Copolymers, block synthesis

Four block copolymers were synthesized in the current study and they were characterized by size-exclusion chromatography and H NMR measurements. Their characteristics are smnmarized in Table 1. All polymers are narrow polydispersed from the SEC measurements and they have a PI-PFDMS block ratio ranging from 1-2 to 11.5-1. 

To prepare micelle solutions, diblock copolymers were first dissolved in a small amount of THF, a common solvent for both blocks. Hexane, a precipitant for PFDMS, was added dropwise afterward with gentle stirring, and the solutions were monitored in a light-scattering apparatus. When a strong increase in lightscattering intensity indicated the onset of the aggregate formation, the addition of hexane was stopped. The solution was left to stir for 3 days at 23 °C to equilibrate the micellar structures. All micelle solutions have a final polymer concentration of 1 mg/mL. 

Bragg peaks are seen in the wide-angle X-ray scattering (WAXS) spectra of films formed from both the cylindrical and tapelike structures (Fig. 2). Both show a strong reflection at 6.4 A similar to that seen in PFDMS homopolymer.  

The scaling model of Vilgis and Halperin (VH) provides a theoretical Ifame-woik to help us understand aspects of our observations. VH consider a chain-folded crystalline core in which each chain experiences an integral number of chain 

Several methods can be used to synthesize block copolymers. Using living polymerization, monomer A is homopolymerized to form a block of A then monomer B is added and reacts with the active chain end of segment A to form a block of B. With careful control of the reaction conditions, this technique can produce a variety of well-defined block copolymers. This ionic technique is discussed in more detail in a later section. Mechanicochemical degradation provides a very useful and simple way to produce polymeric free radicals. When a rubber is mechanically sheared (Ceresa, 1965), as during mastication, a reduction in molecular weight occurs as a result of the physical pulling apart of macromolecules. This chain rupture forms radicals of A and B, which then recombine to form a block copolymer. This is not a preferred method because it usually leads to a mixture of poorly defined block copolymers. 

Using living polymerizations, the Shell Company was able to commercialize several poly(styrene-co-butadiene) and poly(styrene-co-isoprene) block copolymers known in the industry as Kraton 1101 and Kraton G. These block copolymers have found many uses in the shoe sole and adhesive industries. The physical properties were dependent on the macrostructure and microstructure of these block copolymers. 

As major examples, let us consider the three monomers butadiene, styrene, and acrylonitrile, and see how they can be block copolymerized together by mechanochemlcal means. From the large number of theoretical possibilities, 11 have been selected for discussion these may be prepared by mastication of the following  

A butadiene-styrene copolymer rubber with acrylonitrile monomer. 

Polyacrylonitrile (plasticized) with a mixture of butadiene and styrene monomers. 

Ionic reactions are particularly successful in preparing well-defined block copolymers by making use of the observation that there is no easily discernible termination 

The main limitation to the method is that the anion of one monomer must be able to initiate the polymerization of a second monomer, and this may not always be the case. Thus, polystyryl lithium can initiate the polymerization of methyl methacrylate to give an (A - B) diblock but, because of its relatively low nucleo-philicity, the methyl methacrylate anion cannot initiate styrene propagation. Best results are achieved when two monomers of high electrophilicity are used, e.g., styrene (St) with butadiene (Bd) or isoprene and (A - B - A) triblocks can be formed as shown in Equation 5.17a and Equation 5.17b. 

The triblock can also be prepared by coupling the two carbanions using an organic dihalide (Equation 5.18), and other coupling agents such as phosgene or dichloro-dimethylsilane are equally effective. This method can also be used to prepare radial blocks with multifunctional compounds, as illustrated with silicon tetrachloride. 

An interesting consequence of the marked differences in reactivity ratios found in some of the anionic systems is that in a mixture of monomers pure blocks of one can be obtained without incorporation of the second monomer. In styrene-butadiene mixtures, the latter reacts most rapidly and can be almost completely polymerized before the styrene begins to react. As the butadiene becomes depleted, styrene is 

Tribloek eopolymers can be eonstructed if a bifunctional initiator is generated, as when sodium naphthalene is used with styrene or a-methylstyroie. Radieal anions are formed, whieh eombine to give a dianion, and growth can then take place from both ends. Addition of a seeond monomer then yields a tribloek structure. 

---

The azo initiator initially present therefore has to be classified as a transfer agent—it is able to combine monomers polymerized by different polymerization modes with each other. Three different modes of block copolymer synthesis via azo transfer agents can be distinguished ... 

