Bromination of polystyrene

Cross-linked polystyrene can be acylated with aliphatic and aromatic acyl halides in the presence of A1C13 (Friedel-Crafts acylation, Table 12.1). This reaction has mainly been used for the functionalization of polystyrene-based supports, and only rarely for the modification of support-bound substrates. Electron-rich arenes (Entry 3, Table 12.1) or heteroarenes, such as indoles (Entry 5, Table 15.7), undergo smooth Friedel-Crafts acylation without severe deterioration of the support. Suitable solvents for Friedel-Crafts acylations of cross-linked polystyrene are tetrachloroethene [1], DCE [2], CS2 [3,4], nitrobenzene [5,6], and CC14 [7]. As in the bromination of polystyrene, Friedel-Crafts acylations at high temperatures (e.g. DCE, 83 °C, 15 min [2]) can lead to partial dealkylation of phenyl groups and yield a soluble polymer. 

Saigusa, T., and R. Oda Bromination of polystyrene with N-bromosuccini-mide, debromination of the brominated polystyrene, and grafting of vinyl acetate on the backbone of debrominated polystyrene. Bull. Ind. Chem. Research, Kyoto Univ. 33, 126 (1955) Chem. Abstr. 50, 1357 (1956). 

Though we have shown only one bromine atom and hence only one Ph2P group on the polymer, almost all of the benzene rings in polystyrene can be functionalized if the bromopolymer is made by bromination of polystyrene in the presence of a Lewis acid. Now the phosphine can be alkylated with an alkyl halide of your choice to form a phosphonium salt, still on the polymer. 

Polymer-supported Wittig reagents were first prepared more than 20 years ago [32]. It has been shown that the success of the reaction depends strongly upon (i) the preparation of the reagent by bromination and phosphination of cross-linked polystyrene rather than by co-polymerization using styryldi-phenyl phosphine, and (ii) the generation of the phosphorane with a base/ solvent system that swells the phosphonium sites in the polymer network (Scheme 6) [33]. Thus, bromination of polystyrene 1 yielded phenyl bromide 32, and this was followed by phosphination with n-butyUithium and chlor-odiphenylphosphine or with Hthium diphenylphosphide to give 33, a compound which is commercially available (Scheme 6). 

The literature reports the synthesis of two types of catalyst based on tungsten and molybdenum chemically bonded with the polymer, and their use in the metathesis of oleflns with internal C=C bonds [264]. The catalysts are synthesized by bromination of polystyrene containing 2% divinylbenzene with subsequent treatment of the brominated polymer by n-BuLi in tetrahydrofuran. Lithium-polystyrene derivatives are thus formed. After reaction with a,a -dipyridyl or Ph2PCl, they are converted, respectively, to dipyridyl (A) or phosphine (B) derivatives of polystyrene that form active complexes after being heated with W(CO)5 or Mo(CO). ... 

Bromination of polystyrene with iV-bromosuccinimide and benzoyl peroxide in CCI4 at room temperature can achieve a 61% conversion in four hours. Considerable degradation, however, accompanies this reaction. 

Silane radical atom transfer (SRAA) was demonstrated as an efficient, metal-free method to generate polystyrene of controllable molecular weight and low polydispersity index values. (TMSlsSi radicals were generated in situ by reaction of (TMSlsSiH with thermally generated f-BuO radicals as depicted in Scheme 14. (TMSlsSi radicals in the presence of polystyrene bromide (PS -Br), effectively abstract the bromine from the chain terminus and generate macroradicals that undergo coupling reactions (Reaction 70). 

A number of N-brominated and N-chlorinated heterocycles also provide sources of electrophilic bromine. Examples include 1-chlorobenzotriazole (82JOC4895 87JOC173 88CHE36) and various HBr and Br2 adducts of pyridines, or pyridine perbromides [84SC939 85JAP(K)60/87264], Polymer-supported reagents of this type include 1-cyclohexylpyridinium perbromide linked to polystyrene, effective for the bromination of 1-methylindole, benzo[fc]furan, and benzo[6]thiophene (89T7869). 

