Precursor synthesis, anionic polymerization

Anionic polymerizations are well suited for the synthesis of polymers fitted at chain end with reactive functions. Block copolymers can result from reactions between suitable functions carried by two different functional precursors. In some cases the carbanionic sites themselves are the reactive functions. In other cases, functional polymers (obtained anionically, or by other methods) can be reacted with low molecular weight coupling agents. Here are a few examples ... 

Anionic polymerization techniques were also critical for the synthesis of a model cyclic triblock terpolymer [cyclic(S-fo-I-fr-MMA)] [196]. The linear cctw-amino acid precursor S-fr-I-fr-MMA was synthesized by the sequential anionic polymerization of St, I and MMA with 2,2,5,5-tetramethyl-l-(3-lithiopropyl)-l-aza-2,5-disilacyclopentane as the initiator and amine generator, and 4-bromo-l,l,l-trimethoxybutane as a terminator and carboxylic acid generator. Characterization studies of the intermediate materials as well as of the final cyclic terpolymer revealed high molecular and compositional homogeneity. Additional proof for the formation of the cyclic structure was provided by the lower intrinsic viscosity found for the cyclic terpolymer compared to that of the precursor. 

The synthesis and purification of polystyrene methacryloyl macromonomers (PS-MA) in the molecular weight range Mn= 1000-2000 g mol 1 by living anionic polymerization of styrene (S), termination with ethylene oxide (EO), and subsequent reaction with methacrylic chloride has already been described in detail elsewhere [180] (see also Scheme 16). In this context it has to be emphasized that the hydroxyethyl-terminated PS-MA macromonomer precursor (PS-OH) as obtained after purification of the crude PS-OH by silica column chromatography (cyclohexane/dichloromethane 1/1 v/v) and as charged in the PS-MA synthesis still contains up to about 15 wt-% of non-functionalized polystyrene (PS-H). This PS-H impurity of the PS-MA macromonomer does not interfere with the PS-MA synthesis and the subsequent TBA/PS-MA copolymerization and is easily and conveniently removed from the resulting PTBA-g-PS graft copolymer (see below). 

ABSTRACT. A new class of protected hydroxyl containing functionalized initiators were recently disclosed by the Defense Evaluation and Research Agency (DERA). These novel initiators have the general structure TBS-0-(CH2)n-Li. Excellent solubility in hydrocarbon solvents was exhibited by these materials which allowed the preparation of telechelic, high 1,4-microstructure polybutadienes. The two-step synthesis of these functionalized initiators from commercially available raw materials will be presented in detail. The first step involved reaction of an omega-haloalcohol with /-butyldimethylsilyl chloride, in the presence of an acid acceptor, to form the precursor. This precursor was then reacted with lithium metal in a hydrocarbon solvent to afford a solution of the functionalized initiator. The thermal stability of these initiators in hydrocarbon solution will also be presented. The application of the precursors and functionalized initiators in anionic polymerization of dienes will be briefly discussed. 

All these methods have several disadvantages multistep endgroup derivatizations are required for the synthesis of the telechelic PIB initiator precursor, and lithiation and thus the subsequent initiation of anionic polymerization is not quantitative or it requires extreme conditions. As a consequence of less than 100 % initiating efficiency these methods yielded mixtures of homopolymers and block copolymers. This is indicated by the fact that it was claimed that PMMA-Z -PIB-6-PMMA was only obtained by selective extraction ... 

Both the 2,2-diphenyl vinyl and the l-methoxy-l,l-diphenylethyl chain ends are potential endgroups for the anionic polymerization of a variety of monomers by metalation. Our earlier results indicate that quantitative metalation of the 2,2-diphenylvinyl endgroups with alkyllithium cannot be achieved, most likely because of steric hindrance. However, as described recently, the ether cleavage of 1-methoxy-l,l-diphenyl-3,3,5,5-tetramethylhexane or electron transfer to 3,3,5,5-tetra-methyl-l,l-diphenylhex-l-ene by K/Na alloy, Cs or Li led to quantitative metalation resulting in nearly quantitative initiation of the polymerization of methacrylic monomers. Both precursors led to identical (macro)initiators verified by H NMR. These compounds can be considered as models of PIB chain ends formed by LCCP of IB and subsequent end-capping with DPE. The present study deals with the application of this method to the synthesis of different AB and ABA block copolymers by the combination of LCCP and living anionic polymerization. 

