Styrene polymerization termination

In styrene polymerization, termination is mostly by combination (coupling), Quite often, a mixture of initiators or multifunctional initiators are used to achieve a high polymerization rate and a high polymer molecular weight [18-20]. Typical operating conditions for styrene homopolymerization processes are presented in Table 3 [21]. 

The free styrene monomer is restrained within the gel and further reaction with fumarate groups is determined by the spacial arrangement the styrene polymerizes in homopolymer blocks as it intercepts fumarate reaction sites. As individual micelles expand and deplete available fumarate sites in the short polymer chains, the remaining styrene forms homopolymer blocks that terminate at the boundaries between overlapping micelles (Fig. 4). 

Most of the LFRP research ia the 1990s is focused on the use of nitroxides as the stable free radical. The main problems associated with nitroxide-mediated styrene polymerizations are slow polymerization rate and the iaability to make high molecular weight narrow-polydispersity PS. This iaability is likely to be the result of side reactions of the living end lea ding to termination rather than propagation (183). The polymerization rate can be accelerated by the addition of acids to the process (184). The mechanism of the accelerative effect of the acid is not certain. 

Polystyrene-Woc -polysulfone-/ /oc -polystyrene and poly(butyl acrylate)-Woc -polysulfone-/ /oc -poly(butyl acrylate) triblock copolymers were prepared using a macroinitiator.214 The hydroxyl-terminated polysulfone was allowed to react with 2-bromopropionyl bromide, an atomic transfer radical polymerization (ATRP) initiator, in the presence of pyridine. The modified macroinitiator could initiate die styrene polymerization under controlled conditions. 

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. 

Case 3 behavior occurs when the particle size is sufficiently large (about 0.1-1 pm) relative to kt such that two or more radicals can coexist in a polymer particle without instantaneous termination. This effect is more pronounced as the particle size and percent conversion increase. At high conversion the particle size increases and k, decreases, leading to an increase in h. The increase in h occurs at lower conversions for the larger-sized particles. Thus for styrene polymerization it increases from 0.5 to only 0.6 at 90% conversion for 0.7-pm particles. On the other hand, for 1.4-pm particles, n increases to about 1 at 80% conversion and more than 2 at 90% conversion [Chatterjee et al., 1979 Gerrens, 1959]. Much higher values of h have been reported in other emulsion polymerizations [Ballard et al., 1986 Mallya and Plamthottam, 1989]. Methyl methacrylate has a more pronounced Trommsdorff effect than styrene and vinyl acetate, and this results in a more exaggerated tendency toward case 3 behavior for methyl methacrylate. 

If the diazonium groups result from the diazotation of poly-/>-amino-styrene, the macroradicals will initiate grafting. Contrarily, if >-(N-acetyl) phenylenediamine is diazotized and used as initiator of a first monomer, a polymer is obtained with an acetamino. phenyl end group (-CGH4-NH-Ac). After hydrolysis of this last and diazotation of the free amine group, the polymeric terminal diazonium salt can be used with ferrous ions for the synthesis of block copolymers. 

It is desirable to discuss more thoroughly polymerizations taking place in liquid ammonia-alkali metal or alkali amide systems. In the course of their investigation of styrene polymerization carried out in liquid ammonia and initiated by sodium amide, Sanderson and Hauser (14) found a constant molecular weight of about 3,000 for the resulting polymer. Its value was unaffected by the concentration of sodium amide, and it was not changed appreciably by the extent of polymerization. This was interpreted by the above workers as evidence for the termination due to a proton transfer from an ammonia molecule to a growing chain, i.e. 

The case of "living polystyrene and methyl methacrylate is somewhat similar. It was shown, as should be expected, that "living polymethyl methacrylate does not initiate styrene polymerization (70), i. e. methyl-methacrylate is a terminator for the latter polymerization, although its addition to living poly-styrene initiates its polymerization. Hence, one may produce a block polymer by adding methyl methacrylate to "living polystyrene but not vice-versa (9,10). 

Block copolymers, polyclhylcnc-fr/oc/c-polystyrene (PE-fc-PS) and PP-fo-PS, were prepared by the polymerization of styrene with terminally lithiated PO macroinitiators [31]. 

Polymers containing benzoin terminal groups can act as photochemical macroinitiators and are effective in photogenerating polymeric electron donor radicals. The initiation of polymerization by means of azo-benzoin initiators yields polymers with one or two benzoin end-groups according to the termination mode of the particular monomer involved [72-74], The general synthetic procedure is depicted below as illustrated for the case styrene polymerization (Scheme 18). 

