Propagation homopolymerization

Table 7.2 lists a few cross-propagation constants calculated by Eq. (7.20). Far more extensive tabulations than this have been prepared by correlating copolymerization and homopolymerization data for additional systems. Examination of Table 7.2 shows that the general order of increasing radical activity is... 

Note that this inquiry into copolymer propagation rates also increases our understanding of the differences in free-radical homopolymerization rates. It will be recalled that in Sec. 6.1 a discussion of this aspect of homopolymerization was deferred until copolymerization was introduced. The trends under consideration enable us to make some sense out of the rate constants for propagation in free-radical homopolymerization as well. For example, in Table 6.4 we see that kp values at 60°C for vinyl acetate and styrene are 2300 and 165 liter mol sec respectively. The relative magnitude of these constants can be understod in terms of the sequence above. 

AlkyUithium compounds are primarily used as initiators for polymerizations of styrenes and dienes (52). These initiators are too reactive for alkyl methacrylates and vinylpyridines. / -ButyUithium [109-72-8] is used commercially to initiate anionic homopolymerization and copolymerization of butadiene, isoprene, and styrene with linear and branched stmctures. Because of the high degree of association (hexameric), -butyIUthium-initiated polymerizations are often effected at elevated temperatures (>50° C) to increase the rate of initiation relative to propagation and thus to obtain polymers with narrower molecular weight distributions (53). Hydrocarbon solutions of this initiator are quite stable at room temperature for extended periods of time the rate of decomposition per month is 0.06% at 20°C (39). 

This chapter is primarily concerned with the chemical microstructure of the products of radical homopolymerization. Variations on the general structure (CHr CXY) are described and the mechanisms for their formation and the associated Tate parameters are examined. With this background established, aspects of the kinetics and thermodynamics of propagation are also considered (Section 4.5). 

Geometric considerations would seem to dictate that 1,4- and 1,5-dicncs should not undergo cyclopolymerization readily. However, in the case of 1,4-dienes, a 5-hexenyl system is formed after one propagation step. Cyclization via 1,5-backbiling generates a second 5-hexenyl system. Homopolymerization of divinyl ether (22) is thought to involve such a bicyclization. The polymer contains a mixture of structures including that formed by the pathway shown in Scheme 4.18. 

In this section, we consider the kinetics of propagation and the features of the propagating radical (Pn ) and the monomer (M) structure that render the monomer polymerizable by radical homopolymerization (Section 4.5.1). The reactivities of monomers towards initiator-derived species (Section 3.3) and in copolymerizalion (Chapter 6) arc considered elsewhere. 

It is usually assumed that propagation rate constants in homopolymerization ( p) arc independent of chain length and, for longer chains (length >20), there is experimental evidence to support this assumption.356 6 However, there is now a body of indirect evidence to suggest that the rate constants for the first few propagation steps p(l), kp(2), etc. can be substantially different from (overall) (refer Scheme 4.45). The effect can be seen as a special ease of a penultimate unit effect (Section 7.3,1.2). Evidence comes from a number of sources, for example ... 

The rate of copolymerization often shows a strong dependence on the monomer feed composition. Many theories have been developed to predict the rate of copolymerization based on the terminal model for chain propagation (Section 7.3.1.1), This usually requires an overall rate constant for termination in copolymerization that is substantially different from that observed in homopolymerization of any of the component monomers. 

Monomers not amenable to direct homopolymerization using a particular reagent can sometimes be copolymcrizcd. For example, NMP often fails with methacrylates (e.g. MMA, BMA), yet copolymerizalions of these monomers with S are possible even when the monomer mix is predominantly composed of the methacrylate monomer,15j This is attributed to the facility of cross propagation and the relatively low steady state concentration of propagating radicals with a terminal MMA (Section 7.4.3.1). MMA can also be copolymerized with S or acrylates at low temperature (60 C).111 Under these conditions, only deactivation of propagating radicals with a terminal MMA unit is reversible, deactivation of chains with a terminal S or acrylate unit is irreversible. Molecular weights should then be controlled by the reactivity ratios and the comonomer concentration rather than by the nitroxide/alkoxyamine concentration. 

The relative rate of cationic homopolymerization is governed by three factors, ie. the concentration of the propagating species, the ring-opening reactivity of the growing species and the nucleophilic reactivity of the monomer. From kinetic studies196 197 of the polymerization of oxazolines and oxazines it was found that the second factor was the most important. On the other hand, the relative reactivity in the cationic copolymerization is mainly determined by the nucleophilicity of the monomer and for 2-substituted 2-oxazolines this is in the order of benzyl > methyl > > isopropyl > H > phenyl195. ... 

Only fragmented data are available on polymerization of other methacrylates. Propagation constants and the respective Arrhenius parameters for the homopolymerization of various methacrylates initiated by sodium metallo-organics were reported recently +3,56) and are given in Table 2. 

In addition to the cationization (Eq. (20)), the first three propagation steps of the ethene homopolymerization (Eqs. (21)—(23)) should be comparatively investigated. 

Table 17. Calculated activation and reaction enthalpies, AH and AH (kJ mol-1), for the n-th propagation step of the homopolymerization of ethene in the gas phase and in dichloro methane solution...