A new method for the synthesis of polyisobutylene-based block copolymers, involving living carbocationic polymerization of isobutylene and subsequent living anionic polymerization of methacrylic monomers has been demonstrated. Di- and triblock copolymers nearly free of PIB precursor and with narrow and unimodal MWD were synthesized under well-controlled conditions. 

Combination of anionic polymerization and post polymerization reactions has been used for the synthesis of poly(acrylic acid-b-N,N-diethylacrylamide) (PAA-PDEA) copolymers [9]. Initially the synthesis of a precursor poly(tert-butylacrylate-b- N,N-diethylacrylamide) (PtBMA-PDEAAm) block copolymer was realized via sequential anionic polymerization of the tert-butyl acrylate and diethylacrylamide monomers. However, an amount of PtBMA homopolymer was detected in the crude reaction product. In order to remove the vast majority of the homopolymer, the authors proposed the precipitation of the crude product in hexane, where the homopolymer is highly soluble, in contrast to the block copolymer. The piuified block copolymer was subjected to deprotection of the tert-butyl group in acidic media, leading to the desirable DHBC. The final block copolymer showed pH and thermosensitive solution aggregation. 

The synthesis of a cydic triblock terpolymer, polystyrene-b-polyisoprene-b-poly(methyl methacrylate), was achieved by an end-to-end intramolecular amidation reaction of the corresponding linear a,o)-amino acid precursor [S-b-I-b-Methyl methacrylale (MMA)] under high-dilution conditions. The linear precursor was synthesized by the sequential anionic polymerization of styrene, isoprene, and MMA with 2,2,5,5-tetramethyl-l-(3-lithiopropyl)-l-aza-2,5-disilacydopentane as an initiator and 4-bromo-l,l,l-trimethoxybutane as a terminator. The separation of the unreaaed linear polymer from the cydic terpolymer was fadli-tated by the transformation of the tmreacted spedes into high-molar-mass polymers under high-concentration conditions. The SEC-estimated yield for the final cyclic terpolymer of molar mass of about lOOOOgmor was 90%. 

The sulfinyl precursor route is distinct from other radical/anionic polymerizations as the monomer is nonsymmetric (Scheme 7.2d). This means the polymer synthesis is more controlled and it is possible to achieve regiore-gular PPV materials. However, the monomers are more difficult to prepare (Scheme 7.3). Starting from the bis-sulfonium monomer 20, reaction with one... 

Method of synthesis polymer In final form cannot be processed, therefore precursor polymer Is synthesized first and then converted into film or the final forms. Precursor polymer can be obtained by one of the following methods Wessling route, ring opening polymerization, chemical vapor deposition, electropolymerization, condensation, phase transfer catalysis, or anionic polymerization Rnk, J K, High Performance Polymers, William Andrew, 2008. 

As a matter of fact, urea moieties, as already mentioned, are present in anionic polymerizations activated by N-carbamoyl lactams or their precursors and in the synthesis of block copolymers when NH2-terminated prepolymers are functionalized by isocyanates. When the substituted ureas are used as activators, two initiation mechanisms are possible. In the first route (Scheme 25),... 

Synthesis of Linear Polymer Precursors by Living Anionic Polymerization... 

Triisocyanates have also been used efficiently in the preparation of well-defined trifunctional PS star-shaped macromolecules, with PS exhibiting hydroxyl or amino groups at one end, obtained by anionic polymerization, being used as precursor [43]. These triisocyantes have also been applied for the synthesis of well-defined PEO star-shaped molecules [44]. 

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