Polystyrene propagation

Fig. 3. Equilibrium constant of living polystyrene propagation in cyclohexane and benzene In if = -AGj,sJRT plotted versus 1/T, Reproduced, with permission, from Bvwater and Worsfold J. Polymer Sci 58,571...

Thus, the lA order observed for the lithium polystyrene propagation might change into A at some sufficiently high concentrations of living polystyrene, or the A order observed for lithium polyisoprenyl should become Yi on their appropriate dilution. However, there are limitations and technical difficulties which could make such a test at least difficult if not... 

Figure 1 Linear dependence of the apparent bimolecular rate constant (kl ) of living polystyrene propagation on the reciprocal of the square root of living polymer concentration in THF as solvent at 25 Different lines refer to different counterions ir, Na K"", Rb and Cs LE, living ends.

If intermediate radicals are consumed in side reactions, such as combination or disproportionation with another radical species, this will also cause retardation. That so-called intermediate radical termination might complicate RAFT polymerization was first proposed by Monteiro and de Brouwer in 2001. " That a polystyrene intermediate radical can combine with a polystyrene propagating radical to form a stable three-armed star has been demonstrated. However, attempts to detect these species in the expeeted concentrations in polymerization have failed. 

It has been demonstrated that with SBR polystyrene blends the rubber should exist in discrete droplets, less than 50 p.m in diameter where a good finish is required, within the polystyrene matrix. It is believed that in such a form the rubber can reduce crack propagation and hence fracture in various ways. The most favoured current explanations of this were discussed in Chapter 3. Suffice it to say here that the following features appear necessary for a suitable blend ... 

Formation of block polymers is not limited to hydrocarbon monomers only. For example, living polystyrene initiates polymerization of methyl methacrylate and a block polymer of polystyrene and of polymethyl methacrylate results.34 However, methyl methacrylate represents a class of monomers which may be named a suicide monomer. Its polymerization can be initiated by carbanions or by an electron transfer process, the propagation reaction is rapid but eventually termination takes place. Presumably, the reactive carbanion interacts with the methyl group of the ester according to the following reaction... 

The order of reactivities could be also reversed by a change of solvent. For example, in THF styrene is more reactive than butadiene towards salts of polystyryl anions, whereas in hydrocarbon solvents butadiene is more reactive than styrene towards lithium polystyrene. This reversal of reactivities presumably is caused by a change in the mechanism of propagation. The monomers react directly with carbanions in THF, but become coordinated to Li+ in hydrocarbon solvents. 

The correct explanation of the peculiar behaviour of the butadiene-styrene system was provided by O Driscoll and Kuntz 144). As stated previously, under conditions of these experiments butadiene is indeed more reactive than styrene, whether towards lithium polystyrene or polybutadiene, contrary to a naive expectation. This was verified by Ells and Morton 1451 and by Worsfold 146,147) who determined the respective cross-propagation rate constants. It is germane to stress here that the coordination of the monomers with Li4, assumed to be the cause for this gradation of reactivities, takes place in the transition state of the addition and should be distinguished from the formation of an intermediate complex. The formation of a complex ... 

The results reported by Helary and Fontanille 84) provide an illustration of the above principles. Coordination of lithium polystyrene in cyclohexane by TMEDA increases the propagation rate for c = 8.3 mM but decreases for c = 0.92 mM. This is seen in the plots shown in Fig. 22. 

Say that at c = 1 mM the propagation rate is not affected by the addition of TMEDA, a reasonable assumption based on the data of Helary and Fontanille. This leads to ktKigs = 0.84 10-2 M1/2 sec-1 as derived from the equation c = k2Kdiss/8k, whereas its value determined from the kinetics of lithium polystyrene polymerization in cyclohexane is 0.7 102 M1/2 sec-1. The agreement is fair. Note, the results are independent of the value of Kso). 

Under these conditions the maximum propagation constant, kpc = 750 M-1 sec-1, gives the absolute rate constant of the monomer addition to the complexed unassociated lithium polystyrene, a value obviously larger than that of the unassociated but also uncomplexed polymer. 

Transfer constants for polystyrene chain radicals at 60° and 100°C, obtained from the slopes of these plots and others like them, are given in the second and third columns of Table XIII. Almost any solvent is susceptible to attack by the propagating free radical. Even cyclohexane and benzene enter into chain transfer, although to a comparatively small extent only. The specific reaction rate at 100°C for transfer with either of these solvents is less than two ten-thousandths of the rate for the addition of the chain radical to styrene monomer. A fifteenfold dilution with benzene was required to halve the molecular weight, i.e., to double l/xn from its value (l/ rjo for pure styrene (see Fig. 16). Other hydrocarbons are more effective in lowering the degree of polymerization through chain transfer. 

Analogous principles should apply to ionically propagated polymerizations. The terminus of the growing chain, whether cation or anion, can be expected to exhibit preferential addition to one or the other carbon of the vinyl group. Poly isobutylene, normally prepared by cationic polymerization, possesses the head-to-tail structure, as already mentioned. Polystyrenes prepared by cationic or anionic polymerization are not noticeably different from free-radical-poly-merized products of the same molecular weights, which fact indicates a similar chain structure irrespective of the method of synthesis. In the polymerization of 1,3-dienes, however, the structure and arrangement of the units depends markedly on the chain-propagating mechanism (see Sec. 2b). 

The use of these initiators to polymerize LA814 and methylglycolide815 has been reported to proceed in a well-controlled fashion. Block copolymers such as PCL-b-PLA have also been prepared. Elimination of PrOH from the reaction of (270) with preformed hydroxyl terminated polymers, followed by lactone polymerization, yields diblocks of CL with polystyrene or polybutadiene.816 The preparation of an ABA triblock has also been reported (A = CL, B = LA) since propagating chains of PLA do not initiate CL ring opening, (270) was pretreated with hydroxy terminated (PCL-b-PLA)-OH 814... 

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