Copolymer composition microstructures

Recognition of these differences in behavior points out an important limitation on the copolymer composition equation. The equation describes the overall composition of the copolymer, but gives no information whatsoever about the distribution of the different kinds of repeat units within the polymer. While the overall composition is an important property of the copolymer, the details of the microstructural arrangement is also a significant feature of the molecule. It is possible that copolymers with the same overall composition have very different properties because of differences in microstructure. Reviewing the three categories presented in Chap. 1, we see the following ... 

Returning to the data of Table 7.1, it is apparent that there is a good deal of variability among the r values displayed by various systems. We have already seen the effect this produces on the overall copolymer composition we shall return to the matter of microstructure in Sec. 7.6. First, however, let us consider the obvious question. What factors in the molecular structure of two monomers govern the kinetics of the different addition steps This question is considered in the few next sections for now we look for a way to systematize the data as the first step toward an answer. 

In the past three decades, industrial polymerization research and development aimed at controlling average polymer properties such as molecular weight averages, melt flow index and copolymer composition. These properties were modeled using either first principle models or empirical models represented by differential equations or statistical model equations. However, recent advances in polymerization chemistry, polymerization catalysis, polymer characterization techniques, and computational tools are making the molecular level design and control of polymer microstructure a reality. 

More recently, the same author [41] has described polymer analysis (polymer microstructure, copolymer composition, molecular weight distribution, functional groups, fractionation) together with polymer/additive analysis (separation of polymer and additives, identification of additives, volatiles and catalyst residues) the monograph provides a single source of information on polymer/additive analysis techniques up to 1980. Crompton described practical analytical methods for the determination of classes of additives (by functionality antioxidants, stabilisers, antiozonants, plasticisers, pigments, flame retardants, accelerators, etc.). Mitchell... 

Polymer Characterization. The copolymer composition and polybutadiene microstructure were obtained from infrared analysis and checked for certain copolymers using 13C NMR. 

The gas chromatographic analysis of the unreacted monomers in the experiments from Table II discloses a constant C5/C8 ratio comparing the starting comonomer composition to the final composition. This means that monomer conversion is the same for 1,5-cyclooctadiene and cyclopentene in the copolymerization so that copolymer compositions are equal to the charge ratios. This result is consistent with the product analysis by 13C NMR spectroscopy where the copolymer composition is nearly identical to the starting comonomer composition. 13C NMR is used to determine the composition of the cyclopentene/1,5-cyclooctadiene copolymers as part of a detailed study of their microstructure (52). The areas of peaks at 29-30 ppm (the pp carbon from cyclopentene units) and at 27.5 ppm (the four ap carbons from the 1,5-cyclooctadiene) are used to obtain the mole fractions of the two comonomers (53, 54, 55). 13C NMR studies and copolymer composition determinations are described by Ivin (51, 56, 57) for various systems. 

The change in melting point and glass transition of the copolymers as a function of copolymer composition are also of particular interest because this reveals information about the copolymer microstructure. This is discussed along with the crystallinity characterization in the following section. 

The phenomenal growth in commercial production of polymers by anionic polymerization can be attributed to the unprecedented control the process provides over the polymer properties. This control is most extensive in organolithium initiated polymerizations and includes polymer composition, microstructure, molecular weight, molecular weight distribution, choice of functional end groups and even monomer sequence distribution in copolymers. Furthermore, a judicious choice of process conditions affords termination and transfer free polymerization which leads to very efficient methods of block polymer synthesis. 

Anionic polymerizations initiated with alkyllithium compounds enable us to prepare homopolymers as well as copolymers from diene and vinylaromatic monomers. These polymerization systems are unique in that they have precise control over such polymer properties as composition, microstructure, molecular weight, molecular weight distribution, choice of functional end groups and even copolymer monomer sequence distribution. Attempts have been made in this paper to survey these salient features with respect to their chemistry and commercial applications. 

Inserted structures indicate copolymer compositions but not microstructures. 

In the copolymerisation of butadiene and isoprene with Ti-based catalysts, both monomeric units of the copolymers obtained are essentially of a ciy-1,4 structure the microstructure of monomeric units in the copolymers does not differ substantially from that in the homopolymers [196-198], Nd-based catalysts provide butadiene/isoprene copolymers with more than 95% cis-1,4 monomeric units [89,199,200], On the other hand, Co-based catalysts give copolymers in which the structure of the monomeric units depends markedly on copolymer composition [19,201,202], Similarly, the structure of the monomeric units depends on copolymer composition in copolymers of butadiene and 2,3-dimethylbutadiene obtained by copolymerisation with Co-based catalysts [201,203],... 

