Monomer, chain transfer

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). 

Recently many subtle effects of the ligand structure, concentrations of alkene, and conditions on the polymerisation have been reported to have significant effects on molecular weight, regioselectivity, branching, stereoselectivity or enantioselectivity, incorporation of other monomers, chain transfer, etc. Often these subtle effects can be understood from the mechanism, or they contribute to the understanding of the detailed processes going on. 

The first term in the denominator denotes termination by a combination of coupling and disproportionation, and the other terms denote chain transfer by monomer, chain-transfer agent, and initiator, respectively. 

A chain-transfer constant C for a substance is defined as the ratio of the rate constant klr for the chain transfer of a propagating radical with that substance to the rate constant kp for propagation of the radical. The chain-transfer constants for monomer, chain-transfer agent, and initiator are then given by... 

The monomer chain-transfer constants are generally small for most monomers—in the range 10 5 to 10-4 (Table 3-4). Chain transfer to monomer places the upper limit to the polymer molecular weight that can be obtained, assuming the absence of all other transfer reactions. Transfer to monomer does not, however, prevent the synthesis of polymers of sufficiently high molecular weight to be of practical importance. Cm is generally low because the reaction... 

Despite the complex interaction between the components of a catalyst recipe, for example consisting of catalyst, co-catalyst, electron donors (internal and external), monomers, chain-transfer agents such as hydrogen, and inert gases and the catalyst support, the local polymer production rate rate (polymerization rate) in a given volume, Rp (kg polymer hr"1), can often be described by a first-order kinetic equation with respect to the local monomer concentration near the active site, cm (kgm"3), and is first order to the mass of active sites ( catalyst ) in that volume, m (kg) ... 

Solvent polarity and temperature also influence ihe results. The dielectric constant and polarizability, however, are of little predictive value for the selection of solvents relative to polymerization rates and behavior. Evidently evety system has to he examined independently. In cationic polymerization of vinyl monomers, chain transfer is the most significant chain-breaking process. The activation energy of chain transfer is higher than that of propagation consequently, the molecular weight of the polymer increases with decreasing temperature. 

As can be seen in Fig. 1, no gel effect was observed even beyond the gel point in contrast to the polymerization of common multivinyl compounds such as EDMA [6]. This unusual polymerization behavior is characteristic of allyl polymerization, in which monomer chain transfer is essentially a termination reaction [42] in contrast to bimolecular termination of growing polymer radicals in common vinyl polymerization. In other words, primary chain length is quite short and kept nearly constant during polymerization as is required for the application of Eq. (4). 

The most significant difference between allyl and vinyl polymerizations is in the length of their primary chains which has a predominant influence on gelation, since in the allyl polymerization an occurrence of monomer chain transfer is quite remarkable and only the oligomer is formed. Therefore, we tried to carry out the telomerization of EDMA in the presence of CBr4 in order to reduce the primary... 

We now use this result in writing the net rate of monomer consumption. As a first approximation we will neglect the nmnotner consumed by monomer chain transfer. The net rate of monomer consmnption, — r, is the rate of consumption by the initiator plus the rate of consumption by all the radicals in each of the propagation steps (r ). 

Problem 6.22 Vinyl acetate has a relatively high monomer chain transfer constant (2x10 at 60°C). What is the upper limit of molecular weight of poly(vinyl acetate) made by radical polymerization at 60°C ... 

The monomer chain transfer constants are generally small for most... 

The relative rates of transfer and propagation are given by the ratio ktr,u/kp, which is the monomer chain transfer constant Cm- The value of... 

Polymer molecular weights are low for anionic polymerizations of propylene oxide (< 5000) since polymerization is severely limited by chain transfer to monomer. Chain transfer to monomer can take place by proton abstraction from the methyl group attached to the epoxide ring ... 

The second column, Presence or Absence of Events that Determine the Number of Polymer Chains (N), comprises 6 sub-columns and shows the 28 scenarios selected for examination. Among the very large number of theoretically possible scenarios we have selected for analysis only those which seemed to be closest to real-life situations. Specifically, we have examined the effects on N of fast and slow initiation (and in the case of slow initiation, the respective effects of slow ion-generation and slow cationation), the effect of initiation by adventitious protic impurity (i.e., HX = H20 ), and the effects of chain transfer to monomer, specifically both monomolecular or zero-order-in-monomer chain transfer and bimolecular or first-order-in-monomer chain transfer. The presence or absence of these effects is indicated by the symbols, 1 or 0 in column two. The organization of the 28 scenarios is as follows ... 

Problem 6.19 Vinyl acetate has a relatively high monomer chain transfer constant (2x 10 ... 

Table 6.5 Some Values of Monomer Chain Transfer Constants...

The ratio ktr,ulkp defines the monomer chain transfer constant Cm and, as in the case of free-radical polymerization (Section 6.8.1), its value determines the polymer molecular weight, in the absence of other chain termination processes. 

Chain Transfer In the absence of any externally added transfer agent, three transfer reactions [Eq. (9.9)-(9.12)] may be considered, viz., chain transfer with monomer, chain transfer with aUcylaluminum, and spontaneous chain transfer. 

Graft epoxy-acrylic copolymer prepared with a free radical initiator is an example of the "grafting from" process. In the case where benzoyl peroxide was used as the free radical initiator, it is determined that about 77% of the free radical initiator instead of causing initiation of monomers, chain transfers with the epoxy resin backbone, followed by the "grafting from" of22onomers onto the epoxy resin. Benzoyl peroxide is known to decompose mostly (90 ) to the benzoyloxy radical and the (10 ) phenyl radical. Mechanisms of grafting can be demonstrated in the following two schemes. (Schemes A and B). 

Chain Transfer Reactions Chain transfer reactions to polymeric organoalkali compounds can occur from solvents, monomers, and additives that have p/f values lower than or similar to those of the conjugate acid of the carbanionic chain end [3]. Relatively few monomers that undergo anionic polymerization exhibit chain transfer to monomer. Chain transfer has been well documented for the anionic polymerization of 1,3-cyclohexadiene. The chain transfer constant was calculated to be 2.9 x 10 at... 

Chain transfer to initiator Chain transfer to monomer Chain transfer to dimer Chain transfer to solvent Chain transfer to polymer... 

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