Molecular weight, control

The original metallocene catalyst, Cp2ZrCl2/MAO, produced a polymer with a molecular weight of less than 5000 g mof. At 50°C, by using a singlesite catalyst, ethylene (l-indenyl)2ZrCl2/MAO, the molecular weight of the polymer could be increased to about 50,000 g mol  

The ability to produce a wide range of polymers with different molecitlar weights is useful and ethylene ohgomers are, of course, produced by the Aufbau reaction and SHOP process. Single-site catalysts are now able to provide a range of stereospecific ohgomers of propylene to replace or extend the use of current petrochemical processes (Chapter 7). They are used as ct-olefins or as intermediates in the manufacture of surfactants.  

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Solution Polymerization. Plant scale polymerizations ia water are conducted either adiabaticaHy or isotherm ally. Molecular weight control, exotherm control, and reduction of residual monomer are factors which limit the types of initiators employed. Commercially available high molecular weight solution polyacrylamides are usually manufactured and sold at about 5% soHds so that the viscosities permit the final product to be pumped easily. 

Emulsion Process. The emulsion polymerization process utilizes water as a continuous phase with the reactants suspended as microscopic particles. This low viscosity system allows facile mixing and heat transfer for control purposes. An emulsifier is generally employed to stabilize the water insoluble monomers and other reactants, and to prevent reactor fouling. With SAN the system is composed of water, monomers, chain-transfer agents for molecular weight control, emulsifiers, and initiators. Both batch and semibatch processes are employed. Copolymerization is normally carried out at 60 to 100°C to conversions of - 97%. Lower temperature polymerization can be achieved with redox-initiator systems (51). 

In addition to the monomers, the polymerization ingredients include an emulsifier, a polymerization initiator, and usually a chain-transfer agent for molecular weight control. 

Quality Specifications. Because of the extreme sensitivity of polyamide synthesis to impurities ia the iagredients (eg, for molecular-weight control, dye receptivity), adipic acid is one of the purest materials produced on a large scale. In addition to food-additive and polyamide specifications, other special requirements arise from the variety of other appHcations. Table 8 summarizes the more important specifications. Typical impurities iaclude monobasic acids arising from the air oxidation step ia synthesis, and lower dibasic acids and nitrogenous materials from the nitric acid oxidation step. Trace metals, water, color, and oils round out the usual specification Hsts. 

In nonaqueous copolymerization, fluorinated acyl peroxides are used as initiators that are soluble in the medium (12) a chain-transfer agent may be added for molecular weight control. 

The majority of thermal polymerizations are carried out as a batch process, which requires a heat-up and a cool down stage. Typical conditions are 250—300°C for 0.5—4 h in an oxygen-free atmosphere (typically nitrogen) at approximately 1.4 MPa (200 psi). A continuous thermal polymerization has been reported which utilizes a tubular flow reactor having three temperature zones and recycle capabiHty (62). The advantages of this process are reduced residence time, increased production, and improved molecular weight control. Molecular weight may be controlled with temperature, residence time, feed composition, and polymerizate recycle. 

AGE-Gontaining Elastomers. The manufacturing process for ECH—AGE, ECH—EO—AGE, ECH—PO—AGE, and PO—AGE is similar to that described for the ECH and ECH—EO elastomers. Solution polymerization is carried out in aromatic solvents. Slurry systems have been reported for PO—AGE (39,40). When monomer reactivity ratios are compared, AGE (and PO) are approximately 1.5 times more reactive than ECH. Since ECH is slightly less reactive than PO and AGE and considerably less reactive than EO, background monomer concentration must be controlled in ECH—AGE, ECH—EO—AGE, and ECH—PO—AGE synthesis in order to obtain a uniform product of the desired monomer composition. This is not necessary for the PO—AGE elastomer, as a copolymer of the same composition as the monomer charge is produced. AGE content of all these polymers is fairly low, less than 10%. Methods of molecular weight control, antioxidant addition, and product work-up are similar to those used for the ECH polymers described. 

The major applications of catalytic chain transfer are in molecular weight control and in synthesis of macromonomcrs based on methacrylate esters. However, they have also been shown effective in polymerizations and copolymerizations of MAA, MAM, MAN, AMS, S and some other monomers. 

The early attempts at NMP of S in emulsion used TEMPO and related nitroxides and needed to be carried out at high temperatures (100-130 °C) necessitating a pressure reactor. Problems with colloidal stability and molecular weight control and limiting conversions were reported.215 217... 

The activity of initiators in ATRP is often judged qualitatively from the dispersity of the polymer product, the precision of molecular weight control and the observed rates of polymerization. Rates of initiator consumption are dependent on the value of the activation-deactivation equilibrium constant (A") and not simply on the activation rate constant ( acl). Rate constants and activation parameters are becoming available and some valuable trends for the dependence of these on initiator structure have been established.292"297... 

HSi(CH3)2CH2CH29)CH2Cl/Me3Al system is strong evidence for molecular weight control by termination, i.e., for polymerization without chain transfer to monomer. The proposition is further substantiated by results of model experiments of Kennedy et al.26 and H1 NMR analysis of HSi-PEB to be discussed in Sect. IH.B.4.C. 

Although molecular weights in typical phenolic novolac syntiieses are intentionally limited, these procedures lack molecular weight control. The reactions are generally terminated prior to full conversion after a certain reaction time or... 

The first paper of this series concerns the effects of f-BuX, Me3Al, Et2AlX and EtAlCl2, and MeX on PIB yields and polymerization rates. The second paper1 will survey and discuss the effects of reaction variables on molecular weights of PIB and molecular weight control in isobutylene polymerization. 

A relation between A jjv and molecular weight controlling mechanisms was discovered and the effect of initiator system, solvent and temperature on a mv was explained. The present work has led to an understanding of the effect of counteranion on PIB molecular weight. These studies provide better insight into the detailed mechanism of isobutylene polymerization, in particular into the initiation and the molecular weight controlling events. 

Analysis of Molecular Weight Controlling Events Using Mayo Plots. 138... 

V. The Relationship Between AEmv and Molecular Weight Control in... 

This section concerns GPC studies carried out to determine the molecular weights and MWD of PIB s prepared with r-BuX/Et2AlX/MeX systems and to investigate the molecular weight controlling mechanisms. 

Mayo plot provides relative rate constants of the molecular weight controlling events22. Transfer and/or termination with solvent, polymer and impurities are assumed to be negligible. Although the Mayo equation is strictly valid only for data obtained at low conversion, Plesch23 has shown that the plot provides remarkably reliable information even with data obtained at relatively high conversions. 

In the course of these investigations it became apparent that the numerical values of AEmv s contained important information concerning molecular weight controlling events. Thus, it was decided to search the literature thoroughly and to assemble and evaluate available information in this regard. 

Table 8 is a comprehensive compilation of AEmv values obtained in this research together with those available from the literature. An examination of the data led to a definition of the effect of initiating system, solvent and temperature, and to a general hypothesis on molecular weight control in isobutylene polymerization. 

Another consequence of this hypothesis is that for every isobutylene polymerization system, there must exist three temperature regions over which molecular weight control is, respectively by termination, a combination of transfer to monomer and termination, and transfer to monomer alone although due to experimental limitations all three regions may not be possible to detect. 

For r-BuCl/Et2AlCl/MeX a change in AEjj from -1.8 to —4.6 kcal/mole brought about by changing the solvent from MeCl to MeBr suggests a change in molecular weight control from termination to a combination of termination and transfer to monomer. 

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