Initiated polymerization of styrene

Anionic polymerization of vinyl monomers can be effected with a variety of organometaUic compounds alkyllithium compounds are the most useful class (1,33—35). A variety of simple alkyllithium compounds are available commercially. Most simple alkyllithium compounds are soluble in hydrocarbon solvents such as hexane and cyclohexane and they can be prepared by reaction of the corresponding alkyl chlorides with lithium metal. Methyllithium [917-54-4] and phenyllithium [591-51-5] are available in diethyl ether and cyclohexane—ether solutions, respectively, because they are not soluble in hydrocarbon solvents vinyllithium [917-57-7] and allyllithium [3052-45-7] are also insoluble in hydrocarbon solutions and can only be prepared in ether solutions (38,39). Hydrocarbon-soluble alkyllithium initiators are used directiy to initiate polymerization of styrene and diene monomers quantitatively one unique aspect of hthium-based initiators in hydrocarbon solution is that elastomeric polydienes with high 1,4-microstmcture are obtained (1,24,33—37). Certain alkyllithium compounds can be purified by recrystallization (ethyllithium), sublimation (ethyllithium, /-butyUithium [594-19-4] isopropyllithium [2417-93-8] or distillation (j -butyUithium) (40,41). Unfortunately, / -butyUithium is noncrystaUine and too high boiling to be purified by distiUation (38). Since methyllithium and phenyllithium are crystalline soUds which are insoluble in hydrocarbon solution, they can be precipitated into these solutions and then redissolved in appropriate polar solvents (42,43). OrganometaUic compounds of other alkaU metals are insoluble in hydrocarbon solution and possess negligible vapor pressures as expected for salt-like compounds. 

To test our model, we set up small and large-scale tests for thermally-initiated polymerization of styrene. 

Figure 13.7 illustrates stability regimes for the thermally initiated polymerization of styrene for laminar flow in a single tube. Design and operating variables... 

The initial rate of polymerization of methyl methacrylate initiated by chromium allyls (12) in toluene showed identical dependences on monomer and catalyst concentrations, as Zr(benzyl)4 initiated polymerization of styrene. Some data for the monomer dependence are shown in Fig. 14. 

Since this reaction is not affected by hydroquinone and galvinoxyl and does not initiate polymerization of styrene, it obviously occurs without the formation of free radicals. The kinetic parameters of the reactions of three hydroperoxides with triphenyl phosphite in different solvents are given in Table 17.2 [21]. 

The additional complexity present in block copolymer synthesis is the order of monomer polymerization and/or the requirement in some cases to modify the reactivity of the propagating center during the transition from one block to the next block. This is due to the requirement that the nucleophilicity of the initiating block be equal or greater than the resulting propagating chain end of the second block. Therefore the synthesis of block copolymers by sequential polymerization generally follows the order dienes/styrenics before vinylpyridines before meth(acrylates) before oxiranes/siloxanes. As a consequence, styrene-MMA block copolymers should be prepared by initial polymerization of styrene followed by MMA, while PEO-MMA block copolymers should be prepared by... 

The work function of the rubbing surfaces and the electron affinity of additives are interconnected on the molecular level. This mechanism has been discussed in terms of tribopolymerization models as a general approach to boundary lubrication (Kajdas 1994, 2001). To evaluate the validity of the anion-radical mechanism, two metal systems were investigated, a hard steel ball on a softer steel plate and a hard ball on an aluminum plate. Both metal plates emit electrons under friction, but aluminum produced more exoelectrons than steel. With aluminum, the addition of 1% styrene to the hexadecane lubricating fluid reduced the wear volume of the plate by over 65%. This effect considerably predominates that of steel on steel. Friction initiates polymerization of styrene, and this polymer formation was proven. It was also found that lauryl methacrylate, diallyl phthalate, and vinyl acetate reduced wear in an aluminum pin-on-disc test by 60-80% (Kajdas 1994). 

