Transoid monomers

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]. 

Maleic anhydride has been used in many Diels-Alder reactions (29), and the kinetics of its reaction with isoprene have been taken as proof of the essentially transoid stmcture of isoprene monomer (30). The Diels-Alder reaction of isoprene with chloromaleic anhydride has been analy2ed using gas chromatography (31). Reactions with other reactive hydrocarbons have been studied, eg, the reaction with cyclopentadiene yields 2-isopropenylbicyclo[2.2.1]hept-5-ene (32). Isoprene may function both as diene and dienophile in Diels-Alder reactions to form dimers. 

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. 

Rh complexes are examples of the most effective catalysts for the polymerization of monosubstituted acetylenes, whose mechanism is proposed as insertion type. Since Rh catalysts and their active species for polymerization have tolerance toward polar functional groups, they can widely be applied to the polymerization of both non-polar and polar monomers such as phenylacetylenes, propiolic acid esters, A-propargyl amides, and other acetylenic compounds involving amino, hydroxy, azo, radical groups (see Table 3). It should be noted that, in the case of phenylacetylene as monomer, Rh catalysts generally achieve quantitative yield of the polymer and almost perfect stereoregularity of the polymer main chain (m-transoidal). Some of Rh catalysts can achieve living polymerization of certain acetylenic monomers. The only one defect of Rh catalysts is that they are usually inapplicable to the polymerization of disubstituted acetylenes. Only one exception has been reported which is described below. 

We have seen from Fig. 3.4 that some initiator remains after all the monomer has reacted with W(=CHCMe3)(Br)2(OCH2CMc3)2/GaBr3. This is still the case when the initiator and monomer are mixed at —78°C in CD2CI2 confirming that this is not due to inefficient mixing. If the temperature is raised slowly, reaction begins at about -53°C but the initial spectra are different from those observed at room temperature. The species formed at low temperature have been identified as intermediate transoid metallacyclobutane complexes 7. 

The tungstacyclobutane complex formed by the addition of W(=CHCMe3) (=NAr)(OCMe3)2 to 2,3-bis(trifluoromethyl)norbomadiene at 0°C has been characterized at low temperature and, as expected, has the transoid structure and square-pyramidal geometry. It rearranges to the tungsten carbene complex trans double bond), but is unstable and does not polymerize the monomer smoothly (Bazan 1990). 

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. 

Q Q Q Q Q - Further addition of the monomer leads to the various hexamers (prismatie trieyelie eisoid and transoid Q Q lQ, tri-eyelie Q Q Q Q and bieyelie Q Q Qi)- Addition of another monomer to the prismatie or to the eisoid tricyclic hexamers leads to the pentacyclic heptamer Q Q Q Q precursor of the cubic octamer Qg and of the pentacyclic nonamer Q%Q -... 

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-... 

Two such systems are of particular interest from the point of view of asymmetric polymers, and both of them would be expected to lead to quantitative optical yields if the contacts shown below are the only ones leading to reaction. Both involve monomers with two, non-identical double bonds, in chiral structures. In the first [9], transoid diene molecules are translationally related, with the non-identical double bonds suitable spaced and oriented to give topochemical photocyclopolymerization. 

Those found in isotactic polymers whose regular perturbation along the chain favors the formation of successive transoidal and gauche conformations. When no strong steric compression is exerted, a heUx with three monomer units per turn (3i helix) is formed which can be denoted (TG)3 to indicate that, in one helix turn, three successive transoidal and gauche conformations (TG) occur ... 

There is also the added possibility of head-to-head rather than head-to-tail arrangements. For the free-radical polymerization of both isoprene and chloroprene it is found that the trans-1,4 structure predominates. The tendency for most simple diene monomers to form the trans structures may be because it is thought that the monomers exist mainly in transoid conformations and that this stereochemistry is retained during polymerization. 

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