Copolymers, block styrene-tetrahydrofuran

Materials. An AB block copolymer of styrene and tetrahydrofuran (ST) and an ABA block copolymer of styrene-tetrahydrofuran-styrene (STS) were synthesized by the ion-coupling reaction between the living ends of polystyryl anions and polytetrahydrofuran cations as reported previously (3, 4). To re-... 

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). 

A block copolymer of styrene and ethylene oxide has also been prepared in this manner (70, 69). a-Methyl styrene in sodium-naphthalene-tetrahydrofuran also polymerized at — 78°. The polymerization is... 

Polymerization Procedure and Characterization. Cyclic ethers and formals were polymerized by adding a measured amount of monomer into the initiator solution at 0°C. The polymer was precipitated with methanol or ethyl ether and freeze-dried from benzene or fractionated by chloroform. The block copolymer of styrene and tetrahydrofuran was dissolved in 1-butanol and refluxed for 12 hours with sodium metal. The solution was washed with water, and the 1-butanol was distilled off. The residual polymer was freeze-dried from benzene, and poly-THF was extracted with 2-propanol in a Soxhlet apparatus. 

The melting temperature of polytetrahydrofuran is ca. 43°C, and its glass transition temperature is ca. — 86°C whereas polystyrene is usually an amorphous polymer at its glass transition temperature of 90°C. Furthermore, the surface tension of polytetrahydrofuran is slightly lower than that of polystyrene. Thus, styrene-tetrahydrofuran is an interesting block copolymer. This paper presents the results of morphological, crystallization, and some surface chemical studies of this block copolymer. 

Block copolymers consisting of segments with widely separated solubility characteristics have generated considerable interest because of their unusual surfactant properties. In fact, one of the earliest commercial block copolymers were the Wyandotte "Pluronics." These were poly(propylene oxide-b-ethylene oxide) prepared by sequential addition of ethylene oxide to sodium alkoxide initiated propylene oxide (37,38). Szwarc (39) and others (40,41) prepared poly(styrene-b-ethylene oxide) by addition of ethylene oxide to polystyrene anions in tetrahydrofuran. Other syntheses of AB or ABA block copolymers of styrene-ethylene oxide include sequential addition in various solvents, and coupling reactions (42,43). 

It has been shown that in highly crystalline polymers having multiple transitions, such as poly(ethylene terephthalate), the glass transitions may be determined only by gas chromatography [170, 174, 177, 204]. The glass transition was also detected by gas chromatography in the copolymers acrylonitrile-vinyl acetate [201], acrylonitrile-a-methylsty-rene and the terpolymers of these monomers with vinyl acetate [207], polystyrene-butadiene [199], and styrene-tetrahydrofuran block copolymers [208]. 

The synthesis of block copolymers of controlled structures is most conventionally accomplished through the use of living anionic polymerization. One can easily imagine, however, desirable block copolymers derived from monomers which are inert to anionic polymerization conditions, or which do not share any common mode of polymerization. In a recent series of papers (24-34), Richards and coworkers have addressed this problem in a general way, and have developed methods which convert one kind of active center into another. Within the context of cyclic ether polymerizations, Richards has focused on the preparation of block copolymers of styrene and tetrahydrofuran (THF) several methods of accomplishing this copolymerization are described in the following paragraphs. 

Fig. 3.26 Universal calibration curve for crosslinked polystyrene gels with tetrahydrofuran as solvent %y linear polystyrene 0 branched polystyrene (comb type) +, branched polystyrene (star type) A, branched block copolymer of styrene methyl methacrylate x, poly (methyl methacrylate) poly (vinyl chloride) V, graft copolymer of styrene methyl methacrylate , polybutadiene (reprinted with permission from Comprehensive Polymer Science, copyright 1989, Pergammon Press pic).

Not all monomers are anionically polymerizable. Nevertheless, one can take advantage of the activity of the living ends to introduce reactive end groups at the extremity of homopolymers and then use such end groups to initiate the polymerization of anionically non polymerizable monomers. This method has been applied to the synthesis of copolymers with polyvinyl and polylactone blocks19 and of copolymers with polyvinyl and polypeptide blocks20-2S). One can at last use both anionic and cationic polymerization to prepare block copolymers of tetrahydrofuran with styrene or methylstyrene2. ... 

Such living conditions are found principally In anlonlcally Initiated systems and Involve common monomers such as styrene, ormethylstyrene, butadiene and Isoprene (1,22). They are far less common In catlonlcally Initiated systems, there being virtually no established example Involving vinyl monomers, but some cyclic monomers such as tetrahydrofuran (THF) and the oxetanes may be polymerized under carefully specified conditions to yield living polymers ( ). Although living free radical systems have also been described In which radicals have been preserved on surfaces. In emulsion, or by precipitation before termination occurs, these are special conditions not easily adapted for clean block copolymer synthesis. 