The rate expression Eq. 3-32 requires a first-order dependence of the polymerization rate on the monomer concentration and is observed for many polymerizations [Kamachi et al., 1978], Figure 3-2 shows the first-order relationship for the polymerization of methyl methacrylate [Sugimura and Minoura, 1966], However, there are many polymerizations where Rp shows a higher than first-order dependence on [M], Thus the rate of polymerization depends on the -power of the monomer concentration in the polymerization of styrene in chlorobenzene solution at 120°C initiated by t-butyl peresters [Misra and Mathiu, 1967]. The benzoyl peroxide initiated polymerization of styrene in toluene at 80°C shows an increasing order of dependence of Rp on [M] as [M] decreases [Horikx and Hermans, 1953], The dependence is 1.18-order at [M] = 1.8 and increases to 1.36-order at [M] = 0.4. These effects may be caused by a dependence of the initiation rate on the monomer concentration. Equation 3-28 was derived on the assumption that Rt is independent of [M], The initiation rate can be monomer-dependent in several ways. The initiator efficiency / may vary directly with the monomer concentration... 

The initiation process appears more complicated than described above, although data are not available in more than a few systems. The benzoyl peroxide initiated polymerization of styrene involves considerable substitution of initiator radicals on the benzene ring for polymerizations carried out at high conversions and high initiator concentrations. About one-third of the initiator radicals from t-butyl peroxide abstract hydrogen atoms from the a-methyl groups of methyl methacrylate, while there is no such abstraction for initiator radicals from benzoyl peroxide or AIBN. 

Fig. 3-6 Determination of initiator chain-transfer constants in the t-butyl hydroperoxide initiated polymerization of styrene in benzene solution at 70°C. After Walling and Heaton [1965] (by permission of American Chemical Society, Washington, DC.

Fig. 3-9 Inhibition and retardation in the thermal, self-initiated polymerization of styrene at 100°C. Plot 1, no inhibitor plot 2, 0.1% benzoquinone plot 3, 0.5% nitrobenzene plot 4, 0.2% nitrosobenzene. After Schulz [1947] (by permission of Verlag Chemie GmbH and Wiley-VCH, Weinheim).

The effect of temperature on the rate and degree of polymerization is of prime importance in determining the manner of performing a polymerization. Increasing the reaction temperature usually increases the polymerization rate and decreases the polymer molecular weight. Figure 3-13 shows this effect for the thermal, self-initiated polymerization of styrene. However, the quantitative effect of temperature is complex since Rp and X depend on a combination of three rate constants—kd, kp, and kt. Each of the rate constants for initiation, propagation, and termination can be expressed by an Arrhenius-type relationship... 

Fig. 3-13 Dependence of the polymerization rate (O) and polymer molecular weight ( ) on the temperature for the thermal self-initiated polymerization of styrene. After Roche and Price [1952] (by permission of Dow Chemical Co., Midland, MI).

For a purely photochemical polymerization, the initiation step is temperature-independent (Ed = 0) since the energy for initiator decomposition is supplied by light quanta. The overall activation for photochemical polymerization is then only about 20 kJ mol-1. This low value of Er indicates the Rp for photochemical polymerizations will be relatively insensitive to temperature compared to other polymerizations. The effect of temperature on photochemical polymerizations is complicated, however, since most photochemical initiators can also decompose thermally. At higher temperatures the initiators may undergo appreciable thermal decomposition in addition to the photochemical decomposition. In such cases, one must take into account both the thermal and photochemical initiations. The initiation and overall activation energies for a purely thermal self-initiated polymerization are approximately the same as for initiation by the thermal decomposition of an initiator. For the thermal, self-initiated polymerization of styrene the activation energy for initiation is 121 kJ mol-1 and Er is 86 kJ mol-1 [Barr et al., 1978 Hui and Hamielec, 1972]. However, purely thermal polymerizations proceed at very slow rates because of the low probability of the initiation process due to the very low values f 1 (l4 IO6) of the frequency factor. 

High pressure can have appreciable effects on polymerization rates and polymer molecular weights. Increased pressure usually results in increased polymerization rates and molecular weights. Figure 3-17 shows these effects for the radiation-initiated polymerization of styrene [Moore et al., 1977]. 

Fig. 3-17 Effect of pressure on the polymerization rate (o) and polymer molecular weight (A) of radiation-initiated polymerization of styrene at 25° C. After Moore et al. [1977] (by permission of Wiley-Interscience, New York.)...

The same initial polymerization rate and degree of polymerization as in Problem 3-15 are obtained at 27°C for a particular AIBN thermal-initiated polymerization of styrene. Calculate the Rp and X values at 77°C. 

