Blocking polymers

Gaines [13] has reported on dimethylsiloxane-containing block copolymers. Interestingly, if the organic block would not in itself spread, the area of the block polymer was simply proportional to the siloxane content, indicating that the organic blocks did not occupy any surface area. If the organic block was separately spreadable, then it contributed, but nonadditively, to the surface area of the block copolymer. 

The first successhil use of lithium metal for the preparation of a i7j -l,4-polyisoprene was aimounced in 1955 (50) however, lithium metal catalysis was quickly phased out in favor of hydrocarbon soluble organ olithium compounds. These initiators provide a homogeneous system with predictable results. Organ olithium initiators are used commercially in the production of i7j -l,4-polyisoprene, isoprene block polymers, and several other polymers. 

The third generation are latices made with independentiy prepared surfactant to mimic the in situ prepared functional monomer surfactant. These emulsifiers are often A—B block polymers where A is compatible with the polymer and B with the aqueous phase. In this way surface adsorption of the surfactant is more likely. These emulsions are known to exhibit excellent properties. 

The anionic polymerization of methacrylates using a silyl ketene acetal initiator has been termed group-transfer polymerization (GTP). First reported by Du Pont researchers in 1983 (100), group-transfer polymerization allows the control of methacrylate molecular stmcture typical of living polymers, but can be conveniendy mn at room temperature and above. The use of GTP to prepare block polymers, comb-graft polymers, loop polymers, star polymers, and functional polymers has been reported (100,101). 

Fig. 4. MiceUular gelation mechanism. A shows micelle nuclei, highly cross-linked B, boundary where micelle growth terminates in styrene block polymers.

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. 

The stmcture of individual block polymers is deterrnined by the nature of the initiator (1,2-propanediol above), the sequence of addition of propylene and ethylene oxides, and the percentage of propylene and ethylene oxides in the surfactant. Thus, when the order of addition is reversed, a different stmcture is obtained in which the hydrophobic moieties are on the outside of the molecule. With ethylene glycol as the initiator, the reactions are as foUows ... 

A number of these stmctures are offered commercially by BASE Corporation under the trade name Tetronic polyols. The products are similar to oxygen block polymers. Although not strongly surface active per se, they are useful as detergents, emulsifiers, demulsifiers, defoamers, corrosion inhibitors, and lime-soap dispersants. They are reported to confer antistatic properties to textiles and synthetic fibers. 

ABA and ) n block polymers exhibit higher melt viscosities than do AB diblock copolymers with similar molecular weights. The former two... 

In most cases, these active defoaming components are insoluble in the defoamer formulation as weU as in the foaming media, but there are cases which function by the inverted cloud-point mechanism (3). These products are soluble at low temperature and precipitate when the temperature is raised. When precipitated, these defoamer—surfactants function as defoamers when dissolved, they may act as foam stabilizers. Examples of this type are the block polymers of poly(ethylene oxide) and poly(propylene oxide) and other low HLB (hydrophilic—lipophilic balance) nonionic surfactants. 

Interesting products may also be produced by introducing boron atoms into the chain. The amount of boron used is usualy small (B Si 1 500 to 1 200) but its presence increases the self-adhesive tack of the rubber, which is desirable where hand-building operations are involved. The products may be obtained by condensing dialkylpolysiloxanes end-blocked with silanol groups with boric acid, or by reacting ethoxyl end-blocked polymers with boron triacetate. 

IBI 1,4-Polyisoprene 1,4-Polybutadiene Poly(ethylene-co- propylene Polyethylene Inverse block polymer— properties dependent on composition... 

FIGURE 22.5 OTHdC study on the temperature range of the dissociation of the micelle of a styrene-isoprene two-block polymer in n-decane. Column 3.70 fim x 300 cm. (Reprinted with permission from Ref. 14. Copyright 1989 American Chemical Society.)... 

Improved polyurethane can he produced hy copolymerization. Block copolymers of polyurethanes connected with segments of isobutylenes exhibit high-temperature properties, hydrolytic stability, and barrier characteristics. The hard segments of polyurethane block polymers consist of 4RNHCOO)-n, where R usually contains an aromatic moiety. 

The dimer behaves simultaneously as a radical and as a carban-ion, and thus the radical end might grow by a radical mechanism, anionic polymerization proceeding from the carbanion end. This behavior is particularly interesting when two monomers are present in the system, one polymerizable by a radical but not by an anionic mechanism, the other behaving in the opposite sense. In such a hypothetical case the resulting product would be a block polymer, -A—A. . . A—B—B. . . B-. 

There are some indications that the situation described above has been realized, at least partially, in the system styrene-methyl methacrylate polymerized by metallic lithium.29 29b It is known51 that in a 50-50 mixture of styrene and methyl methacrylate radical polymerization yields a product of approximately the same composition as the feed. On the other hand, a product containing only a few per cent of styrene is formed in a polymerization proceeding by an anionic mechanism. Since the polymer obtained in the 50-50 mixture of styrene and methyl methacrylate polymerized with metallic lithium had apparently an intermediate composition, it has been suggested that this is a block polymer obtained in a reaction discussed above. Further evidence favoring this mechanism is provided by the fact that under identical conditions only pure poly-methyl methacrylate is formed if the polymerization is initiated by butyl lithium and not by lithium dispersion. This proves that incorporation of styrene is due to a different initiation and not propagation. 