Tuzar and coworkers [294] investigated the micellization behavior of styrene-butadiene star-block copolymers with four arms and polybutadiene inner blocks in the mixed solvent tetrahydrofuran/ethanol, selective for polystyrene blocks. 

Optical densities at 269.5 nm for polystyrene solutions at concentrations of 0-1 X 10"2 mole/liter and for poly(styrene-co-methyl methacrylate) solutions at a total concentration of 1 X 10 2 mole/liter are presented in Figure 1 as functions of styrene content. The solvents were (from the top) dioxane, chloroform, tetrahydrofuran (THF), tetrachloroethane (TCE), and dichloro-ethane (DCE). It is evident that the linear relationship between optical density and styrene concentration that is valid for a polystyrene at all concentrations (open circles) does not hold for the statistical copolymers (solid circles). For example, copolymer (25-80 mole % styrene) solutions in chloroform deviate markedly from linearity the maximum per cent decrease in extinction coefficient (hypochromism) corresponds to a copolymer containing 50 mole % styrene. We define hypochromism as the decrease in absorption intensity at 269.5 nm per chromophore of the statistical copolymer relative to that of the atactic polystyrene. It is also evident from Figure 1 that the alternating copolymer also gives a sharp hypochromism whereas block copolymers and mechanical mixtures of polystyrene and poly (methyl methacrylate) do not deviate from the straight line. Similar results were obtained with the other solvents, but the composition range where hypochromism appears depends on the solvent used. 

While the equilibrium thermodynamic approaches of Meier (1969,1970, 1971) and Inoue et al (1970a,h) predict that particular compositions will have particular fine structures, several investigators have shown that materials cast from different solvents and subsequently dried differ from each other and from materials prepared from the melt. As an example, let us examine the effects of the following solvents on a typical styrene-butadiene-styrene block copolymer benzene/heptane 90/10 tetrahydro-furan/methyl ethyl ketone 90/10, and carbon tetrachloride (Beecher et al, 1969). The particular compositions were chosen to give selective solvating behavior. While benzene dissolves both blocks, the heptane component, which evaporates last, swells only the butadiene block. Tetrahydrofuran is also a mutual solvent it evaporates first, leaving methyl ethyl ketone, which swells only the polystyrene block. Pure carbon tetrachloride is a mutual solvent. (Examples of swelling crystalline block copolymers are considered in Chapter 6.)... 

Near-IR spectroscopy (10000-4000/cm) was successfully used to monitor conversion dining conventional, anionic solution polymerisation of styrene and isoprene to homopolymers and block copolymers. The conversion of the vinyl protons in the monomer to methylene protons in the polymer was easily monitored under conventional (10-20% solids) solution polymerisation conditions. In addition to the need for an inert probe, high sampling frequencies were required since polymerisation times ranged from 5s in tetrahydrofuran to 20 minutes in cyclohexane. Preliminary data indicate that near IR is capable of detecting sequence distribution for tapered block copolymers, geometric isomer content, and reactivity ratios for free-radical copolymerisation. 20 refs. USA... 

Block Copolymers. Well defined block copolymers of 4-hydroxystyrene and styrene were prepared by firstly, the synthesis of low molecular weight blocks of styrene capped with TEMPO. The molecular weights of these styrene blocks were controlled by the ratio of unimolecular initiator (4) relative to styrene monomer (2). For example, 1-phenyl-l-(2, 2, 6, 6 -tetramethyl-r-piperidinyloxy)ethane (4) (1.67g, 0.0064 mol) was added to styrene (2) (20.0g, 0.192 mol) and heated to 125-130°C, under N2, for 48 hours. The reaction was then cooled to room temperature and the polymer dissolved in tetrahydrofuran (100 mL), and isolated by precipitation into methanol (1000 mL). The TEMPO terminated polystyrene (7) was then filtered, washed with methanol and dried in a vacuum oven overnight at 50°C. Isolated yield 90% of theory. M = 2764, M = 3062, PD = 1.10 (Theoretical A.M.U = 3120). 

The methods of both papers assume that the hydrodynamic volumes of the block copolymer sequences are additive, implying a negligible interaction between unlike segments. Chang s method [5] gave lower molecular masses than Runyon s method [4], but in the case of styrene-butadiene block copolymers in tetrahydrofuran (THF), the difference was negligible [5]. 

SMMA Styrene-methyl methacrylate block copolymer TEM Transmission electron microscopy THE Tetrahydrofuran... 

SMMA Styrene-methyl methacrylate block copolymer sPS Syndiotactic polystyrene S-S Simha and Somcynsky cell-hole theory SSE Single-screw extmder SSSE Solid-state shear extrusion TD Transverse direction TEM Transmission electron microscopy Tg Glass transition temperature THE Tetrahydrofuran TIBA fr -Isobutyl aluminum (°C) Melting temperature TMA fr -Methyl aluminum TMS Trimethylsilyl... 

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