Protonic solvents, living polymerization

GTP is a safe operation. A runaway polymerization can be quickly quenched with a protonic solvent. Since the group transfer polymerization goes to completion, no unwanted toxic monomer remains the silicone group on the living end after hydroxylation is removed as inactive siloxane. The living polymer in GTP is costlier than traditional polymerization techniques because of the stringent reaction conditions and requirements for pure and dry monomers and solvents. It can be used in fabrication of silicon chips, coating of optical fibers, etc. 

Termination occurs by transfer of a positive fragment, usually a proton, from the solvent or some transfer agent (often deliberately added), although other modes of termination are also known. Many anionic polymerizations are living polymerizations when the reaction components are appropriately chosen. 

We recently reported that BX3 (X=C1, Br) alone can initiate the living polymerization of IB using polar solvents in the presence of a proton trap to prevent... 

A rather unexpected water- and alcohol-insensitive activator, namely boron trifluoride etherate, was developed by Sawamoto for the living polymerization of p-hydroxystyrene in combination with the water adduct of p-methoxystyrene as initiator [78], The polymerizations proceeded even in large excess of water, which is in large contrast with the absolutely dry conditions that are normally required for carbocationic polymerizations. It is proposed that acetonitrile, which is used as polymerization solvent, stabilizes the short-lived carbocationic propagating species. The same polymerization methodology could be applied for the preparation of statistical and block copolymers consisting ofp-hydroxystyrene and b-methoxystyrene [79], as well as for the homopolymerizations of styrene, p-chlorostyrene and p-methylstyrene in the presence of a proton trap [80]. 

In carefully controlled systems (pure reactants and inert solvents), anionic polymerizations do not exhibit termination reactions. As we shall see shortly, such systems are referred to as living polymers however, because of the reactivity of carbanions with oxygen, carbon dioxide, and protonic compounds, termination occurs according to Equation 7.64. 

Propagation proceeds with complete consumption of monomer and the propagating anionic centers remain intact as long as one employs solvents such as benzene, n -hexane and tetrahydrofuran which are inactive in transferring a proton to the propagating anion. These polymerizations, referred to as living polymerizations, are terminated when desired by the deliberate addition of a proton source such as water or alcohol. 

This is shown in Figure 1, taken from Reference 43l, and illustrating the change of the position of a triplet of the CHrCH2 Protons directly in the living polymerization system, consisting of a deuterated Diox(-dg) and TO" SbFr initiator in CD,N02 solvent. 

Due to the high pK values of initiators and propagating chain ends, anionic polymerizations can undergo a variety of termination and chain transfer reactions. As a result, careful selection of solvent, initiator, and monomer is important for maintaining the living polymerization behavior. For example, acidic protons on monomers or the presence of alcohols will easily terminate carbanions. 

With conventional techniques and electrolytes, it was not possible to obtain living anions because they are rapidly protonated by tetraalkylammonium salts and residual water. The first report of the production of living polymers by an electrolytic method has to be attributed to Yamazald et al. [247], who used tetrahydrofuran as solvent, and LiAlH4 or NaAl(C2H5)4 as electrolyte for the polymerization of a-methylstyrene. A similar technique was used to polymerize styrene as well as derivatives [248-252]. 

The chain can be terminated by a chain transfer reaction with the solvent or by reaction with an impurity in the reaction mixture. If the solvent cannot donate a proton to terminate the chain and if all impurities that can react with a carbanion are rigorously excluded, chain propagation will continue until all the monomer has been consumed. At this point, the propagating site will still be active, so the polymerization reaction will continue if more monomer is added to the system. Such nonterminated chains are called living polymers because the chains remain active until they are killed. Living polymers usually result from anionic polymerization because the chains caimot be terminated by proton loss from the polymer, as they can in cationic polymerization, or by disproportionation or radical recombination, as they can in radical polymerization. 

The discovery of living cationic polymerization has provided methods and technology for the synthesis of useful block copolymers, especially those based on elastomeric polyisobutylene (Kennedy and Puskas, 2004). It is noteworthy that isobutylene can only be polymerized by a cationic mechanism. One of the most useful thermoplastic elastomers prepared by cationic polymerization is the polystyrene-f -polyisobutylene-(>-polystyrene (SIBS) triblock copolymer. This polymer imbibed with anti-inflammatory dmgs was one of the first polymers used to coat metal stents as a treatment for blocked arteries (Sipos et al., 2005). The SIBS polymers possess an oxidatively stable, elastomeric polyisobutylene center block and exhibit the critical enabling properties for this application including processing, vascular compatibility, and biostability (Faust, 2012). As illustrated below, SIBS polymers can be prepared by sequential monomer addition using a difunctional initiator with titanium tetrachloride in a mixed solvent (methylene chloride/methylcyclohexane) at low temperature (-70 to -90°C) in the presence of a proton trap (2,6-dt-f-butylpyridine). To prevent formation of coupled products formed by intermolecular alkylation, the polymerization is terminated prior to complete consumption of styrene. These SIBS polymers exhibit tensile properties essentially the same as those of... 

The NIR in situ process also allowed for the determination of intermediate sequence distribution in styrene/isoprene copolymers, poly(diene) stereochemistry quantification, and identification of complete monomer conversion. The classic one-step, anionic, tapered block copolymerization of isoprene and styrene in hydrocarbon solvents is shown in Figure 4. The ultimate sequence distribution is defined using four rate constants involving the two monomers. NIR was successfully utilized to monitor monomer conversion during conventional, anionic solution polymerization. The conversion of the vinyl protons in the monomer to methylene protons in the polymer was easily monitored under conventional (10-20% solids) solution polymerization conditions. Despite the presence of the NIR probe, the living nature of the polymerizations was maintained in... 

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