Polydienes microstructure

Aromatic radical anions, such as lithium naphthalene or sodium naphthalene, are efficient difunctional initiators (eqs. 6,7) (3,20,64). However, the necessity of using polar solvents for their formation and use limits their utility for diene polymerization, since the unique ability of lithium to provide high 1,4-polydiene microstructure is lost in polar media (1,33,34,57,63,64). Consequendy, a significant research challenge has been to discover a hydrocarbon-soluble dilithtum initiator which would initiate the polymerization of styrene and diene monomers to form monomodal a, CO-dianionic polymers at rates which are faster or comparable to the rates of polymerization, ie, to form narrow molecular weight distribution polymers (61,65,66). 

The method suffers from several disadvantages. Probably the most important is that the rate constants for the reaction of the first and second living polymeric chains with the DPE derivative are different. This leads to bimodal distributions, due to the formation of both the dianion and the monoanion during the first step. This problem can be avoided by the addition of polar compounds (e.g., THF),but these compounds influence dramatically the microstructure of the polydienes. However, it was found that the addition of sec-butoxide to the living coupled product, prior to the addition of the diene monomer, can produce monomodal miktoarm stars while maintaining high 1,4 polydiene microstructure. 

A comprehensive hypothesis has been proposed to explain the effects of the concentrations of active chain ends and monomer on polydiene microstiucture [163], Based on studies with model compounds and the known dependence of polydiene microstructure on diene monomer... 

The anionic polymerization of dienes is also a subject of long-term, continuous interest. Using a variety of initiators, Lewis base additives, and solvent systems, a wide range of polydiene microstructures can be prepared. Several reports have appeared regarding the relationships between polydiene microstructure, monomer concentration and chain-end concentra-tion. In general, the highest cis-1,4-microstructures for either polybutadiene or polyisoprene can be obtained at high ratios of [monomer] to... 

Polydiene microstructure is also dependent on the ratio of concentrations of monomer to organolithium. This is true for both non-polar and polar media. As catalyst levels are reduced in a non-polar system, cis-l,4-values as high as 86% have been observed in high molecular weight polybutadienes prepared in the absence of any solvents. This compares with 96% cw-l,4-content in similarly prepared polyisoprenes. In the case of high-vinyl BRs (HVBRs) prepared with a polar modifier, such as DIPIP, a decrease in alkyllithium (RLi) concentration at constant ratio of DIPIP/RLi produces smaller amounts of vinyl structure. 

Liquid polydienes are more reactive to MA than CARs. This behavior is determined by the higher degree of unsaturation and polydiene microstructure. Modification of polydiene with MA occurs at 180 to 190°C in the case of initiated processes, and at 180 to 230°C in the case of thermal processes. The same processes take place in the case of CARs at 180 to 225°C and at 275°C, respectively [144]. 

Finally it should be stressed that the complexation affects the microstructure of poly dienes. As was shown by Langer I56) small amounts of diamines added to hydrocarbon solutions of polymerizing lithium polydienes modify their structure from mainly 1,4 to a high percentage of vinyl unsaturation, e.g., for an equivalent amount of TMEDA at 0 °C 157) the fraction of the vinyl amounts to about 80%. Even more effective is 1,2-dipiperidinoethane, DIPIP. It produces close to 100% of vinyl units when added in equimolar amount to lithium in a polymerization of butadiene carried out at 5 °C 158 159), but it is slightly less effective in the polymerization of isoprene 160>. 

Table 17. Microstructure of Polydienes Obtained using Organolithium Initiators...

Table 22. Microstructure of Polydienes Prepared in Solvating Media ...

The photo-induced microstructural changes in polydienes have been extensively studied by Golub who is also the author of a recent review of the photochemistry of unsaturated polymers [47]. 

In contrast, in anionic systems in which the solvent may not actually interrupt the propagation process, it may play an active role in controlling both the rate and mode of the chain growth step. This control is perhaps most dramatically illustrated in the case of the organolithium polymerizations in connection with two specific aspects chain microstructure of polydienes and copolymerization of dienes and styrene. 

Table 2. Effect of polymerization temperature on the microstructure of polydienes obtained by free-radical polymerization [46-48]...

It follows from the existence of conformational scissors (Fig. 4) that in the polymerization of symmetric or quasisymmetric dienes in hydrocarbon media on an active center with a slightly polar carbon-metal bond the primary acts of monomer attachment lead to the cis-conformation of the end unit. This conclusion is in good agreement with the modern concepts of the formation mechanism of the cis-structure of anionic polydienes [70]. According to these concepts this is followed by either the attachment of the next monomer molecule or by the cis-trans isomerization of the end unit. The microstructure is fixed at the moment of the attachment of a new monomer unit to the active center, the configuration (cis- or trans-) of the end unit being retained in the polymer chain ... 

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