Molar mass, control

Beside the activating effect aluminum alkyl cocatalysts are also efficient molar mass control agents. Control of molar mass is achieved by the adjustment of the molar ratio of nAi/nN(j (Sects. 2.1.4, 2.2.8 and 4.5). An increase in the amount of cocatalyst results in a decrease of molar mass. A change of the nAi/ Nd-ratio also influences the rate of the polymerization reaction which is a major shortcoming in the large-scale production of Nd-BR, particularly in continuous processes. Detailed discussions of this issue are found in Sect. 2.2.8. Because of this disadvantage research on Nd-BR still strikes out to find efficient non Al-based molar mass control agents which do not influence the rate of polymerization. 

To the best of our knowledge, studies that focus on the impact of other additives or impurities such as vinylcyclohexene (Sect. 2.2.5) are not available. A large number of additives was also tested for molar mass control. In Sect. 2.2.8 this aspect is addressed in detail. 

Molar Mass Control by Variation of the Monomer/Catalyst-Ratio (fWnNd)... 

A major drawback of molar mass control by changing ftAl/ Nd rati°s is the simultaneous alteration of polymerization rates. As shown for the system NdV/DIBAH/EASC, an increase in nDiBAH/ Ndvrati°s from 10 to 30 reduces molar mass by 73% but also doubles the rate of polymerization [178,179]. For NdV/TIBA/EASC the variation of ftTiBA/ Ndv from 10 to 30 reduces molar masses by 78% but increases the polymerization rate even 27-fold (Fig. 11) [179]. As shown by these two examples, on one hand, variations of ftAi/ Nd-ratios have a considerable effect on molar mass, and on the other hand, lead to an undesired side effect regarding reaction rates. Because of these interdependencies, in the large-scale continuous production of Nd-BR, adjustments of the ftAl/ Ndv rati°s have to be counteracted by adaptations of residence time in order to keep monomer conversion per reactor and fi-... 

The aspects of molar mass control by adjustments of the polymerization temperature are covered in Sect. 2.2.7. Therefore this issue is not addressed in this context. 

With the well-established Ti-, Ni- and Co-based catalyst systems molar mass regulation is achieved by the addition of appropriate amounts of hydrogen, 1,2-butadiene or cyclooctadiene. In Nd-catalyzed BD polymerizations these molar mass control agents are not effective [82,206,207]. 

To the present day there is an ongoing search for the magic additive which allows molar mass control of Nd-catalyzed polymerizations without a detrimental effect on polymerization activities. This search is documented in the scientific as well as in the patent literature. In this context ethanol, dihydronaphthaline, chloroform, diethyl aniline, triphenylmethane, octanoic acid, allyl iodide and diallylether were unsuccessfully evaluated [464,465]. Also propylene, oxygen, 1,5-hexadiene, ethyltrichloroacetate and n-butanol resulted in the deactivation of the catalyst system without the desired reduction of molar mass [157]. 

To the best of our knowledge, beside aluminum alkyls and hydridoalu-minumalkyls only vinyl chloride [206,207] and benzyl-H containing compounds such as toluene [157,384,385,409,410] are unambiguously effective in molar mass regulation. The reports on molar mass control by diethyl zinc are controversial [157,180-182,466,467]. 

Molar mass control by vinyl chloride is described for the catalyst system NdlO/TIBA/EtAlCl2 [206,207]. According to this report vinyl chloride is ef-... 

According to Jenkins diethylzinc has no effect on molar mass [157]. In contrast to the negative result published by Jenkins there are reports from two other sources on the successful use of diethyl zinc [180-182,466,467]. These differences are either due to different catalyst systems or are due to differences in the addition order of catalyst components. Strong evidence in favor of molar mass control by diethyl zinc was provided by Lynch who used NdV/MgR2-based catalyst systems [466,467]. In combination with NdV/DIBAH/EASC the use of ZnEt2 also resulted in a reduction of molar mass [ 180-182]. A careful study revealed that the formal number of polymer chains (pexp) formed per Nd atom increases with increasing nznEt2/ Ndv-ratios (Table 24). 

From these observations the similarities of molar mass control by A1R3 and ZnR2 become evident. For both molar mass control agents the Mn-conversion plots are linear, the slopes of which decrease with increasing molar ratios of... 

Table 24 Influence of the amount of added ZnEt2 ( ZnEt2/nNdv) on molar mass control (formal chain number pexp.) in the catalyst system NdV/DIBAH/EASC [180], reproduced by permission of Taylor Francis Group, LLC., http //www.taylorandfrancis.com...

In a later patent (1986) it is disclosed that phase separation of cz s-1,4-BR and BD occurs at 30 to 35 °C. Below 30 °C there is a single phase and above 35 °C there are two distinct phases of BR and BD. By the application of two polymerization steps the first of which is performed below and the second above the critical solution temperature molar mass is decreased and costs for aluminum alkyls which are used for molar mass control are reduced [511,512], Control of molar mass is further improved by the sequen-... 

