Comonomer sequence distribution

In the most common production method, the semibatch process, about 10% of the preemulsified monomer is added to the deionised water in the reactor. A shot of initiator is added to the reactor to create the seed. Some manufacturers use master batches of seed to avoid variation in this step. Having set the number of particles in the pot, the remaining monomer and, in some cases, additional initiator are added over time. Typical feed times ate 1—4 h. Lengthening the feeds tempers heat generation and provides for uniform comonomer sequence distributions (67). Sometimes skewed monomer feeds are used to offset differences in monomer reactivity ratios. In some cases a second monomer charge is made to produce core—shell latices. At the end of the process pH adjustments are often made. The product is then pumped to a prefilter tank, filtered, and pumped to a post-filter tank where additional processing can occur. When the feed rate of monomer during semibatch production is very low, the reactor is said to be monomer starved. Under these... 

The final class of polymers are copolymers containing one or more of the repeat units of classes 2 and 3 (15-18). Copolymer effectiveness would presumably be a function of the chemical structures of each comonomer, comonomer sequence distribution, and polymer molecular weight. The comonomer could be a relatively... 

Monomer reactivity ratios and thus comonomer sequence distributions in copolymers can vary with copolymerization reaction conditions. The comonomer distribution could affect the geometry of the adsorbed polymer - mineral complex and the fines stabilization properties. 

Wu, Ovenall and Hoehn have published a detailed study of the H- ajid 13C-NMR spectra of the QH —CO copolymers with varied QH /CO ratios, comparing them with appropriate model compounds 48). Based on their work, it is possible to determine copolymer compositions, comonomer sequence distributions, end groups, branching and conformational transitions in these copolymers. 

Consequently, there can be a co-elution of species possessing different chain length and chemical composition. The influence of different comonomers copolymerized into the macromolecule on the chain size can be measured by the GPC elution of homopolymer standards of this comonomer. Unfortunately, the influence of the comonomer sequence distribution on the hydrodynamic radius cannot be described explicitly by any theory at present. However, there are limiting cases which can be discussed to evaluate the influence of the comonomer placement in a macro-molecular chain. 

Since the discovery of olefin polymerization using the Ziegler-Natta eatalyst, polyolefin has become one of the most important polymers produeed industrially. In particular, polyethylene, polypropylene and ethylene-propylene copolymers have been widely used as commercial products. High resolution solution NMR has become the most powerful analytieal method used to investigate the microstructures of these polymers. It is well known that the tacticity and comonomer sequence distribution are important factors for determining the mechanical properties of these copolymers. Furthermore, information on polymer microstructures from the analysis of solution NMR has added to an understanding of the mechanism of polymerization. 

On the other hand, it seems that, aside from the overall clustering aspect, little has been discovered, to date, regarding structural features of the polymer itself, particularly from MIR studies. What, for example, can be learned of comonomer sequence distribution, or backbone and sidechain conformations with the accompanying dynamic effects How is conformation affected by counterion type, water content, and temperature, to name a few What can we say about the fluorocarbon/aqueous interface at the molecular level Future studies similiar to those mentioned in the last section may resolve these remaining problems. 

Table 13. Comonomer sequence distribution of the poly(CPP-SA) series...

While some s)uithetic copolymers - alternating, di-, tri-, and mirltiblock copolymers - possess an ordered-sequence distribution of monomeric units along the chain backbone, random copolymers (RCPs) represent a special class of macromolecules, which possess a disordered monomer sequence distribution. A series of papers have been published which established that tuning the copolymer chemical composition and comonomer sequence distribution profoundly afferts the characteristics of RCPs. " Of those characteristics intrinsic to the polymeric materials, most work performed to date addressed the role of the chemical composition. By judiciously... 

A series of ethylene-vinyl chloride (E-V) copolymers was generated [27] by the reductive dechlorination of poly (vinyl chloride) (PVC) [28]. Each E-V copolymer has the same average chain length (approx. 1000 repeat units) and polydispersity. High-resolution solution NMR revealed [27] their random-like comonomer sequence distributions, which were quantitatively determined to the comonomer sequence triad-level (see chapter 2). E-V copolymers with less than 40% V units were observed [29] to crystallize, and the stabilities, structures and morphologies of their crystals were observed [29,30] and analyzed by DSC, X-ray and electron diffraction, and electron microscopy. 

