Equilibrium coordination polymerization

The reaction is considered to involve coordination of the cycloalkene with the transition metal complex and growth by a process of ring enlargement following opening at the double bonds [263, 264], viz. 

The kinetics are undoubtedly complex and not yet studied in detail but one important feature of the polymerization is its reversible character. This is possible because of the small energy differences between polymer and monomer, and both depolymerization of preformed polymers and cis trans isomerization have been demonstrated. 

Kranz and Beck [268] have calculated that the enthalpies of polymerization of liquid cyclopentene to solid cis and trans polypentenamer are 4.2 and 3.2 kcal mole , in good agreement with a calorimetric value of 4.5 kcal mole for a 65% trans polymer. Other than this limited thermodynamic data the quantitative aspects of the polymerization are unknown of particular interest would be the factors influencing molecular weight and molecular weight distribution. 

---

The distinction between coordination polymerization and ionic polymerization is not sharp. Let us consider for example a C—X bond, X being a halogen or a metal. Winstein54 and Evans14 have demonstrated that in a compound containing this type of bond an equilibrium may be established in a suitable solvent between... 

Transition metal complexes functioning as redox catalysts are perhaps the most important components of an ATRP system. (It is, however, possible that some catalytic systems reported for ATRP may lead not only to formation of free radical polymer chains but also to ionic and/or coordination polymerization.) As mentioned previously, the transition metal center of the catalyst should undergo an electron transfer reaction coupled with halogen abstraction and accompanied by expansion of the coordination sphere. In addition, to induce a controlled polymerization process, the oxidized transition metal should rapidly deactivate the propagating polymer chains to form dormant species (Fig. 11.16). The ideal catalyst for ATRP should be highly selective for atom transfer, should not participate in other reactions, and should deactivate extremely fast with diffusion-controlled rate constants. Finther, it should have easily tunable activation rate constants to meet sped c requirements for ATRP monomers. For example, very active catalysts with equilibrium constants K > 10 for styrenes and acrylates are not suitable for methacrylates. 

Let an inactive dimer, P, be in equilibrium with active unassociated polymeric species P. Assume that a coordinating agent forms a 1 1 complex with the unassociated species but not with the dimeric ones, i.e. ... 

For the ROMP of the 5-alkylthiocyclooctenes (115), with R = Et, Bu, Hex, c-Hex, t-Bu, initiated by 12, the most reactive monomers are those with branched alkyl substituents on the sulphur atom for R = t-Bu, reaction is 95% complete in about 10 min. The variations in rate are likely to be connected with the strength of coordination of the sulphur atoms in the monomer and/or the propagating species to the tungsten centre. Coordination of the monomer to the metal centre through the sulphur atom will be impeded when R is t-Bu or c-Hex, allowing a higher equilibrium concentration of the precursor complex that leads to addition of monomer. For R = Bu the rate of polymerization is proportional to both monomer and initiator concentrations189,364. 

Fig. 8 Low-pressure phase diagram for C60 deduced from compression studies. Full curves show observed polymerization and depolymerization coordinates and dashed line the equilibrium line. The fcc-sc phase boundary is also shown, for comparison. Reprinted with permission from IO Bashkin, VI Rashchupkin, AF Gurov, AP Moravsky, OG Rybchenko, NP Kobelev,YM Soifer, and EG Ponyatovskii, WA new phase transition in the T-P diagram of C60 fullerite , J. Phys. Condens. Matter vol. 6 (1994) 7491-98 [58]. Copyright 1994 Institute of Physics Publishing Ltd...

The NMR data for alcohols 300a-c in acctonc-rff, reveal a complex coordination behavior involving five- and six-coordinated species in a fast equilibrium for which HO —> Sn and (CD3)2C = O —> Sn interactions are in evidence. The crystal structure of alcohol 300c reveals a polymeric structure that arises from significant intermolecular HO — Sn interactions of 2.356 A. As a result, the tin atom is five-coordinate and exists in a distorted TBP geometry with the oxygen and one of the chlorine atoms at the axial positions. 

According to Cabassi et al. the reduction of polymerization rates by aromatic compounds is caused by competitive coordination of monomer and arenes to vacant Nd-sites. The following coordination equilibrium was put forward in order to account for the observed effects (Scheme 12) [165,166]. 

In this equilibrium the Nd-species to which a diene is coordinated is active in polymerization, whereas the Nd-species to which an arene is coordinated is inactive. According to the authors the experimentally determined ranking of activities toluene > mesitylene > toluene (+ 7% hexamethylbenzene) correlates with the electron richness (i.e. Lewis basicity) of the aromatic compounds. The polymerization activity decreases with increasing Lewis basicity of the aromatic compound as the equilibrium is shifted and the concentration of the active species is reduced. These considerations were supported by the following experimental results (Table 18). 

Myriad polydentate aza-macrocycles have been reported 41. The extent of the subject forces limitation of this discussion to only macrocycles containing a pyridine or dipyridine subunit. Most of these coronands have been synthesized by a SchifF base condensation of an aldehyde or ketone with a hfc-primary amine in the presence of a metal ion. The metal ion acts as a template, resulting in dramatic increases in yield of the desired cyclic product over linear polymerization products42 46. Lindoy and Busch45 have described this effect in two ways, kinetic and thermodynamic. If the metal ion controls the steric course of a series of stepwise reactions, the template effect is considered to be kinetic. If the metal ion influences an equilibrium in an organic reaction sequence by coordination with one of the reactants, the template effect is termed thermodynamic. It is the kinetic effect that is believed to be operative in most metal ion-assisted (in situ) syntheses of... 

---