Polymerization ammonium-mediated

In the past decade, several new approaches for controlled NCA polymerization based on the classical primary amine initiation have been reported. In 2003, Dimitrov and Schlaad reported the controlled ( ammonium mediated ) polymerization of ZLL-NCA at elevated temperature using primary amine hydrochloride salts as initiators (Figure 4.3). The initiator reactivity most likely is due to the formation of a small amount of free amine by reversible dissociation of HCl. This equilibrium is strongly shifted toward the dormant amine hydrochloride species. Consequently, as soon as a free amine reacts with an NCA, the resulting adduct is immediately protonated and prevented from further reaction. The presence of protons in the system suppresses formation of unwanted NCA anions ( activated monomers ). The obtained polypeptide blocks exhibit a very narrow, close to a Poisson, molar-mass distribution. 

Scheme 13.2 Plausible mechanism for ammonium-mediated polymerization.

The combination of living cationic polymerization and NCA polymerization has also been reported. Schlaad synthesized poly(2-isopropyl-2-oxazoline)-h-PBLG (16) based on an ammonium-mediated polymerization system. By adding an acid in the NCA polymerization, a primary amino (NH2)-propagating chain end could be transformed into an ammonium (NH3 ) chain end via equilibrium and this system suppresses the generation of the aminyl anion of an NCA monomer that causes byproducts via activated monomer mechanism (Scheme 13.2) (Dimitrov and Schlaad, 2003). 

Scheme 13.3 Synthesis of PBLG-h-PIB-h-PBLG and (PBLG-h-PIB-h-PBLG) multiblock copolymer based on ammonium-mediated polymerization.

Meyer M, Schlaad H (2006) Poly(2-isopropyl-2-oxazoline)-poly(L-glutamate) block copolymers through ammonium-mediated NCA polymerization. Macromolecules 39 3967-3970... 

Other polymeric substrates, such as poly(N,N-dimethylacrylamide) and graft polymers derived from hydroxyl group containing crosslinked polystyrene and ethylene oxide are also nsed as solid phases. In the latter example diisopropylcarbodiimide is used in the coupling reaction. Also, diisopropylcarbodiimide/HOBt is used in the solid phase synthesis of polypeptides. Ammonium salts derived from polymer bound N-hydroxysuccinimide are also used in the EDC mediated amidation reaction. ... 

Fe(III) species can also be employed in a so-called reverse or alternative atom-transfer radical polymerization (section II.B.9). A mixture of FeCl3 and PPh3 can mediate a controlled polymerization of MMA in the presence of AIBN to give similarly narrow MWDs MJMn = 1.1—1.3).80 An ammonium halide such as />Bu4NBr can be employed in place of PPh3 as a ligand for FeBr3 in the AIBN-initiated radical polymerization.74... 

Using NMP [114, 115] or reversible addition-fragmentation chain transfer (RAFT) [ 119,120,127], agents with ammonium groups for the ion exchange allowed the attachment of initiators on the clay surface for controlled radical polymerizations (NMP, RAFT). Samakande et al. investigated the kinetics of RAFT-mediated living polymerization of styrene [120] and styrene/BA [119] mixtures in miniemulsion. 

Important chemical reactions mediated by enz3rmes, which are proteins, are often controlled by the interaction of a nucleophile with an electrophile. Proteins are polymeric structures made of amino acids units (called amino acid residues). Important nucleophilic groups include the hydroxyl (OH) group of the amino acid serine (72), the thiol unit (SH) of cysteine (73 see Chapter 27) and the nitrogen of the imidazole group of histidine (74). Note that the term nucleophilic in this biochemical context does not necessarily indicate reaction at carbon, but rather indicates a two-electron donor. The electron-deficient center (called an electrophilic site or electrophilic group) may be a metal such as Mg, Mn+2, Fe", or an ammonium unit (-NH3+). In some cases, the electron-deficient center is a carbonyl group (C=0) in which the electron donor is indeed a nucleophile because the reaction occurs at the carbonyl carbon. 

Jo TS, Yang M, BrowneU LV, Bae C. Synthesis of quaternary ammonium ion-grafted polyolefins via activation of inert C—H bonds and nitroxide mediated radical polymerization. J Polym Sd A Polym Chem. 2009 47 4519-4531. 

Braslau and coworkers [83] synthesized cyclic PSTY through the combination of nitroxide-mediated radical polymerization (NMRP) and CuAAC click reaction. The synthesis procedure was relatively complex compared with other strategies. 1 - [4-(Chloromethyl)phenyl] ethyl alkoxyamine was used to mediate the styrene polymerization, followed by successive azidation and oxidative cleavage with ammonium cerium(IY) nitrite in the presence of propargyl alcohol. The azide and alkyne groups were then introduced to each end of the polymer. Finally, the cyclization reaction was carried out in toluene with CuBr and PMDETA as catalyst at 100°C (Scheme 35). The cyclization results showed about 64% click product, as derived from Gaussian curve fitting. 

More monomers are mentioned in patent literature but details of the reaction conditions are discrete. One such example is the CCTP of a hydrophilic acrylic macromonomer. Sodium acrylate and ammonium acrylate homopolymers are mentioned and most likely to be examples of cobalt-mediated radical polymerization (CMRP) rather than of CCTP. ... 

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