Random monomer sequence

Crystallinity in ECH and ECH—EO finished products increases over time, and may be detected by x-ray analysis or differential scanning calorimetry. In synthesizing ECH—EO, the process is designed to maximize random monomer sequence and minimize crystallinity. The ECH—EO molecular ratio in these products ranges from approximately 3 1 to 1 1. 

Random monomer sequences lead to very complicated liquid crystalline behavior and produce complex morphologies in the polymer solid (25). 

The above calculations show that the X-ray data are consistent with a copolymer structure with completely random monomer sequence. 

The main idea of [66] was to consider a model in a sense even more random than just a random sequence of a limited (small) number of monomer types. Instead of considering just N independent random monomers, the authors assumed there are N(N - l)/2 N2 N independent arbitrary en-... 

The reaction probability model produces a complete calculated C nmr spectrum as a best fit to the observed experimental spectrum. The deduced probabilities provide the following derived quantities (1) "rira", a measure of monomer sequence randomness, (2) the distribution of methylene sequence lengths,... 

Polymers. The polymers used in the blending experiments were prepared by anionic polymerization using an alkyllithium initiator and a chemical randomizing agent to control monomer sequence, in the manner described by Hsieh and Wofford (3). Randomness was checked in each case by measuring the styrene content as a function of conversion. Table I gives descriptive data for these polymers. 

Commercial EPDM rubbers have monomer sequence distributions ranging between random and alternating in the first-order Markovian sense. From our previous experience they must contain at least about 68 wt % ethylene to have runs of ethylene long enough to crystallize above room temperature at rest. 

It is interesting to note that although apparently unaware of the development of molecular imprinting, Pande et al. [28] proposed the use of thermodynamic control for the preparation of synthetic polymer systems with a memory for a template structure. Monte Carlo computer simulations were performed to validate their hypothesis. From these calculations they identified the formation of non-random polymer sequences arising from an evolution-like preferred selection of various monomer components by similar species. These studies have since been expanded upon using statistical mechanics to examine the consequences for protein folding [29]. 

Main Chain LC Polymers. New thermotropic copolyesters with either random or ordered mesogenic sequences have been reported with a wide range of mesophase behaviors. Recent developments in this field have included the use of naphthalene, stilbene and related structures in addition to the traditional phenylene groups to produce the required rigid main chain, and these are described in chapters by Jin, Jackson and Morris, and Skovby et al. Efforts have been undertaken to control transition temperatures and solubility through the use of either substituents or changes in the monomer sequence distribution. Successful application of these efforts have led to the commercialization of several thermotropic aromatic copolyesters (23.24). 

Solution NMR is widely used in polymer processing for the qualitative and quantitative analyses of tacti-city, end-groups, degradation products, chain defects, and monomer sequence distribution.A typical application is in the characterization of monomer sequence distribution by quantitative NMR spec-troscopy. For example. Fig. 7 shows a typical NMR spectrum of ethylene-co-l-butene. From the relative peak areas, it is possible to determine the fractions of the two monomers, their reactivity ratios, the triad distribution, and the blockiness or randomness of the monomer distributions. All of these structure factors play an important role in the polymer s physical and mechanical properties. 

Whereas the stereosequence distribution in isoregic and aregic PVF is nearly ideal random (Bernoullian with p(m) — 0.5), the latter has a first-order Markov regiosequence distribution. Accordingly the monomer sequence isomerism in PVF cannot be described by a single parameter such as the % defect, and requires two reactivity ratios for complete specification. 

Statistical copolymers are those in which the monomer sequence follows a specific statistical law (e.g., Markovian statistics of order zero, one, two). Random copolymers are a special case of statistical copolymers in which the nature of a monomeric unit is independent of the nature of the adjacent unit (Bernoullian or zero-order Markovian statistics). They exhibit the structure shown in Figure 6.1. If A and B are the two monomers forming the copolymer, the nomenclature is poly (A-stat-B) for statistical copolymers and poly (A-ran-B) for the random case. It should be noted that sometimes the terms random and statistical are used indistinctly. The commercial examples of these copolymers include SAN poly (styrene-ran-acrylonitrile) [4] and poly (styrene-ran-methyl methacrylate) (MMA) [5]. 

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