Homopolymer microstructures

The successive repeat units in strucutres [VI]-[VIII] are of two different kinds. If they were labeled Mj and M2, we would find that, as far as microstructure is concerned, isotactic polymers are formally the same as homopolymers, syndiotactic polymers are formally the same as alternating copolymers, and atactic polymers are formally the same as random copolymers. The analog of block copolymers, stereoblock polymers, also exist. Instead of using Mj and M2 to differentiate between the two kinds of repeat units, we shall use the letters D and L as we did in Chap. I. 

Many commercially important polymers are actually mixtures of two or more polymer components that differ from one another in composition (for copolymers) or in microstructure (for homopolymers). Such mixtures may be the deliberate result of polymer blending, polymer synthesis, or the presence of different types of initiators or catalytic sites that produce different polymer chains. The ung spectral data of the whole polymer in such systems would include contributions from all its components, and as such should be treated with care. 

Polymers that are built from the repetition of identical "repeat units" are called "homopolymers" (from ancient Greek "d/iog — same).2 Linear homopolymer chains are obtained upon linking chemically identical units exclusively at both ends. However, repeat units are not always symmetrical in the chain direction. Depending on the orientation of the repeat unit, different microstructural... 

Due to either accidental or intentional events during the synthesis, branching points can occur along the chain (2D microstructure). Although this needs the presence at the branching point of, stricto sensu, a "non-identical" unit, these macromolecules remain classified as homopolymers. 

This microstructure variation has not been observed with homopolymers synthesized with Na+. For Rn > 3000, % (1,2) - 12%. 

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. 

The unique feature about anionic polymerization of diene to produce homopolymer was that the microstructure of the homopolymer could be altered and changed at will to produce unique physical and chemical properties. These microstructural changes can be introduced before, after or during the polymerization. For example, chelating diamines, such as tetramethyl ethylene and diamine (TMEDA) (18), with the alkyl-lithium catalyst have been used to produce polymer with 80 1,2 addition products, while the use of dipiperidine ethane (DPE),with same catalyst has produced polybutadiene with 100 1,2 addition product. 

IV.3. Homopolymers with Different Chain Microstructures. 213 IV.4. Graft Copolymers. . 219... 

For the adsorbed state of macromolecules it has been speculated that the polymer-adsorbent interactions would be concerned not only with the overall chemical constitution but also the monomer arrangement along the chain, as described in Section IV. 1. This suggests that some homopolymers may be distinguished with TLC from one another by a difference in chain microstructure, such as steric and geometrical isomerism, and stereoregularity. This section deals with this possibility, divided into... 

One can have the same type of situation in a blend of two mutually immiscible polymers (e.g., polymethylbutene [PMB], polyethylbutene [PEB]). When mixed, such homopolymers form coarse blends that are nonequilibrium structures (i.e., only kinetically stable, although the time scale for phase separation is extremely large). If we add the corresponding (PEB-PMB) diblock copolymer (i.e., a polymer that has a chain of PEB attached to a chain of PMB) to the mixture, we can produce a rich variety of microstructures of colloidal dimensions. Theoretical predictions show that cylindrical, lamellar, and bicontinuous microstructures can be achieved by manipulating the molecular architecture of block copolymer additives. 

FIG. 1.7 Some of the microstructures produced by the self-association behavior of diblock copolymer solutions. The figure illustrates the (a) spherical, (b) cylindrical, and (c) lamellar structures (among others) that are possible in such solutions. Each diblock polymer chain consists of strings of white beads (representing one type of homopolymer) and strings of black beads (representing the second type of homopolymer). (Redrawn from A. Yu. Grosberg and A. Khokhlov, Statistical Physics of Macromolecules, AIP Press, New York, 1994.)... 

Conformational energies are calculated for chain segments in poly(vlnyl bromide) (PVB) homopolymer and the copolymers of vinyl bromide (VBS and ethylene (E), PEVB. Semlempirical potential functions are used to account for the nonbonded van der Waals and electrostatic Interactions. RIS models are developed for PVB and PEVB from the calculated conformational energies. Dimensions and dipole moments are calculated for PVB and PEVB using their RIS models, where the effects of stereosequence and comonomer sequence are explicitly considered. It is concluded from the calculated dimensions and dipole moments that the dipole moments are most sensitive to the microstructure of PVB homopolymers and PEVB copolymers and may provide an experimental means for their structural characterization. 

