Polymers with completely condensed

Example 13.3 The conversion of a self-condensing reaction can be limited to give polymers with finite lengths. How does the polydispersity of these polymers compare with those in Example 13.2 where the reaction went to completion with imperfect stoichiometry Make the comparison at the same average chain length. 

The experiments were carried out in an inert medium with liquid condensation agents at 18° 1°C results are presented in Table I. Among the polymers shown in that table, only polyethylene terephthalate was systematically studied. Some informative results, yet to be completed, were collected with the other polymers. 

Homopolymerization. In the simplest type of step growth, a bifunctional monomer reacts successively with itself, eventually forming a polymer with a large number of repeating units. The reaction may be an addition, but more commonly is a condensation. Although condensation usually is reversible, its equilibrium is driven toward complete conversion by removal of the small and volatile cast-off molecule ... 

Condensation reactions under acid catalysed conditions are much slower than hydrolysis reactions and generally start when the hydrolysis process is almost complete. The largest differences in reaction rate constants for hydrolysis and condensation are reported for pH = 0.9 and these differences decrease if the pH is increased [56]. As a consequence a large amount of hydrolysed species is present at the moment condensation becomes significant. Further condensation reactions then take place between individual hydrolysed species (clusters) and lead to aggregated clusters. This is schematically represented for a simple case in Fig. 8.22 where dimers react with each other leading to a linear molecule. Further condensation reactions with other condensed polymers will take place preferentially at the end groups [54]. 

Equation (P5.22.3) indicates that the distribution becomes sharper as f increases. The case of / = 1 corresponds to Type 3 stoichiometric i al ce in linear polymerization (see page 346). For / = 1, Eq. (P5.22.3) gives Xu,/X = 2, as would be expected for most probable distribution in linear polymerization. For f = 2, which represents the linear polymer prepared by condensing to completion an A—B monomer with a small amount of A—A (e.g., a dibasic acid), Xuj/Xn = 1-5. Thus, linear polymerization of A—B with small amounts of A—A leads to significantly narrower distribution than A—B alone or A—A plus B —B. 

The importance of substituent identity with respect to thermal life of a number of completely condensed polymers of alkyl- or aryltrichlorosilane has been accomplished by Brown (24) as shown in Table III. 

The acid functionalized copolymer (1.00 g) was dissolved in 25 ml of toluene in a three neck, 100ml flask equipped with condenser, nitrogen bubbler, and a dropping funnel. 25 ml of ethyl alcohol was added, followed by 1 ml of concentrated sulfuric acid. The solution was heated in an oil bath at 85°C for six hours under nitrogen. The product was isolated by adding the reaction mixture to 200 ml of water. The product was washed with water three times, then with acetone, then was dried under vacuum. The ester-functionalized polymer was completely soluble in solvents such as THF, chloroform, and toluene. 

The stability and size of the polyplexes formed between cationic polymers and DNA have also been correlated to primary amine content [10, 129], Low branched PEI (low density of primary amines) requires higher N/P rations to complete condensation compared with highly branched derivatives (high density of primary amines) [130]. Last, complex size tends to decrease with an increasing polymer-to-DNA ratio [10,121,131],... 

Cationic polymers such as poly (amino acids) have also been used in combination with liposomes. DNA is initially complexed with polylysine (PLL) at low charge ratios and cationic lipids are subsequently added to completely condense the DNA. Alternatively, the PLL condensed DNA containing a net positive charge can subsequently be complexed with an anionic lipid [168]. Precondensation with polylysine has been shown to reduce serum inhibition and to enhance the transfection efficiency [186,187]. 

A limited number of reports have appeared in the literature showing the use of water as a solvent for the microwave-promoted synthesis of thermoplastics. An example is the synthesis of water-borne polyimides using the standard polycondensation reaction of a dianhydride with a diamine (Seheme 3.1). In a scientific microwave unit, polymers with high molecular weights (Af up to 35.460 g/mol) were obtained within 22 min of heating using a one-pot two-step procedure. The dianhydride was first hydrolyzed in water to obtain the corresponding tetracarboxylic acid. This was then condensed with the diamine. The obtained polymers were completely comparable in their chemical and thermal properties to those obtained by conventional polymerization in m-cresol as solvent. 

Vinyl acetal polymers are made by hydrolyzing poly(vinyl acetate) [9003-20-7] to poly(vinyl alcohol) [9002-89-5] and the reaction of the latter with an aldehyde in the presence of an acid catalyst. These two reactions, hydrolysis and acetahzation, can he conducted either sequentially or concurrently (1). The acetahzation reaction, shown in Figure 1, strongly favors complete condensation of one molecule of aldehyde with the 1,3-glycol of two vinyl alcohol imits of poly(vinyl alcohol) to form the 1,3-dioxane ring of one vinyl acetal imit. 

Similarly, vinyl acetal polymers are manufactured by the reaction of aldehydes with poly(vinyl alcohol) in the presence of an acid catalyst. Reaction of the aldehyde with an alcohol, in this instance the polymeric alcohol poly(vinyl alcohol), yields the unstable hemiacetal. Complete condensation with loss of water is strongly favored to form the cyclic acetal. Poly(vinyl alcohol) used to manufacture vinyl acetal polymers is produced by hydrolysis of poly(vinyl acetate) homopolymer with methanol or ethanol. Both acids and bases catalyze the reaction, but base catalysis is usually done with methanol (14). The catalyst is either acid or base, depending on the process. Poly(vinyl acetate) is polymerized from vinyl... 

At mixing charge ratio N P of 1 [190], the polydispersity of the complexes of PEG272- -PLLy 19 4g (Fig. 17a) with pDNA decreased dramatically before increasing again, whereas nearly complete condensation seemed to occur at N P of 2 1 (EtBr exclusion assay), where the diameter of all the polyplexes was inferior to 100 nm. Note that condensation of a linearized pDNA by the same polymer was effective at lower N P ratio. The explanation given by the authors is that native pDNA is in a super-coiled circular form, which certainly has a higher molecular... 

---