By the introduction of styrene into trifluoroacetic acid, polystyrene with a molecular mass of the order of 104 is formed. The trifluoroacetate counterions are stabilized by molecules of the acid by means of hydrogen bonds. The rate of their combination with carbenium chain ends is strongly reduced [92]. This enables several hundred styrene molecules to be added to each centre prior to its termination. When the concentration of styrene is much larger than that of the initiator, which can be realized by the addition of trifluoroacetic acid to styrene, polymerization does not take place. In a non-solvating medium, the combination rate is sufficiently high to prevent propagation. 

The expression for u [eqn. (53)] indicates how important it is to consider primary radical dissociation [the term 1fed/(ki[M])] when correct values of kt pr/fkjkp) are to be obtained, even when termination of macroradicals by secondary phenyl radicals is neglected. In Fig. 6, a graphical representation of eqn. (55) for styrene polymerization with dibenzoylperoxide in benzene is shown. When this dependence is measured for two monomer concentrations, fct pr/(kikp) and kj/kj can be calculated from the slope u. To reduce error,... 

If initiation is faster or comparable to propagation and termination is negligible, kinetic plots are straight in semilogarithmic coordinates. Initiation is faster than propagation and not kinetically detectable in polymerizations of isobutene and styrene initiated by cumyl derivatives because the initiator is more easily ionized than the propagating species. However, if the initiator is less easily ionized than the propagating species as in a-methyl-styrene polymerizations initiated by cumyl derivatives, and in isobutene polymerizations initiated by /-butyl derivatives (cf., also Section III. A.5), then initiation may be incomplete and the overall polymerization rate will increase continuously. 

The formation of stable carbenium ions can be observed visually and/ or spectroscopically. For example, styrene and a-methylstyrene polymerizations are generally colorless because the growing carbenium ions absorb at approximately 340 nm (cf., Sections II.B and IV.B.l). However, these systems may turn brown or dark red at longer reaction times due to formation of indanyl carbenium ions (A 440 nm) [14,26,325] and other delocalized carbocations similar to those in Eq. (121). The stable cyclic diaryl carbenium ions are generated by hydride transfer from the initially formed indanyl end groups [Eq. (124)] in styrene polymerizations, and by methide transfer in a-methylstyrene polymerizations. The prerequisite for this termination is therefore intramolecular transfer by Friedel-Crafts alkylation protons liberated in the first stage can then reinitiate polymerization. 

Finally, termination also occurs if and when the Lewis acid is consumed by side reactions. For example, SbCls adds rapidly to double bonds and must therefore be used in large excess to complete styrene polymerizations at all but very low temperatures [144]. In this case, SbCl3 is too weak a Lewis acid to reactivate the corresponding alkyl chlorides [Eq. (129)]. 

There is further evidence that radical termination reactions are diffusion-controlled. For many polymers, the rate of polymerization shows a sudden increase when the fraction of polymer produced reaches values near 15 to 30 per cent. In the case of methyl methacrylate, Matheson et al. found that, at 30 C and 15 per cent conversion, kt has decreased 160 fold, while kp has not changed appreciably. Vaughan " has proposed a simple diffusion model which is in reasonable accord with the data on styrene polymerization at high conversions. 

On the other hand, however, it is not straightforward to calculate the MWDs for intermediate cases using the conventional approach. A notable advantage of using an MC simulation technique is that it can be applied to virtually any type of emulsion polymerization, and can account for the chain-length-dependent bimolecular termination reactions in a straightforward manner [265]. Sample simulation results for instantaneous MWDs were shown [265] that were obtained using parameters for styrene polymerization that were reported by Russell [289]. 

Since the nitroxide and the carbon-centered radical diffuse away from each other, termination by combination or disproportionation of two carbon-centered radicals cannot be excluded. This will lead to the formation of dead polymer chains and an excess of free nitroxide. The build-up of free nitroxide is referred to as the Persistent Radical Effect [207] and slows down the polymerization, since it will favor trapping (radical-radical coupling) over propagation. Besides termination, other side reactions play an important role in nitroxide-mediated CRP. One of the important side reactions is the decomposition of dormant chains [208], yielding polymer chains with an unsaturated end-group and a hydroxyamine, TH (Scheme 3, reaction 6). Another side reaction is thermal self-initiation [209], which is observed in styrene polymerizations at high temperatures. Here two styrene monomers can form a dimer, which, after reaction with another styrene monomer, results in the formation of two radicals (Scheme 3, reaction 7). This additional radical flux can compensate for the loss of radicals due to irreversible termination and allows the poly-... 

As stated before, when a bifunctional initiator is used, a macro peroxide radical containing undecomposed peroxide is generated, which will lead to additional termination reactions (24) and (25), assuming that styrene polymeric radicals terminate only through combination ... 

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

F7-21g Sketch the polypiet concentration, Pj, mole fraction of polymer with j monomer units, yj, and the corresponding weight fraction, Wj, for j = 2, 10, 30 as a function of monomer conversion in Styrene polymerization for (a) Termination by means other than combination. 

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