Copolymer, composition heterogeneity

Where copolymer compositional heterogeneity was a problem, monomer charges and feed ratios were adjusted to produce constant composition homogeneous polymers. While the required composition for the monomer charge and feed ratios may be determined by a random process, it is much more convenient to use an integrated form of the copolymer equation to calculate these quantities (2). 

While the knowledge of the composition drift provides important information on the copolymers compositional heterogeneity, it reveals nothing about their sequence distribution, important in determining different aspects of the copolymer behavior. 

Eree-radical initiation of emulsion copolymers produces a random polymerisation in which the trans/cis ratio caimot be controlled. The nature of ESBR free-radical polymerisation results in the polymer being heterogeneous, with a broad molecular weight distribution and random copolymer composition. The microstmcture is not amenable to manipulation, although the temperature of the polymerisation affects the ratio of trans to cis somewhat. 

Heterogeneous copolymerization of acrylamide causes redistribution comonomers between phases I and II. This leads to a change of copolymer composition in phases I and II. As a result, the values of ri and change. This accounts for anomalous widening of the experimental composition distribution curves as compared with theoretical curves. 

The existence of an azeotropic composition has some practical significance. By conducting a polymerization with the monomer feed ratio equal to the azeotropic composition, a high conversion batch copolymer can be prepared that has no compositional heterogeneity caused by drift in copolymer composition with conversion. Thus, the complex incremental addition protocols that arc otherwise required to achieve this end, are unnecessary. Composition equations and conditions for azeotropic compositions in ternary and quaternary eopolymerizations have also been defined.211,21... 

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. 

Chain compositional heterogeneity is of particular relevance to functional copolymers which find widespread use in the coalings and adhesives industries.13,240,246 In these applications, the functional copolymer and a crosslinking agent are applied together and are cured to form a network polymer. The functional copolymers are based on functional monomers with reactive groups (e.g. OH), it is desirable that all copolymer molecules have a functionality of at least two. Nonfunctional polymer will not be incorporated and could plasticize the network or be exuded from the polymer. Monofunctional polymers are not involved in crosslink formation and will produce dangling ends. 

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... 

This polymeric oxocarbenium salt readily initiates the cationic ring opening polymerization of oxolane to produce a polystyrene-polyTHF block copolymer. Molecular weight control is provided, polydispersity is narrow and compositional heterogeneity is small59). 

The synthesis of well defined block copolymers exhibiting controlled molecular weight, low compositional heterogeneity and narrow molecular weight distribution is a major success of anionic polymerization techniques 6,7,14-111,112,113). Blocks of unlike chemical nature have a general tendency to undergo microphase separation, thereby producing mesomorphic phases. Block copolymers therefore exhibit unique properties, that prompted numerous studies and applications (e.g. thermoplastic elastomers). 

The heterogeneous copolymerization of styrene and acrylonitrile in various diluents as reported by Riess and Desvalois (22). Although the copolymer composition in these studies was not strongly influenced by the diluent choice, the preferential adsorption of acrylonitrile monomer onto the polymer particles shifted the azeotropic copolymerization point from the 38 mole % acrylonitrile observed in solution to 55 mole % acrylonitrile. 

Copolymers synthesized under conversion of less than ten percent exhibit composition heterogeneity substantially exceeding that predicted by the traditional theories. 

Applications Applications of SEC-FTIR include quantitative analysis of copolymers [701] product deformulation of hot melt adhesives characterisation of polymer compositional heterogeneity analysis of complex mixtures of urethane oligomers and eventually also the identification and quantitative analysis of polymer additives... 

The alternating tendency of the copolymers is advantageous in that the polymerizations can be carried out to high conversions with little or no compositional drift. For random copolymerizations in which there is preferential incorporation of one monomer due to a mismatch in reactivity ratios, the compositional variations with conversion can be substantial. Such compositional heterogeneities in resist materials can lead to severe problems during image development. 

Most copolymers are heterogeneous in both molecular weight and composition. The latter of these arises from the mechanism of the copolymerisation (particularly at high conversion) and individual copolymer molecules differ slightly in their value of WA. Solutions of heterogeneous copolymers constitute multicomponent systems... 

The use of simultaneous equations with at least three sets of experimental data can, however, be applied to Eq. (101) or (102) to yield M, MA and MB. This is especially useful, if the specific co-ordinates of the whole parabola cannot be realised experimentally as is often the case. The data159-1 in Table 14 relate to a random copolymer of styrene (A)/di-n-butyl itaconate (B) in which WA = 0.169. Solution of simultaneous equations [Eq. (101)] yields M = 67300, MA = 56800 and MB =64700. The compositional heterogeneity is such that the data conform to the required theoretical relationship149 ... 

Compositional heterogeneity of the substituted PMMA For all systems investigated (36), the substituted PMMA are characterized by a fairly high chemical homogeneity over the whole range of DSm(DSm<0.76), quite comparable to that of a radical azeotropic S-MMA copolymer (40) (a = 1.6x10 ). The mean square standard deviation o related to two copolymers of DPn =... 

A better agreement between experimental and calculated a values cannot be reasonably expected because of the well known difficulties of accurate determination of low compositional heterogeneity on polydisperse copolymers (4 1). Both the high molecular weight of the PMMA precursor and the autoretarded kinetics contribute to the narrowing of the compositional distribution, but their relative influence cannot be estimated separately. 

An early study of the influence of composition heterogeneity on the physical properties of copolymers was undertaken by Nielson (4), but to the knowledge of the authors, there have been no similar Investigations involving tin-based polymers. In this work a range of different acrylic co- and terpolymers has been prepared and the effects of composition and composition distribution on the physical and performance-related properties of the polymers in their native state and In paints have been examined. 

In the commercial use of copolymerization it is usually desirable to obtain a copolymer with as narrow a distribution of compositions as possible, since polymer properties (and therefore utilization) are often highly dependent on copolymer composition [Athey, 1978]. Two approaches are simultaneously used to minimize heterogeneity in the copolymer composition. One is the choice of comonomers. Choosing a pair of monomers whose copolymerization behavior is such that F is not too different from f is highly desirable as long as that copolymer has the desired properties. The other approach is to maintain the feed composition approximately constant by the batchwise or continuous addition of the more reactive monomer. The control necessary in maintaining f constant depends on the extent to which the copolymer composition differs from the feed. 

---