These reactions are particularly useful if a polymer with two (or more) "living ends is formed, since the bifunctional (or polyfunctional) polymer formed may be then used to synthesize block polymers via condensation reactions. Interesting examples of such materials were obtained and the usefulness of this technique is amplified by the fact that each block can be made uniform in size. 

Formation of block polymers is not limited to hydrocarbon monomers only. For example, living polystyrene initiates polymerization of methyl methacrylate and a block polymer of polystyrene and of polymethyl methacrylate results.34 However, methyl methacrylate represents a class of monomers which may be named a suicide monomer. Its polymerization can be initiated by carbanions or by an electron transfer process, the propagation reaction is rapid but eventually termination takes place. Presumably, the reactive carbanion interacts with the methyl group of the ester according to the following reaction... 

Bismuth, excess entropy of solution of noble metals in liquid bismuth, 133 Block polymers, 181 Bond energies in the halogens, 61 Boron fluoride as initiator in polymerization, 156... 

One of the earliest examples of this methodology involves the reaction of a polymeric anion (formed by living anionic polymerization) with molecular oxygen to form a polymeric hydroperoxide which can be decomposed either thermally or, preferably, in a redox reaction to initiate block polymer formation with a second monomer (Scheme 7.25). However, the usual complications associated with initiation by hydroperoxides apply (Section 3.3.2.5). 

Triphenylinethyl terminated polymers (41) are formed in polymerizations conducted in the presence of triphenylmethyl thiol (40).9 5 Transfer constants for 40 are similar to other thiols (17.8 for S, 0.7 for MM A, compare Section 6.2.2.1). When the polymers (41) are heated in the presence of added monomer it is presumed that the S-CPh bond is cleaved and triphenylmethyl-mediated polymerization according to Scheme 9.11 can then ensue to yield chain extended or block polymers (42). 

A few studies have appeared on systems based on persistent nitrogen-centered radicals. Yamada et al.2"1 examined the synthesis of block polymers of S and MMA initiated by derivatives of the triphenylverdazyl radical 115. Klapper and coworkers243 have reported on the use of triazolinyl radicals (e.g. 116 and 117). The triazolinyl radicals have been used to control S, methacrylate and acrylate polymerization and for the synthesis of block copolymers based on these monomers [S,243 245 tBA,243 MMA,243 245 BMA,245 DMAEMA,24 5 TMSEMA,247 (DMAEMA-Wbc/fc-MMA),246 (DMAEMA-Woc -S)246 and (TMSEMA-6/ocfc-S)247]. Reaction conditions in these experiments were similar to those used for NMP. The triazolinyl radicals show no tendency to give disproportionation with methacrylate propagating radicals. Dispcrsitics reported arc typically in the range 1.4-1.8.2"43 246... 

The gel permeation chromatogram shown in Fig. 6 illustrates the purity of a block copolymer obtained by ion coupling. It is seen that about 5% of uncoupled block copolymer contaminates a triblock copolymer of narrow molecular weight distribution. The synthesis of star block polymers owes its recent development to the use of new coupling agents412. ... 

The living nature of ethylene oxide polymerization was anticipated by Flory 3) who conceived its potential for preparation of polymers of uniform size. Unfortunately, this reaction was performed in those days in the presence of alcohols needed for solubilization of the initiators, and their presence led to proton-transfer that deprives this process of its living character. These shortcomings of oxirane polymerization were eliminated later when new soluble initiating systems were discovered. For example, a catalytic system developed by Inoue 4), allowed him to produce truly living poly-oxiranes of narrow molecular weight distribution and to prepare di- and tri-block polymers composed of uniform polyoxirane blocks (e.g. of polyethylene oxide and polypropylene oxide). 

Formation of living polymers is not restricted to norbornene. For example, Grubbs successfully polymerized cyclooctatetraene to polyacetylene, and demonstrated the living nature of this polymer by forming block polymers with cyclooctadiene 19). 

The observation of Tsuji et al. 148) concerned with copolymerization of 1- or 2-phenyl butadiene with styrene or butadiene illustrates again the importance of the distinction between the classic, direct monomer addition to the carbanion, and the addition involving coordination with Li4. The living polymer of 1- or 2-phenyl butadiene initiated by sec-butyl lithium forms a block polymer on subsequent addition of styrene or butadiene provided that the reaction proceeds in toluene. However, these block polymers are not formed when the reaction takes place in THF. The relatively unreactive anions derived from phenyl butadienes do not add styrene or butadiene, while the addition eventually takes place in hydrocarbons on coordination of the monomers with Li4. The addition through the coordination route is more facile than the classic one. 

On the whole, curing procedures appear a promising way to obtain very stable polymer films. Thus, the structure of already mentioned polylysine has been revised as a block polymer involving either the a or e amino groups of lysine Vitamin Bj2 modified carbon electrodes were prepared by thermal curing of a mixture of a diamino functionalized derivative 5 and an epoxy prepolymer 6 of the araldite... 

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