Living 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). 

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... 

Coupling of GTP living polymers with halide-terminating agents to form star polymers has been achieved [Hertler, 1996 Webster and Sogah, 1989]. Star polymers are also synthesized by using polyfunctional initiators or by copolymerization with dimethacrylate monomers. 

There are many electrophiles which not only terminate living polymer chains but also produce end-group substitution. For example, macromolecules with hydroxyl, carboxyl, thiol, or chlorine termini can be prepared by reacting living polymers with such compounds as epoxides, aldehydes, ketones, carbon dioxide, anhydrides, cyclic sulfides, disulfides, or chlorine (15-23). However, primary and secondary amino-substituted polymers are not available by terminations with 1° or 2° amines because living polymers react with such functionalities (1.). Yet, tert-amines can be introduced to chain ends by use of -N-N-di-methylamino-benzaldehyde as the terminating agent (24). 

A series of at least 14 papers [200-208] have been published dealing with the synthesis of telechelic polymers or block copolymers from the radical polymerization of various vinyl monomers with substituted 1,1,2,2-tetraphenyl ethanes. These aromatic compounds, known for over a century [209], are efficient in radical polymerization [201,210], They behave as both initiators and terminating agents [200] that can be involved in living radical polymerization as illustrated in the following reaction ... 

Although in this example the authors claimed no living character to the synthesis, Opresnik et al. [227,228] described a similar synthesis in which some living character is seen. They also used disulfides as reversible termination agents in the presence of styrene, MMA and ethyl acrylate (EA). The first step involves the synthesis of polymeric precursor 48 under UV cleavage ... 

The Inifer technique enables us to fulfil some requirements of polymer architecture even in some radical processes. An amplified form may be applied, the Iniferter variant, where the radical initiator simultaneously acts as a transfer and terminating agent. Otsu et al. used sulphides and disulphides (tetraethylthiuram disulphide, PhSSPh, Ph2S, PhCH2SSCH2Ph) [96] and carbamates (benzyl-A,A-diethyldithiocarbamate, p-xylylene-A,7V-diethyl-dithiocarbamate) [97] in the photopolymerization of methyl methacrylate and styrene, and phenylazotriphenylmethane in the polymerization of methyl methacrylate [98]. Living radical polymerizations yield polymers with defined end groups or the required block copolymers. 

Another route to synthesizing star polymers by living polymerization involves the use of multifunctional end-capping agents, but this method is not suited for metal-catalyzed radical polymerizations, at least so far, due to the lack of universal and convenient terminating agents, as described above (section III.B.2). 

The polyfunctional initiator method provides the possibility to prepare end-functionalized stars by deactivating the living branches by suitable electrophilic terminating agents. Polystyrene (PS) and PEO stars having end hydroxyl groups were prepared by this method.1519... 

Use of Functionalized Terminating Agents.66 68 The living polymer is terminated with a functionalized electrophilic reagent, according to the reaction ... 

Two related procedures have been developed to effect this transformation. Both Involve the Initial synthesis of mono- or dlfunctlonal living anionic polymers of styrene, butadiene, or block copolymers of both. They are then reacted via Grlgnard Intermediates (7 ) with either excess bromine or with excess m-xylylyl dlbromlde (8-10) to yield polymers with reactive halide terminal groups (benzyllc or allyllc depending upon the polymer and terminating agent). The reactions for polystyrene are shown In equations 2 and 3. 

Living polymers do not live forever and even in the absence of terminating agents decays with time [2]. The most stable of all living anionic systems are polystyryl carbanions, as they are stable for weeks in hydrocarbon solvents. The mechanism for the decay of polystyryl carbanions on aging, referred to as spontaneous termination, is not completely established. The generally accepted mechanism consists of hydride elimination ... 

As mentioned earlier, living polymers do not, in reality, have infinite life times even in the complete absence of terminating agents as they undergo decay on aging, a process known as spontaneous termination. Polystyryl carbanions, known to be the most stable of all anionic chains as they can survive for weeks in hydrocarbon solvents, undergo spontaneous termination by a mechanism known as hydride elimination, as shown by the equation (Odian, 1991) ... 

The living polyTHF may then be reacted with a suitable terminating agent to give the desired end-groups, either directly or after prior reaction with a weakly nucleophilic monomer, such as aziridine, to produce a less reactive chain end. Some of these reactions are shown in Scheme 5. ... 

Functionalized block copolymers by sequential monomer addition. Sequential addition of monomers to a living anionic polymerizing system is at present the most useful method for the synthesis of well-defined block copolymers.An AB diblock copolymer is produced by first completely polymerization of one monomer (A) using an initiator such as butyllithium. The second monomer (B) is then added to the living anions. When the second monomer has reacted completely, a terminating agent is introduced into the reaction mixture and the block copolymer is isolated.This method can be used to synthesize any of the various types of block copolymers (di-, tri-, tetra-... 

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