Terminating agent

Acetals. Acetal resins (qv) are polymers of formaldehyde and are usually called polyoxymethylene [9002-81-7]. Acetal homopolymer was developed at Du Pont (8). The commercial development of acetal resins required a pure monomer. The monomer is rigorously purified to remove water, formic acid, metals, and methanol, which act as chain-transfer or reaction-terminating agents. The purified formaldehyde is polymerized to form the acetal homopolymer the polymer end groups are stabilized by reaction with acetic anhydride to form acetate end groups (9). 

Low-molecular weight azo compounds have frequently been used in cationic polymerizations producing azo-containing polymers. Thus, the combination of ionically and radically polymerizable monomers into block copolymers has been achieved. Azo compounds were used in all steps of cationic polymerization without any loss of azo function as initiators, as monomers and, finally, as terminating agents. 

Regarding anion radical transfer, low-molecular weight azo compounds were used as terminating agents in anionic polymerizations. An interesting example is the addition of a living polystyrene chain to one nitrile group of AIBN [71]. The terminal styryl anion is likely to form... 

Most radicals are transient species. They (e.%. 1-10) decay by self-reaction with rates at or close to the diffusion-controlled limit (Section 1.4). This situation also pertains in conventional radical polymerization. Certain radicals, however, have thermodynamic stability, kinetic stability (persistence) or both that is conferred by appropriate substitution. Some well-known examples of stable radicals are diphenylpicrylhydrazyl (DPPH), nitroxides such as 2,2,6,6-tetramethylpiperidin-A -oxyl (TEMPO), triphenylniethyl radical (13) and galvinoxyl (14). Some examples of carbon-centered radicals which are persistent but which do not have intrinsic thermodynamic stability are shown in Section 1.4.3.2. These radicals (DPPH, TEMPO, 13, 14) are comparatively stable in isolation as solids or in solution and either do not react or react very slowly with compounds usually thought of as substrates for radical reactions. They may, nonetheless, react with less stable radicals at close to diffusion controlled rates. In polymer synthesis these species find use as inhibitors (to stabilize monomers against polymerization or to quench radical reactions - Section 5,3.1) and as reversible termination agents (in living radical polymerization - Section 9.3). 

Although the low temperature polymerization of methyl methacrylate in polar solvents such as THF was believed to proceed without termination, a more exhaustive investigation55) revealed some slow termination processes resulting from the formation of a, not yet specified agent, supposedly produced in the initiation step. Since the nature of the terminating agent is still unknown it is premature to discuss here the proposed schemes of this termination process. 

A-Acetylimidazole was found to be a very efficient terminating (capping) agent in the solid-phase synthesis of peptides.t40],[41] A terminating agent is used to block any N-terminal amino groups that have not reacted in the coupling steps.[40]... 

The main difficulty we had to overcome, however, was the propensity of the terminating agent to interact with any of the preceding steps of the sequence, thus causing the formation of a complex mixture. 

Different results were obtained using allylic alcohols as terminating agents. In spite of the expected reactivity of the alcoholic function as hydrogen donor only products deriving from the reaction of the double bond were obtained (23) according to the general pattern previously described for Heck-type reactions with allylic alcohols (24). 

So far we have considered olefins and alkynes as terminating agents. Another reaction, however, was available to effect C-C coupling in the last step, the Suzuki reaction. Results were satisfactory under the conditions shown in the Table (28). 

In addition, hydroperoxyl radicals become the active chain termination agents in the presence of ions and salts of transition metals (see Chapter 17). 

Polar solvents block the O—H bond of phenols in the reaction with peroxyl radicals due to the formation of hydrogen bond and decrease the activity of phenols as chain terminating agents [1,9,10]. 

HL Ball, G Bertolini. A-(Chlorobenzyloxycarbonyloxy)succinimide as a terminating agent for solid-phase peptide synthesis application to a one-step purification procedure. Lett Pept Sci 2, 49, 1995. 

A (amphiphilic core-shell) R =CjHs-i Terminating agent B (ej( situ kinetics) R =CeHg- Terminating agent ... 

There were several attempts to gain better control on the free radical polymerization process [18, 19], One of these methods was named the iniferter method. The compounds used in this technique can serve as m/tiator, trans/er agent and terminating agent [20-22], Another technique is based on the use of bulky organic compounds such as diaryl or triarylmethyl derivatives [23-25], The main disadvantages of these systems comprise slow initiation, slow exchange, direct reaction of counter radicals with monomers, and their thermal decomposition. Therefore, these techniques did not offer the desired level of control over the polymerization. 

Sequential addition of monomer works well in anionic polymerization for producing well-defined block copolymers [Morton, 1983 Morton and Fetters, 1977 Quirk, 1998 Rempp et al., 1988]. An AB diblock copolymer is produced by polymerization of monomer A to completion using an initiator such as butyllithium. Monomer B is then added to the living polyA carbanions. When B has reacted completely a terminating agent such as water or... 

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