Copolymerization monomer distribution

The polymerization of a mixture of more than one monomer leads to copolymers if two monomers are involved and to terpolymers in the case of three monomers. At low conversions, the composition of the polymer that forms from just two monomers depends on the reactivity of the free radical formed from one monomer toward the other monomer or the free radical chain of the second monomer as well as toward its own monomer and its free radical chain. As the process continues, the monomer composition changes continually and the nature of the monomer distribution in the polymer chains changes. It is beyond the scope of this laboratory manual to discuss the complexity of reactivity ratios in copolymerization. It should be pointed out that the formation of terpolymers is even more complex from the theoretical standpoint. This does not mean that such terpolymers cannot be prepared and applied to practical situations. In fact, Experiment 5 is an example of the preparation of a terpolymer latex that has been suggested for use as an exterior protective coating. 

Random copolymers will be formed, or course, if each radical attacks either monomer with equal facility (kn =k 2, kn = 21, 1 = 2 = I). Free-radical copolymerization of ethylene and vinyl acetate is an example of such a system, but this is not a common case. Random monomer distributions are obtained more generally if k /k 2 is approximately equal tok2i/k22- That is to say, r I jr2- This means that k /k22 and A 2i / 22 will be simultaneously either greater or less than unity or in other words, that both radicals prefer to react with the same monomer. 

Ethylene has been co-polymerized with virtually any conceivable a-olefin, from propylene to vinyl-terminated PE and PP macromonomers. Ethylene/propylene (E/P) copolymerization to produce saturated rubbers and ethylene/propylene/diene (EPD) terpolymerization to produce unsaturated, vulcanizable rubbers will be discussed in Section 4.09.4.1.3. 1-Butene, 1-hexene, and 1-octene are the most commonly used co-monomers for the production of LLDPE. Ethylene/octene co-polymers, developed by Dow and marketed under the Engage tradename, have been shown to have improved thermal properties compared to ethylene/butene and ethylene/hexene co-polymers.503 In ethylene/a-olefin (E/O) co-polymeriza-tions, the critical parameters are co-monomer reactivity and co-monomer distribution . The former is most conveniently described by the relative reactivity parameter, R, defined as the ratio between polymer composition and reactor medium composition. 

Mw/M = 2, highly linear Copolymerization random distribution, LLDPE co-monomers propene, 1-butene, 1-octene Elastomers, Terpolymers of Ethene, Propene and Diene low transition metal concentration in the polymer, narrow molecular weight distribution Polymerization to ... 

A value of unity (or nearly unity) for the monomer reactivity ratio signifies that the rate of reaction of the growing chain radicals towards each of the monomers is the same, i.e. kn ki2 and 22 — A 2i and the copolymerization is entirely random. In other words, both propagating species and M2 have little or no preference for adding either monomer. The copolymer composition is the same as the comonomer feed with a completely random placement of the two monomers along the copolymer chain. Such behavior is referred to as Bemoullian. Free-radical copolymerization of ethylene and vinyl acetate and that of isoprene and butadiene are examples of such a system, but this is not a common case. Random monomer distributions are obtained more generally in a situation where both types of radicals have exactly the same preference for the same type of monomer as represented by the relationship... 

Copolymerization. Monomer reactivity ratio parameters (rj and ra) have been defined as the ratio of the rate constant for a reactive species adding to its own type of monomer to the rate constant for its addition to the other monomer. The parameters have been very useful in predicting sequence distributions among different monomers in multicomponent polymerizations and in delineating compositional variations with conversion. 

In random copolymerizations, and certainly multipolymerizations, the FRRPP process can result in products that do not seem to contain random segmental monomer distributions. When polymerizing chains are not prematurely terminated in the FRRPP process, the product distribution can adhere to the predictions of monomer reactivity and monomer concentration ratios. With the incorporation of... 

The sequence distribution of two copolymerizing monomers depends on the catalyst or initiator used, the method of pol5merization, and the concentration and reactivities of the monomers. Reactivity ratios for many monomer pairs have been measured for free-radical, anionic, and coordination polymerization of butadiene (128). 

While living polymerizations can be exploited to produce block copolymers, a copolymerization should give polymer chains that contain both monomers distributed throughout. You might expect that a radical chain polymerization would give a truly random copolymer. Radicals are quite reactive and not known for their selectivity. In fact, though, radical copolymerizations are not totally random, and some quite distinctive polymer compositions can be achieved. Consider a radical polymerization progressing in the presence of two different monomers. 