Fig. 6.7 Domain spacing of blends of a PS-PI diblock with PS homopolymer relative to that for the pure diblock (du = 26.7 nm) (Hashimoto et al. 1990). The diblock has M = 31.6kgmol, and 48wt% PS, the homopolymers have (o) M = 2.3kgmor , (A) Mn = 4.4 kg mol 1, ( ) M = 10.2 kg mol 1, ( ) Mn = 16.7 kg mol 1. Experiments were performed at room temperature. The symbols L, C and S denote lamellar, cylindrical and spherical microstructures.

Fig. 6.10 Phase diagrams for blends of PS homopolymers with PS-PI diblocks of approximately constant molecular weight (average M = 54.3 kgmol"L), annealed at 125 C (Winey et al. 1992ft). (a) Mn (PS) = 5.9kgmor (b) M (PS) = 14kgmol-1, (c) M (PS) = SVkgmor1. Here L, C and S denote lamellar, cylindrical and spherical microstructures respectively, DM indicates disordered micelles, BC a bicontinuous cubic structure and 2

The effect of adding homopolymer to ordered block copolymer melts was interpreted by considering how homopolymer is distributed within the microstructure (Matsen 19956). Within the A-rich domains of a microslructure, the A blocks tend to segregate to the interfaces, whereas homopolymer A is preferentially located in the domain centre due to tension in the A blocks. This is countered by the entropy of mixing which favours a more uniform distribution of homopolymer. To allow homopolymer near the interface, a phase transition may occur to a microstructure where the interface is less curved towards the A-rich domains (this is the reverse of the process shown in Fig. 6.9). Even without such a phase transition, increasing homopolymer content leads to an increasing... 

Homopolymers of a-olefins such as propylene exhibit unusual properties compared with their normal homopolymers. A homopolypropylene would usually have about 1000 methyl groups per 1000 methylene groups. Polypropylenes made using these catalysts typically have about half that many methyl groups, and in addition have some longer chain branches. Other a-olefins often field polymers whose microstructure is analogous to these polypropylenes when the above catalysts are used for the polymerisation. 

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],... 

Difficulties in the extraction may be chiefly attributed to the microstructure of the sample to be extracted, because the efficiency of extraction may depend on whether the polymer molecule to be extracted is extented or collapsed in the outer component phase15. In the experiment shewn in Fig. 10, an extraction of PS from the acetylated sample is attempted with cyclohexane at 50 °C, after the PVAc homopolymer is removed. However, the PS homopolymer can hardly be extracted, as shown by curve 4. Since this sample is recovered by pouring the acetylation mixture i.e., the pyridine-acetic anhydride solution) into cold water, it may have a microstructure as the PS chain is collapsed, being occluded in the continuous phase of the extended PVAc. Therefore, to inyert the microstructure of the sample it should be dissolved in tetrahydrofaran again and recovered by pouring the solution into n-hexane, whose non-solvency is much stronger for PVAc than PS (at B in Fig. 10). Extraction is then carried out with cyclohexane at 50 °C. The result is shown by curve 4h in Fig. 10. It is clear that the extended PS homopolymer can be removed further by this extraction, as expected. 

Thus for the successful remoral of homopolymers it is necessary to take into consideration the microstructure. Alteration of the microstructure by the solution-precipitation may additionally destroy the crystallite or the intimate entanglement among polymer molecules, which makes the extraction difficult. 

In both series of copolymers, the methoxyl resonance is surprisingly complex and appears to offer the best promise of yielding fundamental information concerning the microstructure of the chains. In methyl methacrylate homopolymers, the methoxyl protons appear as a single narrow peak at 6.40t regardless of the stereochemical configuration of the chains. In the copolymers, the methoxyl resonance is evidently very, sensitive to the configuration and proximity of the... 

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