Condensation polymers self-condensing

For these reasons, this variation is not as widely used as the anionic reaction (the aldol condensation). The base catalyzed reaction often leads to dimers, polymers, self-condensation products or a, 5-unsaturated carbonyl derivatives, as described in Section 9.4.A. Mukaiyama and co-workers modified the acid-catalyzed reaction to include silyl enol ethers. He found that they react with carbonyl compounds to produce aldol-like... 

The aldol reaction has long been recognized as one of the most useful synthetic tools. Under classical aldol reaction conditions, in vhich basic media are usually employed, dimers, polymers, self-condensation products, or a,j5-unsaturated carbonyl compounds are invariably formed as byproducts. The lithium enolate-mediated aldol reaction is regarded as one useful synthetic means of solving these problems. Besides the vell-studied aldol reaction based on lithium enolates, very versatile regio- and stereoselective carbon-carbon bond forming aldol-type reactions have been established in our laboratory by use of boron enolates (1971), silicon enolates-Le vis acids (1973), and tin(II) enolates (1982). Here we describe the first t vo topics, boron and silicon enolate-mediated crossed aldol reactions, in sequence. 

YAN D Y, Hou J, ZHU X, KOSMAN J J and wu H s (2000), A new approach to control crystallinity of resulting polymers self-condensing ring opening polymerization ,... 

Liquid crystal polyesters are made by a different route. Because they are phenoHc esters, they cannot be made by direct ester exchange between a diphenol and a lower dialkyl ester due to unfavorable reactivities. The usual method is the so-called reverse ester exchange or acidolysis reaction (96) where the phenoHc hydroxyl groups are acylated with a lower aHphatic acid anhydride, eg, acetic or propionic anhydride, and the acetate or propionate ester is heated with an aromatic dicarboxyHc acid, sometimes in the presence of a catalyst. The phenoHc polyester forms readily as the volatile lower acid distills from the reaction mixture. Many Hquid crystal polymers are derived formally from hydroxyacids (97,98) and thein acetates readily undergo self-condensation in the melt, stoichiometric balance being automatically obtained. 

The main benefit of the Dow process was control of the polymer architecture. The polymer from the self-condensation process possessed a linear stmcture, but there were other difficulties. The monomer was cosdy and removal of the cuprous bromide by-product was difficult (17) ultimately, scale-up difficulties terrninated the Dow PPS development. However, there was a growing recognition that PPS was an attractive polymer with an excellent combination of properties. 

Unfortunately, because self-condensation of silanols on the same silicone can occur almost spontaneously, the reaction of disdanol or trisilanol compounds with telechelic sdanol polymers to form a three-dimensional network is not feasible. Instead, the telechelic polymers react with cross-linkers containing reactive groups such as alkoxysdanes, acyloxysdanes, silicon hydrides, or methylethyloximesilanes, as in the reactions in equations 18—21 (155). 

In this case the Ar and/or Ar group(s) will contain an ether oxygen. As before if Ar = A then basically identical products may be made by the two polysulphonylation routes (see e.g. Figure 21.4). Where a diether and a disulphonyl chloride are co-condensed there may be some reaction at o- and m-positions in addition to the desired reaction at the para-position. It has been found that o- and m-linked polymers are more brittle, at least in the above example. It is of interest to note that self-condensation of the monosulphonyl chloride of diphenyl ether, on the other hand, can proceed so as to give a virtually all-para and hence tough material. Whilst the self-condensation polysulphonylation route has been used commercially it is understood not to be in current commercial use. 

Self-condensation of hydroxypivalic acid (also known as 3-hydroxy-a,a-dimethylpropionic acid and as 2-hydroxy-1,1-dimethylpropionic acid) only yields low molecular weight polymers and it is more convenient to prepare the polymer from pivalolactone (also known as a,a-dimethyl-[3-propiolactone) using tributylphosphine as an initiator Figure 25.28). 

Interesting products may also be produced by introducing boron atoms into the chain. The amount of boron used is usualy small (B Si 1 500 to 1 200) but its presence increases the self-adhesive tack of the rubber, which is desirable where hand-building operations are involved. The products may be obtained by condensing dialkylpolysiloxanes end-blocked with silanol groups with boric acid, or by reacting ethoxyl end-blocked polymers with boron triacetate. 

Nylon 11 is produced hy the condensation reaction of 11- aminounde-canoic acid. This is an example of the self condensation of an amino acid where only one monomer is used. The monomer is first suspended in water, then heated to melt the monomer and to start the reaction. Water is continuously removed to drive the equilibrium to the right. The polymer is finally withdrawn for storage ... 

It is also possible to prepare them from amino acids by the self-condensation reaction (3.12). The PAs (AABB) can be prepared from diamines and diacids by hydrolytic polymerization [see (3.12)]. The polyamides can also be prepared from other starting materials, such as esters, acid chlorides, isocyanates, silylated amines, and nitrils. The reactive acid chlorides are employed in the synthesis of wholly aromatic polyamides, such as poly(p-phenyleneterephthalamide) in (3.4). The molecular weight distribution (Mw/Mn) of these polymers follows the classical theory of molecular weight distribution and is nearly always in the region of 2. In some cases, such as PA-6,6, chain branching can take place and then the Mw/Mn ratio is higher. 

During the aqueous hydrolysis of dichlorosilanes there is always a very important side reaction. It is the self-condensation of silanols which are formed initially during the hydrolysis. These reactions also give rise to the formation of cyclic siloxanes together with the linear oligomers or polymers (Reaction Scheme III). The amount of cyclic products usually depends on the hydrolysis conditions and the degree of the self-condensation attained as well as concentration considerations. 

The initial step of the polymerisation process is reaction of the amine groups with formaldehyde to generate methylol units, as illustrated in Reaction 1.9. Further heating of the polymer then leads to a variety of reactions. For example, the methylol groups can undergo self-condensation (Reaction 1.10). 

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 theories of Miller and Macosko are used to derive expressions for pre-gel and post-gel properties of a crosslinking mixture when two crosslinking reactions occur. The mixture consists of a polymer and a crosslinker, each with reactive functional groups. Both the polymer and crosslinker can be either collections of oligomeric species or random copolymers with arbitrary ratios of M /Mj. The two independent crosslinking reactions are the condensation of a functional group on the polymer with one on the crosslinker, and the self-condensation of functional groups on the crosslinker. 

The process is repeated for the "crosslinker" component (lines 1180-1950). From the calculated expectation values, the program determines the ratio of A to B groups (line 1960) and the gel point in the absence of B group self-condensation (percent reaction of A groups at gel, line 1990). The various expectation values along with the gel point and the ratio of crosslinker to polymer functionality (1/R), are printed at the terminal (lines 2000-2220). 

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