Triphenylmethyl cation, initiation

Several catalysts and initiator systems have been tested for the polymerization of GlcAnBzl3, including the following Lewis acids boron trifluoride and its etherate, phosphorus pentafluoride, titanium tetrachloride, and antimony pentachloride and pentafluoride. Several cationic initiators have also been used, including (triphenylmethyl) antimony hexachloride, 2,3,4,6-tetra-O-acetyl-D-glucopyranosyl hexa-fluorophosphate, acetyl hexafluorophosphate, pentamethylbenzyl hexa-fluorophosphate (most of which were generated in situ), and triethyl-... 

Initiation of tetrahydrofuran polymerisation by Ph C SbClg has been studied speetrophotometrieally by Ledwith and co-workers (20). Decay of the characteristic triphenylmethyl cation absorption, in methylene chloride was represented by the expression... 

The initiation mechanism for cationic polymerization of cyclic ethers, vinyl amines, and alkoxy styrenes has been investigated by A. Ledwith. He used stable cations, like tropylium or triphenylmethyl cations with stable anions, like SbCl6, and distinguished between three initiation reactions cation additions, hydride abstraction, and electron transfer. One of the typical examples of cationic polymerization, in which the propagating species is the oxonium ion, is the polymerization of tetra-hydrofuran. P. and M. P. Dreyfuss studied this polymerization with the triethyloxonium salts of various counterions and established an order of... 

Use of triphenylmethyl and cycloheptatrienyl cations as initiators for cationic polymerization provides a convenient method for estimating the absolute reactivity of free ions and ion pairs as propagating intermediates. Mechanisms for the polymerization of vinyl alkyl ethers, N-vinylcarbazole, and tetrahydrofuran, initiated by these reagents, are discussed in detail. Free ions are shown to be much more reactive than ion pairs in most cases, but for hydride abstraction from THF, triphenylmethyl cation is less reactive than its ion pair with hexachlorantimonate ion. Propagation rate coefficients (kP/) for free ion polymerization of isobutyl vinyl ether and N-vinylcarbazole have been determined in CH2Cl2, and for the latter monomer the value of kp is 10s times greater than that for the corresponding free radical polymerization. 

Initiation with Triphenylmethyl Cation. Triphenylmethyl (trityl) cation derives its stability (7, 30) from resonance between the electro-... 

Initiation with Triphenylmethyl Cation. When tetrahydrofuran (THF) is used to dissolve triphenylmethyl hexachlorantimonate at room temperature, there is almost immediate decomposition of the triphenylmethyl cation (6). On the other hand, solutions of the trityl salt in THF can be prepared and stored as deep yellow solutions if maintained at temperatures around — 80°C. At room temperature the initial decoloration of the catalyst is followed rapidly by polymerization of the monomer to poly(tetramethylene oxide), and the actual percentage conversion depends markedly on the temperature. This behavior is typical of systems exhibiting monomer-polymer equilibria (28), and Table III shows values for the equilibrium conversion of monomeric THF to polymeric THF obtained with a variety of catalysts. As for vinyl ether polymerization, it is most convenient to use the trityl hexachlorantimonate salt however, recourse to Table III shows clearly that above room temperature this anion yields less than the expected equilibrium conversion monomer... 

The triphenylmethyl cation reacts with the THF to form triphenylmethane and a protonic acid. We believe that this acid is the true initiator and that its reaction with THF may be slow. This can account in part for the deviations in DP. As Bawn and co-workers pointed out, in the case of the SbCl6 gegenion transfer reactions are probably also important and result in low DP s. 

A number of years ago triphenylmethyl cation, Ph3C, formed in situ by dissociation of triphenylmethyl chloride, was shown [73] to initiate the polymerization of 2-ethylhexyl vinyl ether in m-cresol solvent. More recently certain stable carbonium ion salts, notably hexachloroantimonate (SbCls) salts of cycloheptatrienyl (tropylium, C7H7) and triphenylmethyl cations have been shown [74, 50] to be very efficient initiators of the cationic polymerization of many reactive monomers [27, 29, 75]. Since the discovery of the effectiveness of the SbClg salt, triphenylmethyl salts with different anions have also been used [76—78]. The most detailed kinetic studies using these initiators have been carried out on alkyl vinyl ethers [27, 30] and A-vinylcarbazole [39] in homogeneous solution in methylene chloride. 

As an example of conformational change induced by the introduction of electric charges along a macromolecule, let us consider the polymer (15) which in its side chains carries triarylmethane groups in their leuco-form (15). Upon photoirradiation this neutral group dissociates into triphenylmethyl cation and hydroxyl anion. The polymer chain of the irradiated sample adopts an expanded conformation to alleviate the unfavorable ionic interactions between adjacent cations. These changes are reflected by a marked increase in solution viscosity upon photoirradiation in aqueous or methanolic solutions of a polyacrylamide with side chains of structure (15). The viscosity recovers its initial value when irradiation is discontinued. 

Photooxidation of the triphenylmethyl cation (as its tetrafluoroborate salt) in the presence of an aromatic donor was similarly found to afford bis(triphenyl-methyl) peroxide [31]. The mechanism was proposed to proceed through initial electron transfer from the aromatic donor to the singlet-excited triphenylmethyl cation to give the triphenylmethyl radical. Reaction of the radical with triplet oxygen, and subsequent coupling with another triphenylmethyl radical gave the observed peroxide. It was noted that electron transfer from the tetrafluoroborate counterion to the excited state cation could not be completely excluded, because the cation was slowly photooxygenated in the absence of an electron donor. 

