Distribution of monomers

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

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... 

The exact distribution of monomer units depends on the initial proportions of the two reactant monomers and their relative reactivities. In practice, neither perfectly random nor perfectly alternating copolymers are usually found. Most copolymers have many random imperfections. 

The experiment can be repeated with the modification that half of the styrene is added, followed by half of the butadiene, then the remaining styrene followed by the remaining butadiene. The product obtained in this experiment has the same composition and the same molecular weight as the previous product, but the distribution of monomers would be different namely,... 

The copolymer composition equation only provides the average composition. Not all chains have the same composition. There is a statistical distribution of monomers determined by the reactivity ratios. When chains are short, compositional heterogeneity can mean that not all chains will contain all monomers. 

The influence of selectivity in the initiation, termination or chain transfer steps on the distribution of monomer units within the copolymer chain is usually neglected. Galbraith et a .u provided the first detailed analysis of these factors. They applied Monte Carlo simulation to examine the influence of the initiation and termination steps on the compositional heterogeneity and molecular weight distribution of binary and ternary copolymers. Spurting et a/.250 extended this... 

When two or more monomers are polymerized into the same molecular chain they produce a copolymer, The distribution of monomers, in terms of their relative concentrations and placements, is responsible for controlling a copolymer s properties. Figure 5.8 illustrates five possible comonomer distributions for a copolymer comprising equal numbers of two types of monomer. The relative concentrations of the different monomers and the lengths of the various blocks can be varied widely. Relatively small changes in comonomer concentration and placement can result in significant changes in physical and chemical properties. Properties that can be modified include such diverse characteristics as extensibility, elastic recovery, modulus, heat resistance, printability, and solvent resistance. 

The transition temperatures that are combined in Figure 2 show the disappearance of crystallinity in the copolymers as the Ter and Tm flow together moving away from either homopolymer. This reflects the random distribution of monomer units in these copolymers. If the copolymer reactions had given homopolymer mixtures, there would be two separate crystalline melting temperatures. In addition, the 13C NMR indicates that the copolymer products contain a random distribution of C5 and C8 units and that the resulting double bonds are cis from the C8 monomer and largely trans from the C5 monomer (52). 

The determination of the microstructure of vinyl polymers is not merely a characterisation tool. Each polymer molecule is unique, and each polymer chain is a record of the history of its formation, including mis-insertions, rearrangements, the incorporation of co-monomers, and the mode of its termination. NMR analysis of polymers can therefore be used to provide detailed mechanistic and kinetic information. This approach has been applied particularly successfully to the microstructure, i. e. the sequence distribution of monomer insertions, of polypropylene, giving rise to a wealth of studies far too numerous to cover here. Progress in this area has recently been summarised in two excellent and very comprehensive review articles [122, 123[. Here we will cover only the most fundamental aspects of stereoselective polymerisations. 

The conclusion can be drawn that in template copolymerization reactivity ratios depend on the nature and concentration of the template used. Template controls composition and sequence distribution of monomer units in copolymers obtained. 

In homogeneous copolymerization, the instantaneous composition of copolymer is decided only by monomer reactivity ratio. On the contrary, in emulsion copolymerization, the copolymer composition depends not only on the monomer reactivity ratio but also on the distribution of monomers between oil (polymer-monomer particles) and aqueous phases (18). 

Indeed, a series of such polymers, composed of equimolar amounts of styrene and isoprene, has been prepared recently by Levy and Schlick 44), who have shown also that some of their properties, such as viscosity and solubility vary as functions of distribution of monomers along the chain. 

Levy, M., and S. Schlick Block-polymers of styrene and isoprene with variable distribution of monomers along the chain. J. Phys. Chem. 64, 883 (1960). 

Formed from the imine using LDA in hexane, NMR studies reveal complex solvent-dependent distributions of monomers, dimers, and trimers in several ethereal solvents, although a mono-solvated dimer can be selected by appropriate choice of solvent. Study of C-alkylation rates suggests that both monomer- and dimer-based mechanisms operate. The lithioimines were compared with the isostructural lithium dialkylamides, but were shown to be not simply vinylogous analogues thereof. 

When butadiene and styrene are mixed in the presence of an organolithium initiator, the resulting copolymerization process and product will be governed by the reaction conditions. The rate of copolymerization, the relative composition of the copolymer, and the distribution of monomer units (i.e., block, random, etc.) will be determined by such factors as solvent, temperature, and monomer feed ratio. 

Distribution of monomer was found to be good only at high loading which resulted in the polymer being mostly in the homopolymer form in the void structure (63, 64, 65, 66, 67, 68). 

In the case of crosslinked EPDM, the spectral resolution was sufficiently good to carry out the monomer sequence analysis [123]. A similar technique was also applied to NBR. The triad sequence distribution of monomers was determined, by simplifying the copolymer to be consisting of acrylonitrile and butadiene, without discriminating between cis, trans, and vinyl isomeric units [123]. The results indicated the anticipated distribution in the rubber, reflecting the reliability of this technique in the analysis. Based on the... 

The work to date has largely verified the approach of Lowry and extended his models in particular systems. In several cases, the satisfying result appears that one is dealing with thermodynamic control of composition since quantitative data appear to be independent of mechanism. Still to be found is whether the distribution of monomer units in the chain conforms to kinetic or thermodynamic control. Alfrey and Tobolsky... 

The sequence distribution of monomer residues in copolycondensation reactions of this type has been studied in detail by Peebles (18,39). If all monomers are assumed to have equal reactivity and if the reaction has gone to completion, the number-average sequence length of 3IG residues(g) is given by ... 

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