Initiation steps styrene polymerization

The initiation of styrene polymerization by r-butyl lithium in benzene is an apparent exception. The rates were found to be proportional to the first power concentration of the lithium compound and independent of the monomer concentration which was varied from 0.1 to 10-3 M172). This strange behavior is illustrated by Fig. 26. The authors proposed that the dissociation of r-butyl lithium into smaller, active aggregates, dimers or monomers, is the rate determining step of this process. However, the rate of initiation of isoprene by r-butyllithium in benzene shows a more conventional behavior it increases with increasing concentration of the monomer, although again it is proportional to first power concentration of r-butyl lithium. 

F. R. Mayo (private communication) has found evidence that thermal polymerization of styrene may actually be of a higher order than second, i.e., about five-halves order. This would suggest a termolecular initiation step. Generation of a pair of monoradicals in this manner, i.e., from three monomer molecules, would be acceptable from the standpoint of energy considerations. 

This quadratic in Rp is of the form required by the data for styrene-benzoyl peroxide shown in Fig. 14. The first term, corresponding to the intercept, represents the creation of chain ends through transfer with monomer. It occurs to an extent which is independent of the polymerization rate. The second term corresponds to 1/2 according to Eq. (27) it represents the pairs of ends created at the initiation step. Its coefficient is given by the initial slope of the line in Fig. 14. The third term, which accounts for the curvature at higher rates, represents the contribution of chain transfer with benzoyl peroxide. This becomes more prominent at higher rates because of the larger amounts of the initiator which are present. The marked rise in the curves for... 

Based on the literature data available for styrene polymerized with benzoyl peroxide, (10,12,14) transfer to monomer and termination by disproportionation will be neglected. For the Initiation step, only primary and Induced decomposition reactions will be considered. 

The latter mechanism is supported by evidence obtained from the initiation and termination steps in the syndiospecific polymerization of styrene [190]. The 13C-enriched titanium catalyst afforded polystyrene with a CH(Ph)CH213CH3 end group, which indicates that the initiation step proceeded by secondary insertion (2,1-insertion) of styrene into the Ti-13C bond of the active species (Eq. 10). In contrast to this mechanism, termination by the addition of 13C-enriched methanol or tert-butyl alcohol afforded polymers without 13CH30 or tertbutoxy end groups. 

The effect of the nitrone stmcture on the kinetics of the styrene polymerization has been reported. Of all the nitrones tested, those of the C-PBN type (Fig. 2.29, family 4) are the most efficient regarding polymerization rate, control of molecular weight, and polydispersity. Electrophilic substitution of the phenyl group of PBN by either an electrodonor or an electroacceptor group has only a minor effect on the polymerization kinetics. The polymerization rate is not governed by the thermal polymerization of styrene but by the alkoxyamine formed in situ during the pre-reaction step. The initiation efficiency is, however, very low, consistent with a limited conversion of the nitrone into nitroxide or alkoxyamine. 

The process begins in a prepolymerizer, which is a water-jacketed reactor with a mixer in it. See Figure 23—12.) The styrene is partially polymerized by adding the peroxide initiator and heating to 240—250°F for about four hours. About 30% of the styrene polymerizes and the reactor contents become syrupy goo. Thats about as far as the prepolymer step can go—30% conversion— because the mixing and heat transfer gets very inefficient as the goo gets thicker, and the polymerization becomes hard to control. 

Fractional kinetic orders of homogenous reactions in solution may point to association of a particular reagent. The kinetics of the initiation step of styrene polymerization in the presence of n-BuLi (equation 33) is in accordance with the assumption that this organolithium compound in a nonbonding solvent forms aggregates of six molecules on the average" . 

Both the initiation step and the propagation step are dependent on the stability of the carbocations. Isobutylene (the first monomer to be commercially polymerized by ionic initiators), vinyl ethers, and styrene have been polymerized by this technique. The order of activity for olefins is Me2C=CH2 > MeCH=CH2 > CH2=CH2, and for para-substituted styrenes the order for the substituents is Me—O > Me > H > Cl. The mechanism is also dependent on the solvent as well as the electrophilicity of the monomer and the nucleophi-licity of the gegenion. Rearrangements may occur in ionic polymerizations. 

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. 

The first method, which is used in many instances, proceeds by anionic block copolymerization. In a first step styrene is anionically polymerized using an efficient bifunctional initiator. The polymer obtained has a well-defined molecular weight, it exhibits a rather narrow molecular weight distribution and it has carbanionic living sites at both ends of its linear chains7. ... 

