Control of Molar Mass

Molar mass strongly influences the performance of raw (unvulcanized) rubbers during the preparation of rubber compounds, e.g. addition of fillers and other ingredients. Also the processing characteristics of the compounded rubbers as well as the physical properties of the vulcanized rubbers significantly depend on the molar mass of the unvulcanized rubbers. In order to better meet various requirements, there is not only one BR grade available but 

As the influence of polymerization temperature on molar mass and MMD is addressed in Sect. 2.2.7 this issue is not discussed in this context. The residual aspects are reviewed in the following subsections. 

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

The control of molar mass by variation of the M/ Nd-ralio is directly linked with the (partially) living character of Nd-catalyzed diene polymerization. Studies which focus on nM/ Nd-ratios in the context of living polymerizations are addressed in Sect. 4.4. In this subsection only the influence of nM/ Nd-ratios on molar mass is discussed. 

For binary NdCl3-based catalyst systems there are two studies available for which the impact of nM/ Nd-ratios on intrinsic viscosities was investigated. These studies deal with NdCb 2THF/TEA [35] and with NdCb n 2-ethylhexanolate/TEA (n = 1.5 or 2.5) [114]. For both systems increasing M/ Nd-ratios result in increases of molar mass. 

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Numerous studies on Nd-carboxylate-based catalyst systems address hydrocarbon solubility, catalyst activity, stereospecificity and control of molar mass [49,89, 111, 141,156-183]6. 

Nd-boranate-based catalysts were also used in the polymerization of dienes. Nd(BH4)3 (THF)3/TEA yields poly(butadiene) with a frans/cz s-ratio 50/50. If Nd(BH4)3 (THF)3 is combined with a stoichiometric amount of MgBu2 catalyst activities are increased, control of molar masses is improved and poly(diene)s with a trans- 1,4-content of up to 99% are obtained. [348, 349]. 

The cocatalyst s prime function is the activation of the Nd-precursor in order to form the active catalyst. Secondly, the cocatalyst is often used for control of molar mass. This particularly applies to A1-, Mg- and Zn alkyls. 

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. 

Variations of the amount of cocatalyst which are usually expressed by the molar ratio W Nd have a significant influence on polymerization rates, molar masses, MMDs and on the microstructures of the resulting polymers. These aspects are addressed in the following sections with a special emphasis on ternary catalyst systems. For ternary systems it has to be emphasized, however, that in many reports the ratio Ai/ Nd only accounts for the amount of aluminum alkyl cocatalyst and not for other Al-sources such as alkyl aluminum halides. Variations of the Ai/ Nd-ratios are also used for defined control of molar mass. This aspect is addressed in separate sections (Sects. 2.2.8 and 4.5). 

As variations in Ai/ Nlj-ratios are used for the control of molar mass and the MMD this aspect is addressed in separate sections on molar mass regulation (Sect 2.2.8 and 4.5). 

If proton transfer from appropriate substrates leads to stabilized Nd allyl or Nd benzyl species allyl or benzyl proton donors allow for the control of molar mass (Sect. 2.2.8). On the basis of this consideration hexane, toluene and terf-butylbenzene (TBB) were comparatively tested in the polymerization of BD. In this study the catalyst system NdV/DIBAH/EASC was used. The rate of polymerization decreases in the order hexane > TBB > toluene. Only in hexane does the monomer conversion proceed to full completion (Fig. 6) [422]. 

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 in Nd-catalyzed solution processes in gas-phase polymerization of BD regulation of molar mass is a serious problem as there are no agents for the control of molar mass readily available. Vinyl chloride and toluene are no viable options. Vinyl chloride is ruled out due to ecological reasons and toluene is not applicable due to low transfer efficiencies and the required low concentrations if applied in a gas-phase process. For the control of molar mass and MMD in the polymerization of dienes a combination of different methods is recommended [457,458] (1) temperature of polymerization, (2) partial pressure of BD, (3) concentration of cocatalyst (or molar ratio of Al/MNd)> (4) type of cocatalyst, (5) residence time of the rare earth catalyst in the polymerization reactor. 

In a scientific paper on the control of molar mass in gas-phase polymerization, the importance of Ai/ Nd is also emphasized, hi addition, a combination of the cocatalysts TIBA and DIBAH is recommended. By the use of two aluminum alkyl compounds the concentration ratio of two different active Nd-species is adjusted. As these two species produce different molar masses and MMDs the combination of TIBA and DIBAH allows for the control of these two parameters [229,230]. 

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. 

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... 

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]. 

Metallocene catalysts provide PE with control of molar mass and a narrower molar mass distribution than other catalysts. These PE will have precise control of... 

Riccitiello et al. have synthesized preceramic polymers with Si—B bonds in their backbones by a Wurtz analogous reaction of dialkyldichlorosilanes and boron halides, either with or without adding methyl iodide for the control of molar masses of the condensation products. Most of the polymers obtained are solid, and soluble in hydrocarbons. Even though the Wurtz reaction is not specific, the IR spectra of the polymer clearly indicate that Si—B bonds have formed preferentially, but do not provide any evidence for the presence of Si—Si or B—B bonds. Based on these results the authors suggest that the backbones generated mainly consist of an alternating sequence of Si and B [55-59]. 

The MMD of a polymer is of prime importance in its application. In most instances, there is a molar-mass (MM) range for which a given polymer property will be optimal for a particular application. The control of molar mass (expressed in g/mole) and of its distribution is essential for the practical application of a polymerization process, since its utility is greatly reduced unless the reaction can be carried out to yield polymer of a sufficiently high and specified molar mass. 

With the development of controlled radical polymerization techniques like nitroxide-mediated radical polymerization (NMRP), atom transfer radical polymerization (ATRP), and reversible addition fragmentation chain transfer (RAFT) polymerization (see Section 3.2), the field of linear glycopolymers has significantly flourished, especially as control of molar mass and monomer sequence has become available, even for functionalized monomers. This enables incorporation of new and more complex glycomonomers as well as allows controlled dispersity, end group functionality, and monomer sequences in block, star-shaped, and graft copolymers, and eventually... 

Controlled free radical polymerization of acrylonitrile has been achieved by ATRP (atom transfer radical polymerization) [741-747]. Control of molar mass was shown to be possible up to values of = 30,000 g/mol with polydispersities as low as 1.06. However, end group control was limited. This was attributed to oxidation of the free radical to an anion by reaction with Cu [741-743]. 

Alhamad B, Romagnoli JA, Gomes VG. On-hne multi-variable predictive control of molar mass and particle size distributions in free-radical emulsion copolymerization. Chem Eng Sci 2005 60 6596-6606. 

Owing to the monomer activation, polymerization undergoes a drastic acceleration while keeping a good control of molar masses and distributions. For instance, the polymerization rate of POx is multiplied by a factor of 460 after the addition of 0.25% methylaluminum bis(2,4,6-tri-tert-butylphenolate). ... 

A wide spectmm of copolymers can be prepared via spontaneous or sequential ATRP of two or more monomers with precise control of molar mass, composition, and functional-... 

Attempts for industrial processes using anionic polymerization in bulk and at high temperature are in progress. The corresponding materials offer many advantages as compared to the radical PS better thermal stability due to perfectly regular sequences, absence of oligomers, absence of any residual monomer, and better control of molar masses. 

Control of molar mass and molar mass distribution... 

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