Molar Mass Regulation

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. 

As early as 1980 Shen et al. demonstrated that aluminum compounds such as DIBAH are active chain transfer agents [92], This result was subsequently confirmed by several authors, e.g. [87,134,141,165,166,178,460]. It was also recognized that cocatalysts of the type HA1R2 are more active chain transfer agents than the respective hydrogen free A1R3 cocatalysts [ 134,179]. 

In contrast to these observations, no influence on molar mass was found when the amount of aluminum alkyl cocatalyst was varied for two allyl Nd-based catalyst systems (1) Nd(tj3- CsHs Cl 1.5 THF and (2) Nd( y3- C3H5)C12- 2 THF [292]. 

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 

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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). 

Polymerization temperature has an influence on polymerization rate, molar mass, MMD and cis- 1,4-content. As molar mass regulation by adjustment of polymerization temperature is important from an industrial point of view this aspect is given special emphasis in this subsection. 

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 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]. 

In various studies on molar mass regulation by variation of the amount of cocatalyst different interpretations concerning the rate of chain transfer between Mg, A1 and Zn on one hand and Nd on the other hand are given. Detailed kinetic studies on the complex molar mass regulation processes between Nd centers and the cocatalyst are still lacking. What are the relative rates of chain transfer and chain propagation ... 

Insulin is a hormone responsible for the regulation of glucose levels in the blood. An aqueous solution of insulin has an osmotic pressure of 2.5 mm Hg at 25°C. It is prepared by dissolving 0.100 g of insulin in enough water to make 125 mL of solution. What is the molar mass of insulin ... 

The dependence of molar mass on polymerization temperature is exploited for the regulation of molar mass in the large-scale production of BR [81,82]. 

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 Co- and Ni-mediated polymerizations of dienes the molar mass is commonly regulated by metal-free agents such as hydrogen, cyclooctadiene, 1,2-butadiene etc. In Nd-catalysis these additives do not result in a reduction of molar mass. What is the reason for this difference and are there metal-free chain modification agents which work with Nd-catalysts ... 

A method for the controlled emulsion polymerization of chloroprene using dithiocarbamic esters as sulfur-based chain transfer agents is described. The method provides industrially relevant molar masses with Mn s> 50,000 daltons with good yields in acceptable times. It was further determined that when pKa values for the dithiocarbamic acid precursors were less than 12, the thioester was ineffective as a regulator. 

Figure 1.2 Regulation of hyaluronan amount and chain length by expression of a specific HAS protein. Biochemical characterizations of the vertebrate HAS enzymes expressed in mammalian cell culture have revealed similarities and differences between the respective mammalian hyaluronan synthase enzymes. The differences are depicted in this cartoon. HASl produces small amounts of high-molar-mass hyaluronan. HAS2 produces significantly more high-molar-mass hyaluronan. HAS3 is the most active of the hyaluronan synthases, yet produces low-molar-mass hyaluronan chains. The physiological significance of these differences in enzymatic activity is not yet known [33].

Chain transfer agents (CTA) are added to a latex formulation to help regulate (i.e., decrease) the molar mass and molar mass distribution of the latex polymer. The extent of chain transfer can be predicted, if the chain transfer constants (Cs) are known for a given monomer system. Riddle [114] presents a table summarizing some of the chain transfer constants for methyl methacrylate with a wide variety... 

Metallocene catalysts, for example dicyclopentadienyl zirconium dichloride, in combination with the cocatalyst methylalumoxane, helped establish a regulated set of properties, making it possible to customize molar mass, molar mass distribution, tacticity, heat resistance, rigidity, hardness, cold impact strength, and transparency. Added to these advantageous physical properties is the reactivity of this catalyst... 

After drawing air through the acid solution for 10.0 min at a rate of 10.0 L/min, the acid was titrated. The remaining acid needed 13.1 mL of 0.0588 MNaOH to reach the equivalence point, (a) How many grams of NH3 were drawn into the acid solution (b) How many ppm of NH3 were in the air (Air has a density of 1.20 g/L and an average molar mass of 29.0 g/mol under the conditions of the experiment.) (c) Is this manufec-turer in compliance with regulations ... 

Isobutylene, CH2=C(CH3)2, is predominantly produced from cracked petroleum gases, and also, partially, by the dehydration of /-butanol. In industrial polymerizations, isobutylene is liquefied on addition of some diisobutylene, and mixed with about the same quantity of liquid ethylene and then cationically polymerized at -80°C with BF3/H2O. The diisobutylene acts as chain transfer agent and regulates the molar mass. The ethylene does not polymerize under these conditions on the other hand, it dissipates the heat of polymerization by volatilizing. 

The alfin polymerization yields extremely high-molar mass poly (butadienes) with 65%-75% transAA structures. 1,4-Dihydro benzene or 1,4-dihydro-naphthalene serve as regulators to control the molecular weights. Industrially produced butadiene copolymers contain 5%-15% styrene or 3%-10% isoprene. 

The molar mass is additionally fixed by these regulators so that mastication is no longer necessary. The cold polymerization is more favorable than the warm polymerization, since more /rnns-rich structures are produced. Ci5-rich polymers, of course, tend more to cyclization, which produces "stringiness,"" that is, an undesirable increase in viscosity, during subsequent processing. Buna S can be mixed directly with natural rubber. It is primarily used for the running surfaces of car tires. 

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