stereo- and regioselective formation of 16 in the solid state irradiation of 13 according to the topochemical rules. 

---