Anionic polymerization technique

As previously described, all microspheres discussed in this chapter were synthesized from AB type diblock copolymers. Precursor block copolymers, poly(styrene-b-4-vinyl pyridine) (P[S-b-4VP]) diblock copolymers, were synthesized using the additional anionic polymerization technique [13]. The basic properties of the block copolymers were determined elsewhere [24,25] and are listed... 

As these block copolymers were synthesized using the anionic polymerization technique, their molecular weight distributions were narrow. The microspheres with narrower size distribution are better for well-ordered self-organization. Actually, all block copolymers synthesized for these works formed poly(4-vinyl pyridine) (P4VP) spheres in the PS matrices with narrow size distributions. 

The poly(styrene-b-isoprene) (P(S-b-IP)) and poly(-styrene-b-2-vinyl pyridine) (P(S-b-2VP)) block copolymers with narrow molecular weight distributions for blending with the microspheres were also synthesized using the additional anionic polymerization technique. The number-average molecular weights (Mns) and PS contents are also shown in Table 1. 

The synthesis of well defined block copolymers exhibiting controlled molecular weight, low compositional heterogeneity and narrow molecular weight distribution is a major success of anionic polymerization techniques 6,7,14-111,112,113). Blocks of unlike chemical nature have a general tendency to undergo microphase separation, thereby producing mesomorphic phases. Block copolymers therefore exhibit unique properties, that prompted numerous studies and applications (e.g. thermoplastic elastomers). 

Anionic polymerization techniques can also be applied to the synthesis of graft copolymers 6 7 87 1U). Kennedy s classification 134) is used here as shown in Scheme 5. 

The purpose of this review is to show how anionic polymerization techniques have successfully contributed to the synthesis of a great variety of tailor-made polymer species Homopolymers of controlled molecular weight, co-functional polymers including macromonomers, cyclic macromolecules, star-shaped polymers and model networks, block copolymers and graft copolymers. 

Star shaped macromolecules are polymers, where the one end of f > 2 (f functionality of the star) linear chains is chemically attached by covalent bonds to a small central linker unit, are the simplest form of branched polymers. Modern anionic polymerization techniques allow us to synthesize star systems with a large number of nearly monodisperse arms [133, 134],... 

An interesting procedure has been proposed for the synthesis of amylose-b-PS block copolymers through the combination of anionic and enzymatic polymerization [131]. PS end-functionalized with primary amine or dimethylsilyl, -SiMe2H groups were prepared by anionic polymerization techniques, as shown in Scheme 56. The PS chains represented by the curved lines in Scheme 56 were further functionalized with maltoheptaose oligomer either through reductive amination (Scheme 57) or hydrosilyla-tion reactions (Scheme 58). In the first case sodium cyanoborohydride was used to couple the saccharide moiety with the PS primary amine group. 

PS-b-PEO) , n = 3, 4 star-block copolymers were synthesized by ATRP and anionic polymerization techniques [149]. Three- or four-arm PS stars were prepared using tri- or tetrafunctional benzylbromide initiators in the presence of CuBr/bipy. The polymerization was conducted in bulk at 110 °C. The end bromine groups were reacted with ethanolamine in order to generate the PS stars with hydroxyl end groups. These functions were then activated by DPMK to promote the polymerization of ethylene oxide and afford the desired well-defined products (Scheme 73). 

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. 

Unlike the PEG molecules formed from anionic polymerization techniques, there now exist highly discrete forms of the polymer made by controlled addition of small PEG units to create chains of exacting molecular size. These discrete PEGs have a single molecular weight and do not display the polydispersity of the traditional PEG polymers. See Chapter 18 for a complete discussion of discrete PEG-based reagents and their applications. 

Lactomes may also be polymerized by ring-opening anionic polymerization techniques. While the five-membered ring is not readily cleaved, the smaller rings polymerize easily producing linear polyesters (structure 5.46). These polymers are commercially used as biodegradable plastics and in PU foams. 

Fast growing Interest in block copolymers originates from the Intramolecular phase separation, which Is responsible for some unique properties involving many potential applications. In this field again major progress has been accomplished due to the availability of the "living" anionic polymerization techniques. 

Some Block Copolymers Obtained by Anionic Polymerization Techniques... 

Anionic polymerization techniques have contributed to a very large extent to the development of tailot made molecules of various types. The long life time of the active sites is a factor of decisive importance for such synthesis it enables one to choose at will the molecular weights of the polymers to be made, and it ensures narrow molecular weight distribution of the samples. It involves the possibility of functionalizations at one or more chain ends, and of coupling reactions with bifunc-... 

