Monomer placement

Spectroscopic analyses are widely used to identify the components of copolymers. Infrared (IR) spectroscopy is often sufficient to identify the comonomers present and their general concentration. Nuclear magnetic resonance (NMR) spectrometry is a much more sensitive tool for analysis of copolymers that can be used to accurately quantify copolymer compositions and provide some information regarding monomer placement. 

Some chiral initiators have structures such that alternate monomer placements occur with opposite faces of the monomer to yield the syndiotactic polymer. This is syndioselective polymerization proceeding with catalyst site control and is usually observed only with some homogeneous initiators, both traditional Ziegler-Natta and metallocene. 

Fig. 15. The dependence of af( 1 — o) on IJT for free radical polymerization of methyl methacrylate, where T is the polymerization temperature (° K.) and a the probability of iso tactic monomer placement...

Fig. 14. Monomer placement in particles during emulsion polymerization, a, b, c, Various stages of polymerization (see text) d monomer particle stabilized by emulsifier diam. = 104 nm e monomer-polymer particle f, monomer solubilized in a micelle q, emulsifier. See p. 180 of ref. 126.

Wicke and Elgert studied the mode of a-methylstyrene placement in chains polymerized with butyllithium in tetrahydrofuran. They called attention to the possibility of various addition and depropagation rates for isotactic and syndiotactic monomer placement. Isotactic attachment is harder to form, and easier to decompose. For reversible processes this circumstance must be manifested at higher temperatures by thermodynamic, and at lower temperatures by kinetic, control of product stereoregularity [363],... 

Similarly, an atactic triad is produced by opposite monomer placements, i.e., (mr) or (rni). The two atactic triads are indistinguishable in an NMR analysis. 

The dyads in commercial poly(vinyI chloride) (PVC) are about 0.55% racemic, indicating short runs of syndiotactic monomer placements. The absence of a completely atactic configuration is reflected in the low levels of crystallinity in this polymer, which have a particular influence on the processes used to shape it into useful articles. 

The exact sequence of monomer units along the chain can vary widely depending upon the relative reactivities of each monomer during the polymerization process. At the extremes, monomer placement may be totally random or may be perfectly alternating. If repeating units are represented by A and B, then the random copolymer might have the structure shown as ... 

Our discussion above on polymer conformations in single chains and in crystals has assumed regularity of macromolecular structure. However, irregularities such as inversions of monomer placements (head-to-head instead of head-to-tail), branches, and changes in configuration may occur. These irregularities, which are considered in a later section, may Inhibit crystallization and have a profound effect on polymer properties. 

Figure 2.15 The probabilities (or fractions) of isotactic (mm), heterotactic (mr), and syndiotactic (rr) triads as a function of Pm, the probability of isotactic monomer placement during propagation. (From Ref. 18.)...

These words are based on the Greek roots iso- = same, a- = not, tactos = form. Isotactic chains contain exclusively mm monomer placements syndiotactic chains contain exclusively mr or rm placements, while atactic chains contain 25% rr, 25% mm and 50% mr placements. Special catalysts are necessary to produce isotactic polypropylene, which is highly crystalline, whereas atactic polystyrene is produced without the use of special catalysts. It is possible to produce isotactic polystyrene and atactic polypropylene, but these have very limited markets. 

The polymer that forms is crosslinked, but spectroscopic analysis shows that 90% of the monomer placement is through ring formation. Formation of six-membeied rings is also well documented. Two examples were shown above in the polymerization of a quaternary diethyldially-lamine and in the polymerization of 2,6-disubstituted, 1,6-heptadiene. Many other 1,6-heptadienes yield linear polymers containing six-membered rings. " ... 

Ionic polymerizations yield highly stereoregular polymers when control is exercised over monomer placement. The earliest stereospecific vinyl polymerizations were observed in preparation of poly isobutyl vinyl ether) with a BFa-ether complex catalyst at -70 °C. An isotactic polymer formed. °The same catalyst was employed later to yield other stereospecific poly(vinyl ether)s. " The amount of steric placement increases with a decrease in the reaction temperature, and, conversely, decreases with an increase in the temperature. ... 

Use of hydrocarbon solvents has an advantage in polymerizations of conjugated dienes, because they yield some steric control over monomer placement. This is true of both tacticity and geometric isomerism. As stated earlier, the insertions can be 1,2 3,4 or 1,4. Furthermore, the 1,4-placements can be cis or trans. Lithium and organolithium initiators in hydrocarbon solvents can yield polyisoprene, for instance, which is 90% cw-1,4 in structure. The same reaction in polar solvents, however, yields polymers that are mostly 1,2 and 3,4, or trans-lA in structure. There is still no mechanism that fully explains steric control in polymerization of dienes. 

NMR spectra of poly(A, Ar-dimethylacrylamide) formed with ec-butyllithium in both polar and nonpolar solvents show that the penultimate unit does affect monomer placement. Also, a coordination was observed with both heteroatoms,the one on the ultimate and the one on the penultimate unit. 

The above means that the active sites act as templates or molds for successive orientations of the monomers. The monomers are forced to approach these site with the same face. This sort of monomer placement is called enantiomorphic site control or catalyst site control. 

Polymerizations of ketenes yield varieties of structures, because monomer placements are possible through either the carbon-to-carbon double bond or through the carbon-to-oxygen double bond. Dimethylketene polymerizes by anionic mechanism to a polymer with the following struc-... 

Analyses of polymers to determine stereosequence distributions and understand the propagation mechanism can be carried out with NMR spectroscopy aided by statistical propagation models. 75 detailed discussion of the subject is beyond this book. The following is a brief explanation of the concepts. The Bernoulli, Markov, and Colman-Fox, models describe propagation reactions with chain end control over monomer placement. The Bernoulli model assumes that the last monomer unit in the propagating chain end determines the stereochemistry of the polymer. No consideration is given to the penultimate unit or other units further back. In such an event, two modes... 

There are many reports in the literature of preparations of polymers from various other substituted dienes. Most have no commercial significance. Some are, however, interesting materials. An example is a polymer of 2-r-butyl- 1,3-butadiene formed with TiCU and either alkylaluminum or aluminum hydride catalysts.The polymer is crystalline and melts at 106 C. It can be dissolved in common solvents. Based on X-ray data, the monomer placement is high ci5-l,4. 

The occurrence frequency of sequences of two, three, etc., monomeric units of the same kind is determined from the incorporation probability of the monomers concerned into the copolymer chain. A given active chain end can add on either a monomer A or a monomer B. The probability of forming an MaMa constitutional diad is consequently greater with greater A-monomer placement rate in comparison to the sum of all possible placement rates ... 

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