Friedel-Craft alkylation reactions polymers

Beside borosilicate and fused silica capillaries, PS/DVB monoliths have been fabricated within the confines of steel and PEEK tubings [52]. In order to increase the hydrophobic character of the supports, a Friedel-Crafts alkylation reaction was used for the attachment of Cig-moieties to the polymer surface. The derivatized material was demonstrated to be more retentive and to provide more efficient peptide separations compared with the original, nonderivatized monolith. 

Bakelite, the first synthetic polymer, is an example of a thermoset polymer. It is prepared by the polymerization of phenol and formaldehyde in the presence of ail acid. Carbocations produced by protonation of formaldehyde bond to the ortho and para positions of the highly reactive phenol molecules in a Friedel-Crafts alkylation reaction. The benzylic alcohols that are produced in this step react to produce carbocations that then alkylate additional phenol molecules. A mechanism for the first few steps of this polymerization process is shown in Figure 24.4. 

We have omitted from the above brief survQ tho studies dealing with the cationic grafting of a polymer onto another since these Friedel-Crafts alkylation reactions, although proceeding via carbenium ion intermediates, do not initiate any polymerisation and fall therefore outside the realm of the present review. Kennedy has recently summarised this type of work ° ... 

HCPs [12, 15] represent a family of robust microporous organic materials which are low cost and mainly prepared by the Friedel-Crafts alkylation reaction. The permanent porosity in HCPs is a result of extensive crosslinking reactions which prevent the polymer chains from collapsing into a dense, non-porous state. Such highly... 

The Friedel-Crafts alkylation reaction is a well-developed method to synthesize hypercrosslinked organic polymers. Using benzene and aniline as monomers, Cooper has used this method to synthesize copolymers with increased gas selectivity." Recently, the Friedel-Crafts all lation reaction has also been used to synthesize PAF by Jing et By choosing tetra-... 

Sogah (24) and McGrath (25) have reported tert-butyl and phenyl-substituted poly(aryl ether ketones) respectively, which are prepared by nucleophilic substitution reaction of the corresponding substituted hydroquinones and 4,4 -difluorobenzophenone. The resulting polymers were amorphous and highly soluble in common organic solvents as a result of the bulky substituents which suppressed crystallization. The bulky substituents were cleaved with acid in a reverse Friedel-Crafts alkylation reaction to produce the semi-crystalline PEEK (24). 

As catalysts Lewis acids such as AICI3, TiCU, SbFs, BF3, ZnCh or FeCl3 are used. Protic acids such as FI2SO4 or FIF are also used, especially for reaction with alkenes or alcohols. Recent developments include the use of acidic polymer resins, e.g. Nafion-Fl, as catalysts for Friedel-Crafts alkylations and the use of asymmetric catalysts. ... 

Mohanty et al. were the first to introduce pendent r-butyl groups in die polymer backbones. The resulting material was quite soluble in aprotic dipolar solvents.83 The PEEK precursors were prepared under a mild reaction condition at 170°C. The polymer precursor can be converted to PEEK in die presence of Lewis acid catalyst A1C13 via a retro Friedel-Crafts alkylation. Approximately 50% of die rerr-butyl substitutes were removed due to die insolubility of the product in die solvent used. Later, Risse et al. showed diat complete cleavage of f< rf-butyl substitutes could be achieved using a strong Lewis acid CF3SO3H as both die catalyst and the reaction medium (Scheme 6.15).84... 

The topics analysed here include reversible termination and the formation of p-tolyl end-groups on polystyrenes made in toluene. For unknown reasons, most authors have very largely ignored this Friedel-Crafts alkylation, which in a polymer context is a transfer reaction. It was unfortunately termed molecular termination by Overberger and was explored by his and Smets groups. 

Friedel-Crafts alkylation of benzo[6]thiophene has received little attention. The published results, which deserve reexamination, indicate that exclusive 3-substitution occurs in some cases, whereas in others, 2-substitution predominates. Benzo[6]thiophene is alkylated with isopropyl chloride, isopropanol, or propene in the presence of various acid catalysts under a variety of reaction conditions to give a mixture of 2- and 3-isopropylbenzo[6]thiophene in which the 2-isomer predominates (78-92%).358 410 In contrast, alkylation with isobutene in the presence of either 80% sulfuric acid415 or 100% phosphoric acid416 is said to afford exclusively 3-/er<-butylbenzo[6]thiophene in yields of 100 and 75%, respectively. In neither case was the structure of the product rigorously confirmed. Likewise, 3-Jeri-amylbenzo [63-thiophene is the exclusive product of alkylation with tert-amyl alcohol in the presence of stannic chloride414 alkylation with pent-l-ene, hex-l-ene, and a Ci8 propylene polymer is also claimed to give... 

Friedel-Crafts alkylation of aromatics with CMF to give rise to benzyl derivatives 34a and 34b was first demonstrated in 1909 [139] and reprised by Rauchfuss and co-worker in 2013 to provide feedstock 34c for hydrodeoxygenation to diesel-range hydrocarlxMis [140]. Similar reactions have been used to produce a range of novel monomers and polymers. Thus, Szmant et al. reported as far back as 1981 the application of CMF to the synthesis of polymeric building blocks 35 and 36, among others (Fig. 1) [141]. 

Chain Transfer. Chain transfer is the most important chain-breaking reaction in carbocationic polymerization. It can be considered as a subcase of termination, when this latter is accompanied by reinitiation. In chain-transfer reactions the active center is transferred to a new site to imreacted monomer or initiator, the preformed polymer, or the solvent. This can happen by /3-proton elimination from the growing chain, or Friedel-Crafts alkylation of aromatic rings. /8-Hydrogen atoms of the propagating carbenium ions are quite acidic owing to the delocalization of the positive charge. In isobutylene, vinyl ether, and styrene... 

The synthesis of alltyl-bridged poly(p-phenylene)s is based on typical poly(p-phenylene) chemistry, followed by a second reaction in which the rigidifying bridge is created. Scherf et al. [314, 315,704,705] used Suzuki coupling to create a ketone-substituted poly(p-phenylene). In a next step, the ketone functions in this polymer were reduced and, finally, the ladder polymer was obtained by Friedel-Crafts alkylation (Fig. 43). 

However, transfer reactions are nearly unavoidable side reactions in cationic polymerization and may involve yS-proton transfer even to rather weak bases like the monomer itself, to transfer agents and solvent as well as Friedel-Crafts alkylation of aromatic rings, if monomers like styrene are polymerized. Proton transfer yields unsaturated chains [Eq. (58)], with the double bond in either the endo or exo position. These may lead to branched polymers, if the double bonds are accessible to homo- or copolymerization. 

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