Of diene polymerization

When the cnolate of an enone is brought into reaction with an enone, usually a carbocyclic system is prepared by two consecutive Michael additions (M1MIRC reactions). Due to the lower temperatures employed and the absence of diene polymerization these reactions are useful alternatives for Diels-Alder reactions and proceed in general with high diastereoselectivities. When neither enolate nor enone is cyclic a monocyclic system is formed 338 which can be converted into a bicyclic system when the Michael addition is followed by an aldol reaction339. When, however, the enolate is cyclic a bicyclic or a tricyclic system is formed340 341. 

The stereochemistry of diene polymerizations is also affected by solvent polarity. For instance, the proportion of cA-1,4 units is increased by using organolithium or lithium itself as the initiator in the polymerization of isoprene or 1,3-butadiene in nonpolar solvents. 

The stereochemistry of diene polymerization is somewhat dependent upon temperature note however the results of Morton and Rupert209 (Table 19). In general, linear Arrhenius plots are obtained. Some pertinent data are summarized in Table 21 for convenience results are presented for both solvating and non-solvating media. The structures obtained in solvating media are very different from those obtained in hydrocarbon solvents (cf Tables 17 and 22). The sensitivities to tempera-... 

III. Mechanism of Diene Polymerization with Polar Ligands. 65... 

The lithium and alkyllithium initiation of diene polymerization has, from the earliest times, remained in the shadow of other, apparently more important, initiator systems. However, it has now become clear that the alkyllithium catalyst is the most efficient, initiator system at present available for diene polymerization. That organolithium initiators are not used much more widely is due largely to economic considerations,... 

The earliest mention of lithium initiation of diene polymerization was in 1910-1913 (1-3). It is quite likely that this was conceived in the fertile imagination of a patent lawyer, rather than the scientists involved, as all of the experimentation reported involved sodium and potassium. 

Neodymium phosphate-based catalysts were used as early as in 1978 for the polymerization of IP by Monakov et al. [249,250]. At a later stage neodymium phosphate-based catalyst systems were claimed by Asahi in a patent issued in the mid 1980s [251,252]. A neodymium-phosphate which is predominantly mentioned in the context of diene polymerization is neodymium bis(2-ethylhexyl)phosphate (NdP). In Chinese scientific literature NdP (Scheme 5) is often abbreviated by its commercial name Nd(P204)3-... 

As early as in 1985, supported catalysts were described for the use in a solution process [399]. Bergbreiter et al. used catalyst supports on the basis of divinylbenzene-styrene-copolymers as well as on polyethylene. These authors found that the use of supported catalysts has no influence on the stereospecificity of diene polymerization. 

In addition to the experimental evidence published, the reaction given in Scheme 12 is supported by the fact that -toluene complexes of Nd could be isolated, characterized and used for the initiation of diene polymerization [424]. 

Most reaction models which describe the mechanism of diene polymerization by Nd catalysts have been adopted from models developed for the polymerization of ethylene and propylene by the use of Ti- and Ni-based catalysts systems. A monometallic insertion mechanism which accounts for many features of the polymerization of a-olefins has been put forward by Cossee and Arlman in 1964 [624-626]. Respective bimetallic mechanisms date back to Patat, Sinn, Natta and Mazzanti [627,628]. The most important and generally accepted mechanisms for the polymerization of dienes by Nd-based catalysts are discussed in the following. 

A detailed review on the multiple aspects of the reaction mechanisms of diene polymerization with transition-metal catalysts was published by Porri, Giarrusso and Ricci in 1991 [486]. 

Butenyllithium and butenylmagnesium chloride were used as "dynamic allylic compounds. The former was selected because of the ability of lithium catalysts to provide high rates of diene polymerization and to give stereoregular polymers the latter was selected for its availability and simplicity of synthesis. 

The high rate of diene polymerization initiated by lithium compounds and the availability of much data reported in the literature prompted us to undertake a more detailed investigation. Polymerization was carried out in dimethyl ether in the presence of butyllithium and butenyllithium, with polymerization conditions similar to those used in the investigations of the NMR spectra of butenyllithium. 

According to present ideas, the initiation mechanism of diene polymerization on ZN centres does not substantially differ from that of alkene initiation. Experiments support the idea according to which the chain is propagated on the transition metal atom it may be bound to the metal either by a a, or by a n allylic bond... 

It is apparent that the bridged ring monomers in which the double bond is made more reactive by steric strain are comparable in reactivity with ethylene, whereas, as would be expected, the aliphatic dienes are more like a-olefins in reactivity. Baldwin and van Strate have critically discussed this work [244], drawing attention to the possibilities of E/P reactivity ratios being changed by the presence of the diene and of diene polymerization by cationic mechanisms. They identify as... 

The only work on the catalysis of diene polymerization by structurally defined organotitanium compounds that can be mentioned is that of Dolgoplosk and coworkers [50]. As these authors have found, tetrabenzyltitanium and the tribenzyl-titanium halides can catalyze the stereospecific diene polymerization without any co-catalyst. 

The mechanism of diene polymerization using catalysts such as CpTiCls/MAO is not completely understood. It is reasonable that there are mechanistic similarities of this system with syndiospecific... 

Recently, several lanthanide complexes, based on Nd, Ln, Sm, or Yb, have been smdied for controlling stereoregularity and activity of diene polymerizations and copolymerizations. Specifically, Nd-based catalysts have shown very good stereoregularity control over the production of high CM-polybutadiene rubber [36]. 

The first section will deal with the preparation and characterization of polymer-bound catalysts, focusing on the methods used to anchor metals to the polymer. We will also describe a classification that is based on the types of bonds used to link the metal to the carrier. Several spectroscopic techniques have been used to characterize the polymer-attached species. Infrared spectroscopy was the most useful method, especially when the attached species incorporated carboxylic group ligands. The second section is related to aspects of diene polymerization catalyzed by polymer-supported catalysts. Catalyst activities and factors that influence these activities will be discussed. 

---