The polymerization of norbornene, Eq. (19), is stopped by cooling the reaction mixture to room temperature. The active polymer 11 can be stored for long periods of time. Heating 11 to temperatures above 65 °C in the presence of monomer causes renewed chain propagation. The subsequent addition of different cyclic olefins, such as endo- and exo-dicyclopentadiene, benzonorbomadiene and 6-methylbenzonorbornadiene resulted in the formation of well-defined AB- and ABA-type block copolymers, Eq. (21) [38]. Triblock copolymers 13 with narrow molecular weight distributions (polydispersity = 1.14) were prepared. Thus, the living character enables the preparation of new uniform block copolymers of predictable composition, microstructure and molecular weight. 

In contrast to the above mentioned models, the similar statistical description of the products of the complex-radical copolymerization occurring through the scheme (2.5) has been carried out quite recently [37, 49, 55-60]. Within the framework of this Seiner-Litt model, both copolymer composition [37,49, 55-58] and fractions of the different triads and blocks of the monomer units in the macromolecules were calculated [57]. The probability approaches which were applied in these works, are regarded as being of limited applicability in contrast to the general statistical method [49, 59, 60], By means of the latter method, the sequence distribution and composition inhomogeneity of the copolymer were completely described [49, 60] and also thorough calculations of its microstructure with the account for the tactidty were carried out [59, 60]. 

The thermodynamic parameters ASP° and AHP° may also be affected by the microstructure of the resulting polymer or copolymer. In particular, low values of AHP° lead to reversible propagation, which in turn results in significant deviation of the copolymer composition as described by the terminal copolymerization model discussed below. On the other hand, the microstructure of the polymer affects ASP°, with atactic polymers and more random copolymers having higher entropies than tactic polymers and more regular copolymers, respectively. 

Spectrometric Analysis. Spectroscopy has been extensively used for polymer and copolymer analysis. (59-69). The kind of information available from different spectroscopic techniques as well as the instrumentation required depends on the region of the electromagnetic spectrum in which absorption is taking place. Recent investigations (63) on the use of spectrophotometers for copolymer analysis have shown that the response from spectrophotometers is sometimes sensitive to the microstructure of the polymer molecules and that calibration of spectrophotometers with absolute measurements on the microstructure (i.e. NMR) may be necessary in order to obtain reliable quantitative information on concentration and copolymer composition determinations. 

The problem of copolymer characterization becomes more acute when on-line process measurements are required. Most of the process instrumentation available yields measurements which are functions of the molecular weight, copolymer composition and, in some cases, of the microstructure of the polymer molecules. Such is the case for on-line viscosimeters, densitometers, torque meters etc. Clearly, the development of copolymer characterization techniques in general and on-line instrumentation in particular requires extensive development. 

Holmqvist, P. Alexandridis, P. Lindman, B. Modification of the microstructure in block copolymer-water- oil systems by varying the copolymer composition and the oil type small-angle x-ray scattering and deuterium-NMR investigation. J. Phys. Chem. B 1998, 102 (7), 1149-1158. 

The copolymer equation (7.11) describes the copolymer composition only on a macroscopic scale, that is, the overall mole ratio or mole fraction of monomer units in a copolymer sample produced from a comonomer feed. It does not reveal details of molecular level composition or microstructure, that is, the manner in which the monomer units are distributed in the... 

Table I Composition, microstructure of BD units and molecular parameters for copolymers EPM-BD used for determination of sensitivity on ionizing radiation...

It should be noted that the copolymer equation (7.11) describes the instantaneous copolymer composition on a macroscopic scale, that is, composition in terms of the overall mole ratio or mole fraction of monomer units in the copolymer sample produced, but it does not reveal its microstructure, that is, the manner in which the monomer units are distributed along the copolymer chain. Thus for two monomers Mi and M2, the ratio Fi/(I — F ) gives the overall mole ratio of Mi and M2 units in the copolymer but no information about the average lengths (i.e., number of monomer units) of Mi and M2 sequences, as illustrated (Allcock and Lampe, 1990) for a typical copolymer by... 

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

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