Chain growth polymerization transfer agent

The cobalt hydride in turn reacts with a new monomer molecules to regenerate the Co(II) [235]. One pubUcation describes a chain transferring agents that can be used in controlled polymerization of methacrylate monomers where reductive elimination of cobalt hydride from the neighboring methyl group deflects further chain growth [236]. The agent was illustrated as follows ... 

Use of Dialkylmagnesiums as Reversible Transfer Agents in Chain Growth Polymerizations... 

Scheme 1 Chain Growth Polymerization illustrated in the case of a neodymium complex ([Nd] [Nd]-Pol stands for species carrying a growing polymer chain) associated with a dialkylmagnesium chain transfer agent. Reversible and fast CGP, achieved with < transfer > Vop results in m as n as p...

An example of a commercial semibatch polymerization process is the early Union Carbide process for Dynel, one of the first flame-retardant modacryhc fibers (23,24). Dynel, a staple fiber that was wet spun from acetone, was introduced in 1951. The polymer is made up of 40% acrylonitrile and 60% vinyl chloride. The reactivity ratios for this monomer pair are 3.7 and 0.074 for acrylonitrile and vinyl chloride in solution at 60°C. Thus acrylonitrile is much more reactive than vinyl chloride in this copolymerization. In addition, vinyl chloride is a strong chain-transfer agent. To make the Dynel composition of 60% vinyl chloride, the monomer composition must be maintained at 82% vinyl chloride. Since acrylonitrile is consumed much more rapidly than vinyl chloride, if no control is exercised over the monomer composition, the acrylonitrile content of the monomer decreases to approximately 1% after only 25% conversion. The low acrylonitrile content of the monomer required for this process introduces yet another problem. That is, with an acrylonitrile weight fraction of only 0.18 in the unreacted monomer mixture, the low concentration of acrylonitrile becomes a rate-limiting reaction step. Therefore, the overall rate of chain growth is low and under normal conditions, with chain transfer and radical recombination, the molecular weight of the polymer is very low. 

Fig. 5.8 Examples of polymers grafted from nanocarbons, (a) An ATRP initiator covalently attached to RGO via nitrene and carbodiimide chemistry was used for the growth of poly(2-(ethyl (phenyl)amino)ethyl-methacrylate). (b) A RAFT chain transfer agent is covalently attached to GO prior to polymerization of vinylcarbozole.

Figure 5.10. Mechanism of atom-transfer radical living polymerization. In this process, addition of atom-transfer agents results in initiating radicals that react with monomers. Rather than terminating polymer growth, halogenated end units are formed that are capable of propagating chain growth when additional monomer is added.

In bulk polymerizations, the initial reaction mixture consists essentially of monomer. If the process is a chain-growth reaction, the mixture will also contain initiator and chain transfer agent, if needed. If the polymer and monomer are miscible, the system remains homogeneous during the polymerization reaction. 

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