The data in Table III illustrate that the addition of polypropylene showed an improvement in physical properties similar to the commercially available SBS triblock copolymer. This TPE, shown in Table III, shows outstanding physical properties. This unique block copolymer can only be made using the anionic polymerization technique. 

A second method, using also anionic polymerization techniques was developed to achieve a better knowledge of the functionality of the branch points. In this method the bifunctional living polymer species is prepared under the same conditions as above. It is then reacted with a stoichiometric amount of a plurifunctional electrophilic reagent14,1S. The chief difficulty is to find a reagent the electrophilic functions of which are isoreactive, to ensure that the reaction will go to completion. 

Dispersion Polymerization. AB block copolymer stabilizers were prepared using anionic polymerization techniques by the addition of hexamethylcyclotrisiloxane to "living" polystyryl-... 

Anionic polymerization techniques and naphthalene chemistry were used by Teyssie et al. to prepare A2B miktoarm stars, where A is poly(ethylene oxide) (PEO) and B is PS, PI, poly(a-methyl styrene) or poly(tert-butyl styrene) [25]. The reaction sequence is shown in Scheme 7. 

Star polymers having several PS branches and only one poly(2-vinyl naphthalene), PVN branch were prepared by Takano et al. using anionic polymerization techniques [31]. Sequential anionic block copolymerization of (4-vinyl-phenyl) dimethylvinylsilane (VS) and VN was employed. The double bonds attached to silicon have to remain unaffected during the polymerization of VS. This was ac-... 

A very popular and useful TPE is made from blocks of styrene and butadiene monomers using anionic polymerization techniques, which was described in the solution SBR section above. They are made up of short chains of polystyrene (usually 8000-15,000 MW), followed by a much longer chain of polybutadiene (about 60,000 MW), and capped off by another short chain of polystyrene, hence the name SBS. Similar polymers are prepared using isoprene instead of butadiene (SIS). The differences between SBS and SIS will be discussed later in the subsection Uses. ... 

Bonding of hydrophobic plastic materials to wood to create new wood-plastic (polystyrene) materials with improved mechanical and physical properties that incorporate the desirable features of each constituent is difficult to achieve. This is due to poor interfacial adhesion between the wood and polystyrene components because of their inherent incompatibility. New, well-defined, tailored cellulose-polystyrene graft copolymers have recently been prepared using anionic polymerization techniques. Preliminary bonding studies showed that these graft copolymers can function effectively as compatibi-lizers or interfacial agents to bond hydrophobic plastic (polystyrene) material to wood, evolving into a new class of composites. 

In order to synthesize polyfpropylene imine) dendrimers onto polystyrene via the divergent method, well-defined primary amine functionalized polystyrene had to be prepared as the core molecule. The anionic polymerization technique was chosen for the preparation of polystyrene (PS), because of the possibility of control over molecular weight and end group functionalization. An indirect amination procedure was developed [50], In this procedure a standard quantitative end-cap reaction with CO2 is used and a spacer is created between polystyrene and the primary amine function. The obtained polystyrene-COOH (PS-COOH), with Af around 3kgmol and values around 1.05, could be quantitatively reduced to the... 

Owing to the stability of the DPE radical, S/DPE polymers cannot be prepared using free radical polymerization, but they can be easily produced using the anionic polymerization technique. The copolymers are, however, limited to a maximum DPE content of 50 mol% because two consecutive DPE units are not possible in the polymer chain for steric reasons. This leads to a reactivity ratio rDPEC dd/ ds) = 0. 

The use of supercritical-fluid-extraction techniques in the fractionation of polysiloxanes has been demonstrated by the data presented. The poly-dispersities of the fractions were comparable with those generally attainable only by anionic-polymerization techniques, with which the incorporation of two functional groups is often difficult to attain. The ability to isolate these well-defined fractions will lead to important fundamental studies on structure-property relationships in multiphase copolymer systems. 

As discussed so far in this section, the helical polymethacrylates are synthesized predominantly using anionic polymerization techniques. However, recently, more versatile, inexpensive, and experimentally simple free-radical polymerization has been proved to be an alternative, effective way to prepare helical polymethacrylates from some monomers. Although the stereochemical control of radical polymerization is generally more difficult compared with that in other types of polymerization,69 an efficient method would make it possible to synthesize helical, optically active polymers having functional side chains by direct radical polymerization without using protective groups. In the radical polymerization of bulky methacrylates, helix-sense selection is governed by the chirality of a monomer itself or an additive. 

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