Block copolymers polymerizations

B.C. = DMA-TrMA block copolymer Polymerization time (minutes)... 

Recent advances in synthetic polymer preparation and formulation have enabled a rapid advancement of nanomedicine, with over 40 formulations being approved for routine human use in the last 20 years and many more currently in clinical development [6]. Examples include polymer-drug and protein conjugates [5], block copolymer polymeric micelles [7] and polyplexes developed for... 

Mechanochemical degradation creates free macroradicals in pairs, practically without any side reactions, and most potential applications of this technique are centered around the formation and subsequent reactions of these reactive species. Elongational flow-induced degradation breaks polymer chains exactly at their center [42, 46]. This remarkable propensity is being explored in the author s laboratory as a simple means of obtaining well-defined block copolymers. Polymerization and... 

Rigid Chain Polymers Synthesis and Properties Structure and Properties of Block Copolymers Polymeric Drugs... 

The existence of living polymer radicals in emulsion polymerization of styrene [37] in the presence of oxidized polypropylene and triethylene tetraamine was ascertained by preparing a MMA—St block copolymer. Polymerization of styrene proceeded after removal of the initiator fi-om the... 

The desired form in homopolymers is the isotactic arrangement (at least 93% is required to give the desired properties). Copolymers have a random arrangement. In block copolymers a secondary reactor is used where active polymer chains can further polymerize to produce segments that use ethylene monomer. 

A brief review has appeared covering the use of metal-free initiators in living anionic polymerizations of acrylates and a comparison with Du Font s group-transfer polymerization method (149). Tetrabutylammonium thiolates mn room temperature polymerizations to quantitative conversions yielding polymers of narrow molecular weight distributions in dipolar aprotic solvents. Block copolymers are accessible through sequential monomer additions (149—151) and interfacial polymerizations (152,153). 

GopolymeriZation Initiators. The copolymerization of styrene and dienes in hydrocarbon solution with alkyUithium initiators produces a tapered block copolymer stmcture because of the large differences in monomer reactivity ratios for styrene (r < 0.1) and dienes (r > 10) (1,33,34). In order to obtain random copolymers of styrene and dienes, it is necessary to either add small amounts of a Lewis base such as tetrahydrofuran or an alkaU metal alkoxide (MtOR, where Mt = Na, K, Rb, or Cs). In contrast to Lewis bases which promote formation of undesirable vinyl microstmcture in diene polymerizations (57), the addition of small amounts of an alkaU metal alkoxide such as potassium amyloxide ([ROK]/[Li] = 0.08) is sufficient to promote random copolymerization of styrene and diene without producing significant increases in the amount of vinyl microstmcture (58,59). 

Polymerization ofiVIasked Disilenes. A novel approach, namely, the anionic polymerization of masked disilenes, has been used to synthesize a number of poly(dialkylsilanes) as well as the first dialkylamino substituted polysilanes (eq. 13) (111,112). The route is capable of providing monodisperse polymers with relatively high molecular weight M = lO" — 10 ), and holds promise of being a good method for the synthesis of alternating and block copolymers. 

Polymerization of methacrylates is also possible via what is known as group-transfer polymerization. Although only limited commercial use has been made of this technique, it does provide a route to block copolymers that is not available from ordinary free-radical polymerizations. In a prototypical group-transfer polymerization the fluoride-ion-catalyzed reaction of a methacrylate (or acrylate) in the presence of a silyl ketene acetal gives a high molecular weight polymer (45—50). 

Nonaqueous Dispersion Polymerization. Nonaqueous dispersion polymers are prepared by polymerizing a methacryhc monomer dissolved in an organic solvent to form an insoluble polymer in the presence of an amphipathic graft or block copolymer. This graft or block copolymer, commonly called a stabilizer, lends coUoidal stabiUty to the insoluble polymer. Particle sizes in the range of 0.1—1.0 pm were typical in earlier studies (70), however particles up to 15 pm have been reported (71). 

Ionic polymerizations are almost exclusively solution processes. To produce monodisperse polymers or block copolymers, they must be mn batchwise, so that all chains grow for the same length of time under identical conditions. 

Butadiene copolymers are mainly prepared to yield mbbers (see Styrene-butadiene rubber). Many commercially significant latex paints are based on styrene—butadiene copolymers (see Coatings Paint). In latex paint the weight ratio S B is usually 60 40 with high conversion. Most of the block copolymers prepared by anionic catalysts, eg, butyUithium, are also elastomers. However, some of these block copolymers are thermoplastic mbbers, which behave like cross-linked mbbers at room temperature but show regular thermoplastic flow at elevated temperatures (45,46). Diblock (styrene—butadiene (SB)) and triblock (styrene—butadiene—styrene (SBS)) copolymers are commercially available. Typically, they are blended with PS to achieve a desirable property, eg, improved clarity/flexibiHty (see Polymerblends) (46). These block copolymers represent a class of new and interesting polymeric materials (47,48). Of particular interest are their morphologies (49—52), solution properties (53,54), and mechanical behavior (55,56). 

Anionic polymerization, if carried out properly, can be truly a living polymerization (160). Addition of a second monomer to polystyryl anion results in the formation of a block polymer with no detectable free PS. This technique is of considerable importance in the commercial preparation of styrene—butadiene block copolymers, which are used either alone or blended with PS as thermoplastics. 

Mass-polymerized PVC also has a skin of compacted PVC primary particles very similar in thickness and appearance to the suspension-polymerized PVC skin, compared in Figure 3. However, mass PVC does not contain the thin-block copolymer membrane (7). 

Group-Transfer Polymerization. Living polymerization of acrylic monomers has been carried out using ketene silyl acetals as initiators. This chemistry can be used to make random, block, or graft copolymers of polar monomers. The following scheme demonstrates the synthesis of a methyl methacrylate—lauryl methacrylate (MMA—LMA) AB block copolymer (38). LMA is CH2=C(CH2)COO(CH2) CH2. 

The second front originates in the polymer synthesis community. Efforts are mainly directed toward production of monodisperse block copolymers by living polymerizations. These stmctures typically result in microphase separated systems if one block is a high T material and the other is elastomeric in... 

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