Vinyl-type addition polymerization. Many olefins and diolefins polymerize under the influence of heat and light or in the presence of catalysts, such as free radicals, carbomum ions or carbamons. Free radicals are particularly efficient in starting polymerization of such important monomers as styrene, vinylchloride, vinylacetate, methylacrylate or acrylonitrile. The first step of this process—the so-called initiation step—consists in the thermal or photochemical dissociation of the catalyst, and results in the formation of two free radicals-. 

Hall U3>, Hsieh 106>, Roovers and Bywater107), Tanlak and co->workers114), and Bordeianu and co-workersI1S) followed the initiation of styrene under polymerization conditions in aromatic or alkane solvents using ethyllithium, z-propyllithium, or isomers of butyllithium. Without exception, these authors found a first power dependency of initiation rate on total active center concentration. Hsieh s results106) and those of Roovers and Bywater 107, also indicate that the first order character for initiation is independent of the degree of association (4 or 6) of the alkyllithium. The first order dependence of the initiation step on total active center concentration is also maintained over the period where cross-aggregated structures, PSLi (RLi)x, are present. 

Frechet and coworkers recently described how living free radical polymerization can be used to make dendrigrafts. Either 2,2,6,6-tetramethylpiperidine oxide (TEMPO) modified polymerization or atom transfer radical polymerization (ATRP) can be used [96] (see Scheme 10). The method requires two alternating steps. In each polymerization step a copolymer is formed that contains some benzyl chloride functionality introduced by copolymerization with a small amount of p-(4-chloromethylbenzyloxymethyl) styrene. This unit is transformed into a TEMPO derivative. The TEMPO derivative initiates the polymerization of the next generation monomer or comonomer mixture. Alternatively, the chloromethyl groups on the polymer initiate an ATRP polymerization in the presence of CulCl or CuICl-4,4T dipyridyl complex. This was shown to be the case for styrene and n-butylmethacrylate. SEC shows clearly the increase in molecu-... 

The dimer cation was supposed to have a sandwich structure in which the orbitals of one molecule overlapped with those of the other molecule. The band at 450 nm (B) is due to the bonded dimer cation (St—St T) the formation of this species corresponds to the initiation step of the polymerization. The bonded dimer cation may be formed by the opening of the vinyl double-bonds. Egusa et al. proposed that the structure was a linked head-to-head type I or II, by the analogy of the dimeric dianions of styrene and a-methylstyrene. Table 1 summarizes the assignment of absorption bands observed in pulse radiolysis of 1,1-diphenylethylene in dichloromethane, which is a compound suitable for studying monomeric and dimeric cations [28],... 

It must be stated that up to now models that can describe detailed styrene polymerization including all kinds of initiation step are rare. The work of Dhib ei al. [31] is so far the most comprehensive in this respect. It is a common practice tc fit the model to experimental data under different reaction process conditions. 

Polymerization reactions proceed via initiation, propagation, and termination steps as illustrated in Section 4.1. A simplified network to describe the styrene polymerization is ... 

Polymerization of styrene and dienes in hydrocarbon solvents 3.1. THE INITIATION STEP... 

In many cases, the catalyst is not able to initiate polymerization without a prior activation step. Azo or peroxy catalysts must be heated to effect dissociation. This is illustrated in Reaction 1 for 2,2 -azobisiso-butyronitrile. Decomposition produces free radicals (Reaction la) that are then capable of reacting with monomer (in this case styrene) in the polymerization-initiation step (Reaction lb) ... 

Using this method asymmetric stars of the type (PSa) (PSb)/7 were prepared.77 Living PS chains were obtained by s-BuLi initiation and reacted with a small amount of DVB to give a living star polymer. The anionic sites of the star core were subsequently used to initiate the polymerization of a new quantity of styrene. This initiation step was accelerated by the addition of a small quantity of THF. It was revealed by SEC analysis that high molecular weight species were also present, probably due to the formation of linked stars. These structures can be obtained when living anionic branches of one star react with the residual double bonds of the DVB-linked core of another star. 

In anionic polymerization, B-propiolactone initiated with potassium alcoholate, gives,in initiation,both alco-holate and carboxylate anions. Alcoholate ions in every next step convert partially into carboxylate whereas carboxylate reproduce themselves quantitatively. Thus, after a few steps only carboxylate anions are left (14). Related situation was observed in the polymerization of styrene oxide (15). Here, however, it is only due to the structure of the initiator used. Thus, when in the initiation step both secondary and primary alcoholate anions are formed, due to the low steric requirements, in the next step apparently only the attack on the least substituted carbon atom takes place and already in the second step exclusively secondary alcoholate anions are present. 

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