Quite different is the case of induced transfer to the polymer. Functions that are known to exhibit high transfer constants (such as trichloromethyl or diethylaminoethyl), introduced by chemical modification or by copolymerization, are distributed at random along the backbone (Scheme 7). The polymerization of a monomer carried out in the presence of such polymer species involves a large number of transfer steps whereby grafts are formed, together with a small amount of homopolymer. The advantage is that crosslinking reactions are not expected to occur and have never been observed. 

During usual radical-initiated copolymerization of a mixture of two or three monomers, distribution of the composition of the copolymers obtained is dependent upon the temperature gradient and different reactivity of monomers with different refractive indices. 

There are certain applications in which homopolymers or their blends are not adequate. In such cases, copolymers are synthesized. In this chapter, we have presented the analysis of copolymerization. By developing relations, the rate of polymerization and the monomer distribution on polymer chains can be determined. 

For a growing radical chain that has monomer 1 at its radical end, its rate constant for combination with monomer 1 is designated and with monomer 2, Similady, for a chain with monomer 2 at its growing end, the rate constant for combination with monomer 2 is / 22 with monomer 1, The reactivity ratios may be calculated from Price-Alfrey and e values, which are given in Table 8 for the more important acryUc esters (87). The sequence distributions of numerous acryUc copolymers have been determined experimentally utilizing nmr techniques (88,89). Several review articles discuss copolymerization (84,85). 

During copolymerization, one monomer may add to the copolymer more rapidly than the other. Except for the unusual case of equal reactivity ratios, batch reactions carried to completion yield polymers of broad composition distribution. More often than not, this is an undesirable result. 

Suspension Polymerization. At very low levels of stabilizer, eg, 0.1 wt %, the polymer does not form a creamy dispersion that stays indefinitely suspended in the aqueous phase but forms small beads that setde and may be easily separated by filtration (qv) (69). This suspension or pearl polymerization process has been used to prepare polymers for adhesive and coating appHcations and for conversion to poly(vinyl alcohol). Products in bead form are available from several commercial suppHers of PVAc resins. Suspension polymerizations are carried out with monomer-soluble initiators predominantly, with low levels of stabilizers. Suspension copolymerization processes for the production of vinyl acetate—ethylene bead products have been described and the properties of the copolymers determined (70). Continuous tubular polymerization of vinyl acetate in suspension (71,72) yields stable dispersions of beads with narrow particle size distributions at high yields. 

Regular copolymer—These copolymers have an ordered sequence in the distribution of monomers. These are produced by controlled feeding of the monomers during copolymerization. 

Currently, more SBR is produced by copolymerizing the two monomers with anionic or coordination catalysts. The formed copolymer has better mechanical properties and a narrower molecular weight distribution. A random copolymer with ordered sequence can also be made in solution using butyllithium, provided that the two monomers are charged slowly. Block copolymers of butadiene and styrene may be produced in solution using coordination or anionic catalysts. Butadiene polymerizes first until it is consumed, then styrene starts to polymerize. SBR produced by coordinaton catalysts has better tensile strength than that produced by free radical initiators. 

Several different types of copolymers can be defined, depending on the distribution of monomer units in the chain. If monomer A is copolymerized with monomer B, for instance, the resultant product might have a random... 

Copolymerizations are processes that lead to the formation of polymer chains containing two or more discrete types of monomer unit. Several classes of copolymer that differ in sequence distribution and/or architecture will be... 

Any understanding of the kinetics of copolymerization and the structure of copolymers requires a knowledge of the dependence of the initiation, propagation and termination reactions on the chain composition, the nature of the monomers and radicals, and the polymerization medium. This section is principally concerned with propagation and the effects of monomer reactivity on composition and monomer sequence distribution. The influence of solvent and complcxing agents on copolymerization is dealt with in more detail in Section 8.3.1. 

Tire simplest model for describing binary copolyinerization of two monomers, Ma and Mr, is the terminal model. The model has been applied to a vast number of systems and, in most cases, appears to give an adequate description of the overall copolymer composition at least for low conversions. The limitations of the terminal model generally only become obvious when attempting to describe the monomer sequence distribution or the polymerization kinetics. Even though the terminal model does not always provide an accurate description of the copolymerization process, it remains useful for making qualitative predictions, as a starting point for parameter estimation and it is simple to apply. 

Cases have been reported where the application of the penultimate model provides a significantly better fit to experimental composition or monomer sequence distribution data. In these copolymerizations raab "bab and/or C BA rBBA- These include many copolymerizations of AN, 4 26 B,"7 MAH28" 5 and VC.30 In these cases, there is no doubt that the penultimate model (or some scheme other than the terminal model) is required. These systems arc said to show an explicit penultimate effect. In binary copolynierizations where the explicit penultimate model applies there may be between zero and three azeotropic compositions depending on the values of the reactivity ratios.31... 

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