Copolymerization compositional heterogeneity

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. 

Hence, in macromonomer copolymerization the ratio of the molar amounts of A and of M incorporated instantaneously into the copolymer is proportional to the ratio of their molar amounts in the feed. This does not prevent compositional heterogeneity in samples obtained in the case of high conversions however, the amplitude of the fluctuations in the composition generally remains acceptable for conversions up to 50-70 %, because of the low molar concentrations of M usually chosen. 

During recent years an increasing evidence has accumulated that discernible types of centers exist in Z—N catalysts, particularly in their heterogeneous versions. The centers may differ in their kp values, monomer coordination abilities, stereospecificities and reactivities in copolymerization. This concept can explain — at least qualitatively — wide MWD of polyolefins, composition heterogeneity of copolymers and specific responses of the catalyst performance to electron-donor additives. The origin of the differently behaving centers should be seen in a diversity of chemical processes... 

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 copolymerizations have also been defined. " ... 

Determining the composition and sequence of comonomer units is essential in the case of copolymers, since both parameters influence the physical and chemical properties of these materials. Furthermore, comonomer sequence is related to the mechanism of copolymerization, and to the reactivity ratios of the comonomers. Among the techniques developed for polymer characterization. Mass Spectrometry (MS) is one of the most powerful. The mass spectrum of a polymer contains plenty of information on polymer properties such as the structure, the repeat units which constitute the mac-romolecular backbone, the length of macromolecular chains, the end groups which terminate the chains, the chemical heterogeneity, the sequence of copolymers and their composition heterogeneity. 

In the absence of an azeotrope, and when one monomer is more reactive than the other in a binary batch copolymerization (e.g., rj > 1 and T2 < 1), the instantaneous copolymer composition will decrease in monomer A with increase in conversion. The extent of composition drift, which leads to a copolymer heterogeneous in composition, depends on the ratio of reactivity ratios ri/r2 (increasing with any increase in ri/ra), the initial monomer composition and the monomer conversion. A copolymer which is heterogeneous in composition usually has inferior properties, therefore industrial processes have been developed to reduce composition heterogeneity. These processes are usually semibatch, but sometimes continuous as well. 

The alkyllithium-initiated anionic copolymerization of diene and styrene monomers continues to be of interest because one can tailor-make copolymers with a wide range of compositional heterogeneity. Recently, kinetic studies have provided rate constant data to further clarify the factors responsible for the predominant incorporation of the less reactive diene monomer in styrene/diene copol3naerizations carried out in hydrocarbon media.They confirm that the magnitude of the rate constants for butadiene-styrene copolymerizations fall in the order results of several... 

In most copolymerizations, ri r2 and one monomer is preferentially incorporated into the initially growing polymer. This leads to depletion of the preferentially incorporated monomer in the feed and the composition of the copolymer formed changes with conversion. For systems undergoing continuous initiation, propagation, and termination, the resulting compositional heterogeneity is intermolecular, that is, the copolymer formed initially has a different comonomer composition from... 

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 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. 

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. 

Heterogeneous Copolymerization. When copolymer is prepared in a homogeneous solution, kineiic expressions can be used to predict copolymer composition Bulk and dispersion polymerization are somewhat different since the reaction medium is heterogeneous and polymeri/aiion occurs simultaneously in separate loci. In bulk polymerization, for example, the monomer swollen polymer particles support polymerization within the particle core us well as on the particle surface, lit aqueous dispersion or emulsion polymeri/aiion the monomer is actually dispersed in two or three distinct phases a continuous aqueous phase, a monomer droplet phase, and a phase consisting of polymer particles swollen at Ihe surface with monomer. This affect the ultimate polymer composition because llie monomers are partitioned such that the monomer mixture in the aqueous phase is richer in the more water-soluble monomers than the two organic phases. 

Copolymerization. The importance of VDC as a monomer results from its ability to copolymerize with other vinyl monomers. Bulk copolymenzations yielding high VDC-content copolymers are normally heterogeneous. During copolymerization. one monomer may add to the copolymer more rapidly than the other. Batch reactions carried to completion usually yield polymers of broad composition distribution. More often than not, this is an undesirable result,... 

Attempts have been made to copolymerize conjugated dienes with olefins but there are no data on polymerization rates or reactivity ratios. They are ill suited for copolymerization in that polymerization rates are markedly reduced by the presence of the conjugated diene and the copolymers are heterogeneous in composition and may be crosslinked. The reasons for this behaviour have not been established but a possible explanation is that the conjugated diene coordinates preferentially with the catalyst and so excludes the olefin, but has a slow insertion rate compared with the more reactive olefin. 

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