In order to rank performance of catalysts in copol)unerization with respect to comonomer incorporation and comonomer sequence distribution, copolymerization parameters have proven to be very useful. They are determined by means of nuclear magnetic resonance spectroscopy (NMR copolymer sequence analysis) taking into account the Markovian statistics of chain growth, as reviewed by Randall [4], Galimberti and coworkers [5] described the analysis of EPM prepared by means of... 

Early attempts to produce E/NB copolymers utilized heterogeneous TiCl4/AlEt2Cl or vanadium catalysts, but real progress was achieved utilizing metallocene catalysts for this purpose. Metallocenes are about ten times more active than vanadium systems, and by choosing the metallocene, the norbornene/ethylene comonomer sequence distribution in the copolymer may be varied from statistical (random) to alternating. 

Today, the development of a new polymeric material requires a keen understanding of how to manipulate the most intimate features of individual polymer chains—tacticity, branching, comonomer sequence distribution, block length, regioerrors—to obtain desirable physical properties and performance. The modem polymer chemist must possess a good understanding of fundamental microstmctural stmcture-property relationships for any system under study, both from the synthetic perspective (relationships between polymerization catalyst ligand/active site stmcture, polymerization mechanism, and chain microstructure) and the performance perspective (relationships between chain microstmcture, phase behavior, and bulk properties). 

Nuclear magnetic resonance (NMR) spectrometers offer spectral capabilities to elucidate polymeric structures. This approach can be used to perform experiments to determine comonomer sequence distributions of polymer products. Furthermore, the NMR can be equipped with pulsed-liied gradient technology (PFG-NMR), which not only allows one to determine self-diffusion coefficients of molecules to better understand complexation mechanisms between a chemical and certain polymers, but also can reduce experimental time for acquiring NMR data. Some NMR instruments can be equipped with a microprobe to be able to detect microgram quantities of samples for analysis. This probe has proven quite useful in GPC/NMR studies on polymers. Examples include both comonomer concentration and sequence distribution for copolymers across their respective molecular-weight distributions and chemical compositions. The GPC interface can also be used on an HPLC, permitting LC-NMR analysis to be performed too. Solid-state accessories also make it possible to study cross-linked polymers by NMR. 

Monomer composition and comonomer sequence distribution were determined by high resolution and NMR spectroscopy. In the NMR spectrum, MATRIF a-CH3 resonance pattern is observed in 0.82-1.67 ppm (Figure 20.6), three signals were observed and can be associated to the (M) centered VMV, MMV, and MMM triads (where V and M stand for VCN and MATRIF, respectively) [86]. Integration of the three sets of resonances yields the following ratio VMV/MMV/MMM = 37/55/8. 

COPOLYMER COMPOSITION AND COMONOMER SEQUENCE DISTRIBUTION FOR STYRENE/BUTADIENE... 

COMONOMER COMPOSITION, BUTADIENE MICROSTRUCTURE AND COMONOMER SEQUENCE DISTRIBUTION... 

The broad range of control of solution polymer structure and macrostructure of styrene-butadiene rubbers that is only possible using lithium catalysts was discussed in Section 2. We have seen how the microstructure of the butadiene units in the chain and comonomer sequence distribution can be controlled with the addition of polar modifiers and/or variations in pol5onerization process variables. Additionally, the unique control of macrostructure features and the new possibilities offered by reactive functional groups were discussed as part of the molecular engineering capabilities of solution anionic polymerizations. 

A major development in solution SBRs was first disclosed at the International Rubber Conference in Kiev by R. A. Livigni et and in subsequent publications. " As described in Section 3 of this chapter, these solution SBRs have a butadiene portion of trans-1,4-content 80-90%, vinyl content 2-4%, and a random comonomer sequence distribution. Additionally, the polymerization process, which uses a new catalyst of Mg-Al alkyls complexed with barium alkoxide salts, has all the distinguishing characteristics of organoUthium-initiated polymerization (Tables 13 and 14). 

Kinetic studies of the solution (benzene) and bulk polymerization of methyl methacrylate with MA have been run at 60°C and 70°C, using AIBN initiator. The microstructure of the purified copolymers was determined by H-NMR and IR spectroscopy. Analysis of the comonomer pair sequence distribution for the solution-prepared copolymers supported a copolymerization mechanism involving participation of an association species between the two monomers. A terminal model or the classical Mayo-Lewis concept more adequately explained the results of bulk copolymerization, where the comonomer sequence distribution was more random. Theoretically, the concentration of associating species should be greatest in bulk, which was... 

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