In a third type of block copolymer formation. Scheme (3), the initiator s azo group is decomposed in the presence of monomer A in a first step. The polymer formed contains active sites different from azo functions. These sites may, after a necessary activation step, start the polymerization of the second monomer B. Actually, route (3) of block copolymer formation is a vice versa version of type (1). It has been shown in a number of examples that one starting bifunctional azo compound can be used for block copolymer synthesis following either path. Reactions of type (3) are tackled in detail in Section III of this chapter. 

Although most examples reported in the literature used azo initiators with two transformation sites F per molecule, in a few cases block copolymer synthesis was accomplished with transfer agents containing only one reactive site. The resulting macro-azo-initiators did contain exclusively terminal azo groups. 

II. MACRO-AZO-INITIATORS (MAIs)—KEY ELEMENTS IN BLOCK COPOLYMER SYNTHESIS... 

The number and location of azo functions in the polymeric azo initiator is substantial regarding its application for block copolymer synthesis. The cases illustrated in Table 1 will be discussed here. 

The oxidizing capability of Ce(IV) has also been used for block copolymer synthesis starting from hydroxyl functional azo compounds, but not proceeding via the formation of MAIs vide infra). 

III. BLOCK COPOLYMER SYNTHESIS WITH LOW-MOLECULAR WEIGHT AZO COMPOUNDS... 

Moreover, block copolymers with two radically polymerizable monomers can be synthesized with a combination of thermal and photochemical polymerizations. Regarding their utilization in block copolymer synthesis, azocompounds with photoactive benzoin [103,109-111] and azyloximester groups [112] have been described. Two low-molecular weight azo benzoin initiators of the general formula (Scheme 32) were synthe-... 

The reactions of polymeric anions with appropriate azo-compounds or peroxides to form polymeric initiators provide other examples of anion-radical transformation (e.g. Scheme 7. 6). ""7i However, the polymeric azo and peroxy compounds have limited utility in block copolymer synthesis because of the poor efficiency of radical generation from the polymeric initiators (7.5.1). 

The hindered carbon-centered radicals are most suited as mediators in the polymerization of 1,1-disubstituled monomers e.g. MMA,78,95 other methacrylates and MAA,06 and AMS97). Polymerizations of monosubstituted monomers are not thought to be living. Dead end polymerization is observed with S at polymerization temperatures <100°C.98 Monosubstituted monomers may be used in the second stage of AB block copolymer synthesis (formation of the B block).95 However the non-living nature of the polymerization limits the length of the B block that can be formed. Low dispersities are generally not achieved. 

Living polymerization processes immediately lend themselves to block copolymer synthesis and the advent of techniques for living radical polymerization has lead to a massive upsurge in the availability of block copolymers. Block copolymer synthesis forms a significant part of most reviews on living polymerization processes. This section focuses on NMP,106 A TRP,265,270 and RAFT.- 07 Each of these methods has been adapted to block copolymer synthesis and a substantial part of the literature on each technique relates to block synthesis. Four processes for block copolymer synthesis can be distinguished. 

Hydrosilylation was successfully applied to block copolymer synthesis. Polystyrene with two H—Si end groups derived from the polystyrene dianion and di-... 

Another consequence of the absence of sponataneous transfer and termination reactions is that the polymer chains formed remain living 3), i.e. they carry at the chain end a metal-organic site able to give further reactions. Block copolymer synthesis is probably the major application 12 14), but the preparation of co-functional polymers, some chain extension processes, and the grafting onto reactions arise also directly from the long life time of the active sites. 

Polystyrene-polyamide block copolymer synthesis 63> also involves some kind of site transformation. The polystyrene precursor is fitted at chain end with a function... 

Kim KT, Vandermeulen GWM, Winnik MA, Manners I (2005) Organometallic-polypeptide block copolymers synthesis and properties of poly(ferrocenyldimethylsilane)-b-poly (gamma-benzyl-L-glutamate). Macromolecules 38 4958 961... 

Acid-containing polymers, hydrogen bonding, 260 Acrylic monomers photografting, 172,173/,174 UV curing, 212-213 Acrylic-acrylic block copolymers, synthesis, 259... 

Quasi-living Carbocationic Polymerization of Alkyl Vinyl Ethers and Block Copolymer Synthesis... 

Colfen, H. (2001) Double-hydrophilic block copolymers synthesis and application as novel surfactants and crystal growth modifiers. Macromotecular Rapid Communications, 22, 219-252. 

Block copolymer synthesis from living polymerization is typically carried out in batch or semi-batch processes. In the simplest case, one monomer is added, and polymerization is carried out to complete conversion, then the process is repeated with a second monomer. In batch copolymerizations, simultaneous polymerization of two or more monomers is often complicated by the different reactivities of the two monomers. This preferential monomer consumption can create a composition drift during chain growth and therefore a tapered copolymer composition. 

---