As discussed in Sects. 2.1 and 2.2.8 control of molar mass is an important aspect in the large-scale polymerization of dienes. In Nd-catalyzed polymerizations the control of molar mass is unique amongst Ziegler/Natta catalyst systems as standard molar mass control agents such as hydrogen, 1,2-butadiene and cyclooctadiene which are well established for Ni- and Co-systems do not work with Nd catalysts [82,206,207]. The only known additives which allow for the regulation of molar mass without catalyst deactivation are aluminum alkyls, magnesium alkyls, and dialkyl zinc. 

Scheme 31 TIBA-mediated molar mass control in the polymerization of BD by NdO/TIBA/DEAC depicted as given in [188], reprinted with permission of John Wiley Sons, Inc.

Recently, the impact of the metal alkyls TIBA, DIBAH and ZnEt2 on molar mass was comparatively studied. In these studies ternary NdV-based catalyst systems were used [178-182]. The first two of these studies focus on molar mass control by DIBAH and by TIBA. Linear dependencies of Mn on monomer conversion were obtained. In addition, PDIs decreased with increasing monomer conversion. On the basis of these observations it was concluded that chain transfer of living poly(butadien)yl chains between Nd and Al is fully reversible. A reaction mechanism which accounts for these features is outlined in Scheme 32. 

The lengths of the arrows in Scheme 33 give an estimation of the equilibrium positions in order to account for the different molar mass control activities of DIBAH and TIBA. In these studies it was also demonstrated that /3-hydride-elimination from TIBA which resulted in the formation of DIBAH... 

The addition of ZnEt2 to the catalyst system NdV/DIBAH/EASC had the same effect on the decrease of molar mass as aluminum alkyls. Therefore, a reversible exchange of polybutadienyl chains between Nd and Zn was also assumed to apply for molar mass control by ZnEt2 (Schemes 34 and 35) [180]. 

A comparison of the molar mass control efficiencies of ZnEt2, DIBAH and TIBA allowed for the establishment of the ranking DIBAH > ZnEt2 > TIBA [180,205]. The reaction mechanism which accounts for the control of molar mass by reversible transfer of (poly)butadienyl chains between Nd and Al on one hand and between Nd and Zn on the other hand was established for the catalyst system NdV/DIBAH/EASC. In addition, it was concluded that in this system the (poly)butadienyl chains are only active during the period in which they are attached to Nd. The (poly)butadienyl chains are dormant in the period during which they are attached to Al or Zn. In the context of these results it is not clear whether the irreversible transfer of polybutadienyl... 

Availability of metal-free agents for molar mass control yes yes no no... 

Application of metal-free molar mass control agents. 

From a theoretical point of view the application of readily available metal free molar mass control agents should allow the reduction of overall catalyst costs for BR-grades with low molar masses. Unfortunately, to the present day such additives are not known for Nd-catalyzed solution processes. The only exception seems to be slurry technology in which control of molar mass by the use of 1,2-butadiene is possible [517,518]. 

Chemicals. The four-armed star St-Bd block copolymers with different Bd compositions were synthesized in our laboratories at PSS (Mainz, Germany) by anionic polymerization according to standard procedures (25, 26) modified to give samples with well-known structure and molar mass control. 

Fig. 8 Molar mass control using on-line multivariable constrained MPC (A) conversion and (B) MWD. (View this art in color...

Polyolefins are widely used in different applications since 1950 after the development of Ziegler-Natta catalysts which make it easier to produce them with low cost and high quantities and better molar mass control. 

Ashby R, Solaiman D, FogUa T (2002) Poly(ethylene glycol)-mediated molar mass control of short-chain- and medium-chain-length poly(hydroxyalkanoates) from Pseudomonas oleovorrms. Appl Microbiol Biotechnol 60 154-159... 

Bokias, G. Durand, A. Hourdet, D. Molar mass control of... 

The above two examples of the polymerization of styrene contrast the prototypical controlled and uncontrolled polymerizations. Controlled polymerizations offer simple molar mass control, the ability to define the polymer end groups, and give polymer samples with narrow molar mass distributions. Molar mass definition in uncontrolled polymerizations is more difficult, polymer end groups are determined by inherent termination (and transfer) reactions, and the molar mass distributions are typically broader. There is much contemporary interest in developing polymerization reactions that are controlled because of the precision with which macromolecules can be designed. Many chapters are dedicated to these endeavors with controlled radical polymerization receiving the most attention recently. 

Several papers report studies on the anionic polymerization of EO in DMSO. The use of potassium tert-butoxide (t-BuOK) as initiator was reported to yield living poly(EO)s with molar masses controlled by the ratio [monomer]/[initiator] although dimsyl ion resulting from transfer to the solvent... 

Extent of Molar Mass Control in Processes Initiated with Multivalent Metal Alkoxides... 

The new generation of the single-site catalysts explored until now in CL polymerization do not show any particular advantage over the multiple-site ones, like Al(0 Pr)3, with respect to the molar mass control as well as molar mass distribution (MMD) or the end group control in the resulting Results reported for polymerization... 

---