Evidence for the absence of termination or transfer reactions in the organolithium-initiated polymerization of styrene and iso-prene is shown in Table I for representative examples of these polymers. It can be seen that these polymers exhibit the expected low kfo/Mn values, except in the case of the isoprene polymerized in the H4furan, where a slow side reaction seems to occur between the solvent, on the one hand, and both the initiator (Ifo vs Mg) and the growing chains (1% vs Mn)>on the other hand. 

This observation has been used by Kargin, and Plate (127) who initiated polymerization and grafting with the help of mechanically disrupted inorganic materials. Many metals, oxides, and salts which never normally act as initiators, when mechanically disrupted, are able to initiate polymerization of styrene, methyl methacrylate, acrylonitrile, and other vinyl monomers. The surface of the active inorganic substance can also be used as a site for grafting to already existing polymer chains if joint dispersion of polymer and monomer, such as cellulose and styrene, is performed. 

Although NH2 ion initiates polymerization of styrene it is known (17) that this monomer, as well as some other compounds containing C=C double bonds, are rapidly reduced by a solution of alkali metals in liquid ammonia. Undoubtedly, the first step of such a reduction is represented by the equation... 

Welch, F. J. Effect of Lewis acids and bases on the rate of butyl lithium initiated polymerization of styrene. Read in St. Francisco ACS Meeting. 

The ability of toluene to serve as a transfer agent was further demonstrated by Bower and McCormick 275) and Brooks 2761 for the organosodium initiated polymerization of styrene in that solvent. Both groups reported molecular weights lower than the values calculated from the monomer-initiator ratio. 

Poly (benzyl ether) [G-2]-TEMPO, [G-3]-TEMPO, and [G-4]-TEMPO compounds have been synthesized and used as additives in the benzoyl peroxide initiated polymerization of styrene [127] (see Scheme 15c). After an induction period, chain growth is observed. However, the MWD is larger than in a dendrim-er-free TEMPO modulated system (Mw/Mn 2). The expectation that the den-drimer would isolate the growing chain end and prevent side reactions is not borne out. Polymerizations of methylmethacrylate, vinylacetate, and n-buty-lacrylate with the same initiator/TEMPO recipe are disappointing. 

It is evident that the tendency for free radical polymerization is a function of the monomer. Natta (287) stated that these radicals can initiate polymerization of styrene and diolefins to high molecular weight products but not that of aliphatic alpha olefins. North (339) showed that the decomposition of phenyl tri-isopropoxy titanium initiates radical polymerization of styrene but not of ethylene. 

Equation adequately describes the course of the thermally initiated polymerization of styrene (see Fig. 4). 

Thiocarbamate derivatives, (III), prepared by Rink [2] were effective as free radical regulators in the 2,2-azobisisobutyronittiIe-initiated polymerization of styrene. 

Boron trichloride and tribromide successfully polymerize styrenes and isobutene. These Lewis acids are typically used in combination with water or alkyl chlorides, acetates, ethers, and alcohols [105,153]. In contrast to earlier reports, BC13 can self-initiate polymerization of styrene and isobutene [137] by haloboration, and subsequent activation of the resulting alkyl chlorides by excess Lewis acid. Direct initiation was confirmed by the formation of lower molecular weight polymers than pre-... 

As outlined in Section III. A.3. a, the strength of the Lewis acid with mixed chloride and alkoxy derivatives decreases as the number of chloride ligands are replaced with alkoxy groups. Titanium chloride with one alkoxy group polymerizes styrene and a-methylstyrene Lewis acid with two alkoxy groups is too weak to initiate polymerization of styrene, but will initiate polymerization of a-methylstyrene and vinyl ethers. The Lewis acidity of titanium chloride derivatives with three alkoxy groups are so low that only vinyl ether polymerizations reach reasonable conversions. 

No comparative data are available for these temperatures in Lewis acid-initiated polymerizations of styrene. However, DP increases linearly with polymer yield in TiCl4-initiated polymerizations in CH2CI2 at temperatures from -90° C to -25° C (DP 60 at -25° C, DP 200 at -60° C, [M]o 0.25 mol/L), indicating that transfer to monomer is small (CtrM - 4-10-4 and <5 I0-5) [308]. 

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