The initiation mechanisms, however, by many carbon cations as, for instance, by triphenylmethyl cations, are not straightforward. Initially, hydride ions are abstracted from the monomers to form triphenylmethanes [51-53]. Simultaneously, acids are released from the counterions. The acids... 

Another differential reaction is copolymerization. An equi-molar mixture of styrene and methyl methacrylate gives copolymers of different composition depending on the initiator. The radical chains started by benzoyl peroxide are 51 % polystyrene, the cationic chains from stannic chloride or boron trifluoride etherate are 100% polystyrene, and the anionic chains from sodium or potassium are more than 99 % polymethyl methacrylate.444 The radicals attack either monomer indiscriminately, the carbanions prefer methyl methacrylate and the carbonium ions prefer styrene. As can be seen from the data of Table XIV, the reactivity of a radical varies considerably with its structure, and it is worth considering whether this variability would be enough to make a radical derived from sodium or potassium give 99 % polymethyl methacrylate.446 If so, the alkali metal intitiated polymerization would not need to be a carbanionic chain reaction. However, the polymer initiated by triphenylmethyl sodium is also about 99% polymethyl methacrylate, whereas tert-butyl peroxide and >-chlorobenzoyl peroxide give 49 to 51 % styrene in the initial polymer.445... 

A general method for the synthesis of pyrylium salts is the cyclodehydration of 1,5-dicarbonylalkanes (Scheme 4.4). Acetic anhydride is commonly used as both solvent and reagent, but since the initial product is a 4/f-pyran, an oxidant such as the triphenylmethyl (trityl) cation in the form of triphenylmethyl hexachloroantimonate is added (Ph3C + [H ] -> Ph3CH). In certain cases, however, it is advantageous to isolate the pyran and to oxidize it in a separate step. 

The first use of the ion as an initiator was by Eley and Richards (73) who polymerised 2-ethylhexyl vinyl ether in chloroform/liquid sulphur dioxide and m-cresol solutions using triphenylmethyl chloride as initiator. The appearance of the characteristic intense yellow colour proved the presence of the carbo-cation. 

AH attempts to convert dimer 263 into a dimeric 2-benzopyryIium salt, on treatment with triphenylmethyl or acetyl perchlorate, lead only to the rupture of the newly formed C—C bond and to the regeneration of the initial monomeric salt 261, unlike the behavior of dimers of monocyclic pyrylium cations [73DOK(212)370]. Dimerization may be considered a typical reaction for benzo[c]pyrylium-4-oxides of type 19, which react in dimerizations as 1,3-dipoles by analogy with their behavior in cycloadditions (Section III,E,2). 

Mechanism and kinetics of cationic poiymerization initiation. Unlike free-radical and anionic polymerization, initiation in cationic polymerization employs a true catalyst that is restored at the end of the polymerization and does not become incorporated into the terminated polymer chain. Initiation of cationic polymerization is brought about by addition of an electrophile to a monomer molecule. TVpical compounds used for cationic polymerization include protonic acids (e.g., H2SO4, H3PO4), Lewis acids (e.g., AICI3, BF3, TiCl4, SnCl4), and stable carbenium-ion salts (e.g., triphenylmethyl halides, tropylium halides) ... 

Led with and his co-workers [50] have also studied the mechanism of initiation by triphenylmethyl salts in some detail, by following the decay of the characteristic visible absorption maximum of the initiating cation. The kinetic equation which has been established for both methylene chloride and THF solvents is... 

Problem 8.20 Give plausible explanation for the following facts. Primary and secondary alkyl halides are generally ineffective as initiators of cationic polymerization of monomers such as isobutene and styrene, but t-butyl and cumyl chlorides are effective. On the other hand, triphenylmethyl chloride and cyclo-heptatrienyl (tropylium) chloride are not very efBcient in polymerizing isobutylene and styrene but produces rapid polymerization of p-methoxystyrene, vinyl ethers and N-vinylcarbazole. 

Although styrene polymerized by ionic mechanism is not utilized commercially, much research was devoted to both cationic and anionic polymerizations. An investigation of cationic polymerization of styrene with an A1(C2H5)2C1/RC1 (R = alkyl or aryl) catalyst/cocatalyst system was reported by Kennedy.The efficiency (polymerization initiation) is determined by the relative stability and/or concentration of the initiating carbocations that are provided by the cocatalyst RCl. A/-butyl, isopropyl, and j c-butyl chlorides exhibit low cocatalytic efficiencies because of a low tendency for ion formation. Triphenylmethyl chloride is also a poor cocatalyst, because the triphenylmethyl ion that forms is more stable than the propagating styryl ion. Initiation of styrene polymerizations by carbocations is now well established. 

Advantage can be taken of the ionizing and dissociating effects produced by a solvent to activate inert molecules and initiate a polymerization. For instance, triphenylmethyl chloride in pure sulfuric acid solution undergoes an instantaneous ionization and produces a triphenylmethylium cation with a characteristic red color ... 

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