Cross-linked polystyrene can be directly brominated in carbon tetrachloride using bromine in the presence of Lewis acids (Experimental Procedure 6.2 [55-58]). Thal-lium(III) acetate is a particularly suitable catalyst for this reaction [59]. Harsher bro-mination conditions should be avoided, because these can lead to decomposition of the polymer. Considering that isopropylbenzene is dealkylated when treated with bromine to yield hexabromobenzene [60], the expected products of the extensive bromi-nation of cross-linked polystyrene would be soluble poly(vinyl bromide) and hexabromobenzene. In fact, if the bromination of cross-linked polystyrene is attempted using bromine in acetic acid, the polymer dissolves and apparently depolymerizes [61]. 

A sample of polystyrene prepared by heating styrene with tribromobenzoyl peroxide in the absence of air has the formula Br3C6H3(C jHg)w. The number n varies with the conditions of preparation. One sample of polystyrene prepared in this manner was found to contain 10.46% bromine. What is the value of n ... 

Farall MJ, Frechet JMJ, Bromination and lithiation two important steps in the functionalization of polystyrene resins, J. Org. Chem., 41 3877-3882, 1976. 

Brominated indans are also a non-DBDPO option. They are soluble flame retardants, but do not lower the heat resistance of the styrenic polymer because the glass transition temperature of, e.g., FR-1808 is close to that of polystyrene [26,27]. 

The bromination of 1% cross-linked polystyrene was done in the presence of Tl(0Ac)3 or TICI3, which are preferable to the inefficient FeCl3 and the inconvenient BF3 (15). The resin (2 g) was swollen in CCI4 (30 ml) and contacted with TICI3 (0.2 g). The reactants were stirred in the dark for 30 min, then 1.36 g of Br2 in 2 ml of CCI4 were added slowly. After stirring fori hr at room temperature in the dark, the mixture was heated to reflux for 1.5 hr. The reaction mixture was filtered, and the beads were washed in sequence with CCI4, acetone, water, benzene, and methanol. The beads were then dried in vacuum. 

A similar well-defined graft copolymer consisting of polystyrene main chain and branches (G-7) can be prepared simply via repetition of copper-catalyzed living radical polymerizations.209 Thus, the synthesis starts with the copolymerization of styrene and />(acetoxymethy 1)styrene or />(methoxymethyl)sty-rene, followed by bromination of the substituent into the benzyl bromide moiety, which then initiates the copper-catalyzed radical polymerization of styrene to give graft polymers with 8—14 branches. 

A combination of metallocene-catalyzed syndiospe-cific styrene polymerization and the metal-catalyzed radical polymerization affords various graft copolymers consisting of syndiotactic polystyrene main chains (G-8).433 The reactive C—Br bonds (7—22% content) were generated by bromination of the polystyrene main chain with AZ-bromos uccimid e in the presence of AIBN. 

Preparation of Macromolecular Dioxolenium Salts. Living polystyrene prepared by the polymerization of styrene in THF with a-methylstyrene tetramer dianion reacted with a 2.1-molar amount of ethylene oxide for three hours at room temperature a 6.6-molar amount of adipoyl chloride was added, and the mixture was stirred for 20 horns a 20-molar amount of ethylenebromohydrin was added. This mixture was stirred for 44 hours. The bromoethylated polystyrene was precipitated in excess methanol and freeze-dried from benzene in a vacuum system. A 1-nitropropane solution of polystyrene dioxolenium salt was prepared by reaction of bromoethylated polystyrene with silver perchlorate in 1-nitropropane. Silver bromide was removed from the reaction mixture by filtration. Molecular weight of the product was measured by a vapor-pressure osmometer it was 1910 for living polystyrene and 5190 for the bromoethylated polystyrene. Bromine analysis of the bromoethylated polystyrene showed 67.9% of the calculated value. 

Prins et al. (21) described the lower flammability of poly-bromostyrene relative to that of polystyrene. On the basis of thermal analysis experiments, they suggested that bromine inhibited most of the oxidative chain reactions, and thus the combustion was not supported (vapor-phase mechanism). Khanna and Pearce (16) and Brauman (22) demonstrated that polystyrene could be flame retarded by appropriately modifying its structure with substituents that promote the char yield of the system (condensed-phased mechanism). 

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