Poly vinyl ether Backbone

The dataset was developed with the characterization of the "incremental differences" between the Tg s of structurally related polymers used as a foremost consideration. Addition of successive backbone methylene units in the linear polyoxides, and addition of successive methylene units in side groups as in the poly(vinyl ethers), are examples of incremental structural changes. Such incremental changes usually cause incremental (gradual) differences... 

Since the effect of the nature of the polymer backbone has not been studied extensively using well-defined polymers, it is worth summarizing the observations from less controlled systems based on comparisons of poly(phosphazene), poly(methylsiloxane), poly(vinyl ether), poly(acrylate), poly(me-thacrylate), poly(chloroacrylate), and polystyrene backbones [233-241]. In general, the mesogen is more decoupled from the polymer backbone as the latter s flexibility increases. This is because the dynamics of ordering increase with increasing flexibility, which increases the ability of the mesogenic side-chains to form more ordered... 

Secondary structural variables, including the nature of the polymer backbone, will only be elucidated by synthesizing complete and homologous series of well-defined polymers. As discussed previously, only a few of the poly(vinyl ether)s, polynorbor-nenes and polymethacrylates contain both identical mesogens and spacers. These systems confirm the above trends. For example, when (4 -n-butoxyphenoxycarbo-nyl)phenoxy mesogens are attached to polymethacrylate and poly(vinyl ether) back-... 

Comparison of the phase diagrams plotted in Fig. 14 of poly(5- [ -[4 -4"-cyano-phenyl)phenoxy]alkyl]carbonyl]bicy-clo[2.2.1]hept-2-ene]s [189] and poly(n-[(4 -(4"-cyanophenyl)phenoxy)alkyl]vinyl ethers [122-127, 212, 213] which contain a single mesogen per repeat unit demonstrates that the glass transition temperature decreases as the flexibility of the polymer backbone increases from polynorbornene to poly(vinyl ether), whereas the isotropiza-tion temperature increases. In addition to revealing additional mesophases at lower temperatures, this increase in polymer flexibility enables the poly(vinyl ether)s to form more ordered mesophases. That is, poly(5- [ -[4 -(4"-cyanophenyl)phenoxy]al-kyl]carbonyl ]bicyclo[2.2.1 ]-hept-2-ene ]... 

The data in Table 10 demonstrates that for a constant spacer length and mesophase, both the change in enthalpy and entropy of isotropization decrease as the flexibility of the polymer backbone increases from poly-norbornene to poly (vinyl ether). However, the change in entropy decreases more rapidly than the change in enthalpy, and the isotropization temperature (T = AH /AS ) therefore increases with increasing flexibility. Since lower entropies of fusion are associated with more rigid structures, the lower entropy of isotropization of poly(vinyl ether)s is obviously not due to a lack of inherent flexibility of its polymer backbone, but rather to the more flexible backbone being more ordered and therefore more... 

We have take the raesogen density into account by multiplying Mi, and AS by 0.5 for the poly(vinyl ethers) (1 mesogenic side chain per 2 atoms in the polymer backbone), 0.4 for the disubsituted polynorbomenes (2 side chains/... 

Scheme 10.6 Synthetic scheme of PS molecular brushes by grafting living PS anions to the poly(chloroethyl vinyl ether) backbone [70]. (Reproduced with permission of the American Chemical Society.)...

Semifluorinated segments with varying number of alkyl and perfluoroalkyl groups have been used as substituents on many different polymeric backbones over the years, among them polyesters [29,30], cross-linkable acrylics [31], polyurethanes [32], polyglutamates [33], poly(vinyl ether)s [34], polystyrene [35], as well as polyacrylates [36], polymethacrylates [37], and i/poly(styrene-flft-maleimide) copolymers [38]. The main difference between these polymers is the flexibility and dynamics of the polymer backbone. 

Aldol group transfer polymerization of ferf-butyldimethylsilyl vinyl ether [62] was initiated by pendant aldehyde functions incorporated along a poly(methyl methacrylate) (PMMA) backbone [63]. This backbone was a random copolymer prepared by group transfer polymerization of methyl methacrylate (MMA) and acetal protected 5-methacryloxy valeraldehyde. After deprotection of the aldehyde initiating group, polymerization proceeded by activation with zinc halide in THF at room temperature. The reaction led to a graft copolymer with PMMA backbone and poly(silyl vinyl ether) or, upon hydrolysis of the ferf-butyldimethylsilyl groups, poly(vinyl alcohol) branches. 

As pointed out by Heller (2), polymer erosion can be controlled by the following three types of mechanisms (1) water-soluble polymers insolubilized by hydrolytically unstable cross-links (2) water-insoluble polymers solubilized by hydrolysis, ionization, or protonation of pendant groups (3) hydrophobic polymers solubilized by backbone cleavage to small water soluble molecules. These mechanisms represent extreme cases the actual erosion may occur by a combination of mechanisms. In addition to poly (lactic acid), poly (glycolic acid), and lactic/glycolic acid copolymers, other commonly used bioerodible/biodegradable polymers include polyorthoesters, polycaprolactone, polyaminoacids, polyanhydrides, and half esters of methyl vinyl ether-maleic anhydride copolymers (3). 

Similarly to small molecules, polymers can be classified based on their chemical nature, i.e. based on the functional groups present in their molecule. However, different from small molecules, one important element in polymer classification is the chemical structure of the polymeric backbone. The attached side atoms or groups of atoms to the polymer backbone play a different role compared to that of the presence of various atom types or groups of atoms in the backbone. For example, it is a significant difference between poly(oxy-1,4-phenylene-oxy-ethylene) that contains phenyl groups and oxygen atoms in the polymer backbone and poly(phenyl vinyl ether) that is a vinyl type polymer with a carbon chain as backbone, although both polymers are ethers. The two polymers are synthesized differently and have quite different properties. Their structures are shown below ... 

Acetoxylation of poly(vinyl chloride) can be carried out under homogeneous conditions. Crown ethers, like 18-crown-6, solubilize potassium acetate in mixtures of benzene, tetrahydrofuran, and methyl alcohol to generate unsolvated, strongly nucleophilic naked acetate anions. These react readily with the polymer under mild conditions. Substitutions of the chlorine atoms on the polymeric backbones by anionic species take place by a Sn2 mechanism. The reactions can also proceed by a Sivl mechanism. That, however, requires formations of cationic centers on the backbones in the rate-determining step and substitutions are in competition with elimination reactions. It is conceivable that anionic species may (depending upon basicity) also facilitate... 

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