Tactic sequence length

NMR not only yields chemical information, but can also be used to analyze polymer tacticity, sequence lengths, short-chain branching, and crystallinity. 

Ke5Words cluster index, comonomer distribution, monomer dispersity, NMR, H NMR, NMR, tacticity, sequence length, copol3uner. 

From appropriate ratios of these sequence lengths, what conclusions can be drawn concerning terminal versus penultimate control of addition The following are experimental tacticity fractions of polymers prepared from different monomers and with various catalysts. On the basis of Fig. 7.9, decide whether these preparations are adequately described (remember to make some allowance for experimental error) by a single parameter p or whether some other type of statistical description is required ... 

B. Measurement of Property Distributions for Copolymers. Figure 12 shows chromatograms of typical products in the copolymerization study (Column Code B2). Since the detector is responding to concentration, composition, and periiaps sequence length, the direct single detector interpretation as described for PMMA is not immediately applicable here. Tacticity variation is yet another consideration but ]s assumed of sa ond order importance for th samples (22). 

A few reviews have dealt with the identification of synthetic polymers by Py-GC/MS [76]. In addition to compositional studies, applications of pyrolysis to synthetic polymers include sequence length characterization in copolymers [77] and tacticity measurements in stereoregular homopolymers [78]. 

Using an Ising model for rotational isomeric states the unperturbed dimensions, 0 / nl2, are evaluated for copolymers of propylene and pentene-1. Chemical composition, tacticity, and sequence length distribution are varied. It is found that only for atactic copolymers 0 I n 2 depends linear on the chemical composition. Deviations from linearity cannot be attributed... 

First-order structure (Chain length, tacticity, sequence)... 

In proton spectra of polymers knowledge of the exact values of the coupling constants and the chemical shifts makes it possible to simulate the experimental spectra by computer calculations. In this way it is possible to interpret quantitatively the complex spectra of many homopolymers and copolymers in terms of configuration (or "tacticity") and monomer sequences (copolymers). The obtained sequence lengths can be directly related to the polymerisation mechanism. 

With the development of polymer structural characterizations using spectroscopy, there has been a considerable effort directed to measurements of tacticity, sequence distributions and number average sequence lengths (59 65). Two methods have been traditionally used for microstructure analysis from polymer solutions. Vibrational spectroscopy (infrared) and Nuclear Magnetic Resonance (NMR). Neither of these techniques is absolute. The assignment of absorption bands requires the use of model compounds or standards of known structure. 

Nuclear Magnetic Resonance. The successful study of polymers in solution by high resolution NMR spectroscopy started with the pioneering work on the sequence structure of poly methyl methacrylate in 1960. Since then, an ever-increasing number of investigations have been carried out ranging from the elucidation of the statistics of homopolymer and copolymer structure to the study of conformation, relaxation and adsorption properties of polymers. The aspects of sequence length determination and tacticity have received considerable attention (Klesper 84, for example, reports more than 500 entries). Therefore, a detailed review will not be attempted. (For a detailed description of the NMR Theory and statistics of polymer structure, see Bovey 59, Randall 23, and Klesper 84). 

Similarly, one can speak about mean sequence-length and its dispersity in statistical copolymers, about mean tacticity and its dispersity in the stereoiegular polymers, and so on. 

A configurational or tactic sequence consists of at least one tactic linkage and, hence, at least two monomeric units. The number of successive linkages of a like nature is defined as the sequence length. The transition from an iso- and a syndiotactic sequence involves a heterotactic triad The number-average sequence length L of all iso- and syndiotactic sequences of a polymer is thus given by the inverse of the mole fraction of all heterotactic triads ... 

Crystallization analysis fractionation (Crystaf) fractionates polymer chains according to differences in crystallizability. Crystaf can be used to fractionate polymers due to differences in chemical composition, comonomer sequence length, and tacticity. It may also respond to long-chain branching, provided that the polymer is branched enough to affect its crystallinity. The fractionation principle operative in Crystaf was discussed in the section on batch fractionation for the case of slowly cooling (or warming) solutions of semicrystalline polymers. 

Shulz and collaborators in their kinetic polymerizations done anionically in with cumylcesium initiators. Their main concern was to determine the statistics which best described the tactic addition of monomer. By NMR analysis of sequence lengths they concluded that the system obeyed Markovian rather than Bernoulli an statistics. ... 

If structure elucidation is required, most commonly a NMR spectrum of the bulk sample provides the first structural information. NMR is applied for determination of the chemical constitution of the monomer units, tacticity, end groups, degree of branching, microstructure, and sequence length distribution. One of the main advantages of NMR is the ability to provide exact quantitative results, without detector calibration. 

A. Hiltner Let s look at that as follows. In a chain, there are segments with some kind of order, it could be a sequence of methylene units or it could be a tactic sequence. The idea is that these can come together to form an association. Now, if in fact the/te longer than a mirrimum length, then they can start to crystallize. 

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