Before some detailed remarks about the first propagation step of ethene homopolymerization (Eq. (21)) are given, a general survey of the energetic conditions for the reactions (21)—(23) in the gas phase and in solution (solvent CH2C12) should be provided 131). 

A new rate model for free radical homopolymerization which accounts for diffusion-controlled termination and propagation, and which gives a limiting conversion, has been developed based on ft ee-volume theory concepts. The model gives excellent agreement with measured rate data for bulk and solution polymerization of MMA over wide ranges of temperature and initiator and solvent concentrations. 

The influence of changes in these other variables on MWD in a homopolymerization has not yet been tested, but whatever perturbations are introduced to the feed in a radical polymerization in a laboratory-scale CSTR, they are unlikely to introduce dramatic changes in the MWD of the product because of the extremely short life-time of the active propagating chains in relation to the hold-up time of the reactor. This small change in MWD could be advantageous in a radically initiated copolymerization where perturbations in monomer feeds could give control over polymer compositions independent of the MWD. This postulate is being explored currently. 

Aqueous Phase Hass Balances. The usual material balances for the active species in the aqueous solution are considered. With respect to the case of homopolymerization (4) the conplexity of the resulting equations is increased because of the cross propagation and termination terms. For the batch reactor considered in this wortt, the following equations arise ... 

Stearamide is one of many electron donors which donate an electron to the cationic moiety in excited MAH or in propagating -MAH chains. This results in the inhibition of the homopolymerization of MAH and decreases the crosslinking of polyethylene and the degradation of polypropylene which accompany the peroxide-catalyzed reaction of MAH with these polyolefins (8,9). ... 

The DIS monomer, unlike its iron analogue, did not homopolymerize with SnCl initiator even on heating. A plausible reason for this result is that this monomer contains a lone pair of elec-trons available for donation to Lewis acids.JU Thus side reactions similar to those of the previous two monomers would prevent propagation. However, the DIS monomer also underwent a free radical copolymerization reaction with styrene and AIBN initiation. 

Although the basic mechanisms are generally agreed on, the difficult part of the model development is to provide the model with the rate constants, physical properties and other model parameters needed for computation. For copolymerizations, there is only meager data available, particularly for cross-termination rate constants and Trommsdorff effects. In the development of our computer model, the considerable data available on relative homopolymerization rates of various monomers, relative propagation rates in copolymerization, and decomposition rates of many initiators were used. They were combined with various assumptions regarding Trommsdorff effects, cross termination constants and initiator efficiencies, to come up with a computer model flexible enough to treat quantitatively the polymerization processes of interest to us. 

As an example of a cyclic ether copolymerization, we will briefly discuss the polymerization of THF with OXP initiated with methyltriflate. The homopolymerizations of both cyclic monomers follow a similar mechanism, and both were found to proceed via macrooxonium ion and/or the macroester mechanism depending on the polarity of the polymerization medium. There should then be 8 possible end-groups, i.e. two types of methoxy tails having a penultimate THF or OXP unit, respectively, two covalent macroesters, and four different oxonium ion propagating chain heads two from a THF oxonium center attached to penultimate THF or OXP units, and two from an OXP oxonium center attached to THF and OXP penultimate units (Scheme III). ... 

The potential of transformation reactions for synthesizing a wider range of block copolymers has not been realized because either the reactions are not quantitiative or deterimental side reactions occur. Thus coupling of two propagating carbanions by one phosgene competes with the 1 1 transformation in Eq. 5-123. The anionic-to-radical transformation in Eq. 5-124 involves the formation of trimethyllead radical, which initiates homopolymerization of monomer B. 

Equation 6-12 is known as the copolymerization equation or the copolymer composition equation. The copolymer composition, d M /d Mi, is the molar ratio of the two monomer units in the copolymer. monomer reactivity ratios. Each r as defined above in Eq. 6-11 is the ratio of the rate constant for a reactive propagating species adding tis own type of monomer to the rate constant for its additon of the other monomer. The tendency of two monomers to copolymerize is noted by r values between zero and unity. An r value greater than unity means that Mf preferentially adds M2 instead of M2, while an r value less than unity means that Mf preferentially adds M2. An r value of zero would mean that M2 is incapable of undergoing homopolymerization. 

A special situation arises when one of the monomer reactivity ratios is much larger than the other. For the case of r >> r2 (i.e., r S> 1 and ri propagating species preferentially add monomer M,. There is a tendency toward consecutive homopolymerization of the two monomers. Monomer Mj tends to homopolymerize until it is consumed monomer M2 will subsequently homopolymerize. An extreme example of this type of behavior is shown by the radical polymerization of styrene-vinyl acetate with monomer reactivity ratios of 55 and 0.01. (See Sec. 6-3b-l for a further discussion of this comonomer system.)... 

The order of radical reactivities can be obtained by multiplying the I /r values by the appropriate propagation rate constants for homopolymerization (fen). This yields the values of fei2 for the reactions of various radical-monomer combinations (Table 6-4). The fei2 values in any vertical column in Table 6-4 give the order or monomer reactivities—as was the case for the data in Table 6-3. The data in any horizontal row give the order of radical reactivities toward a reference monomer. (The Q and e values in the last two vertical columns should be ignored at this point they will be considered in Sec. 6-3b-4.)... 

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