Slow addition polymerization

Optimal yields were obtained by slow addition of the alkene substrates to a solution of the ruthenium vinylalkylidene and this allowed just two equivalents of the acyclic alkene to be used without significant formation of polymeric products. Unlike the acyclic cross-metathesis reactions, which generally favour the formation of tram products, the above ring-opening metathesis reactions yielded products in which the cis stereoisomer is predominant. Particularly noteworthy was the absence of significant amounts of products of type 31, formed from metathesis of one cyclic and two acyclic alkenes. In fact, considering the number of possible ring-opened products that could have been formed, these reactions showed remarkable selectivity (GC yields > 80%). 

Since monomer coordination is required for polymerization, the gel modification additive can also slow the polymerization reaction by competing with the monomer for coordination sites on the metal center. 

The tin hydride method suffers from one major disadvantage, namely the efficiency of the reagent as a hydrogen atom donor. For successful synthesis, alkenes have to be reactive enough, otherwise direct reduction of the starting precursor becomes a considerable side reaction. In practice, the yields are increased by slow addition of a solution of tin hydride and a radical initiator into the reaction mixture containing an excess of alkene. However, a delicate balance must be maintained. If a large excess of olefin is used, polymerization can compete. 2,2-Azobisisobutyronitrile is the most commonly employed initiator, with a half-life time for unimolecular scission of 1 h at 80°C. 

In 1994, Rubinsztajn reported the hyperbranched hydrosilylation polymerization of the AB3 carbosiloxane monomers 9 and 10181. Since intramolecular cyclization of these monomers would lead to more highly strained five-membered rings, it was expected that intermolecular reaction would be preferred. This was indeed the case, and molecular weights were highest when polymerizations were performed in bulk. Furthermore, slow addition of monomer resulted in polymers of much higher molecular weight. Functionalization of the polymers could be performed via hydrosilylation reactions, although these... 

In contrast to the slow step-reaction polymerizations, chain-reaction polymerizations are fairly rapid.14 Chain-reaction polymerizations (often referred to as addition polymerizations) require the presence of an initiator for polymerization to occur. Initiation can occur by a free radical, an anionic, or a cationic species, which open the double bond of a vinyl monomer and the reaction proceeds as shown in Fig. 15.11 where may be a radical,... 

The addition of small amounts of radical scavenger (such as benzoquinone and diphenylpicrylhydrazyl) led to the appearance of induction periods in the kinetic curves. The duration of the induction periods are proportional to the concentration of the radical scavenger. The presence of atmospheric oxygen slightly slowed the polymerization. These observations indicate that the polymerization proceeds by a radical mechanism. The radicals are formed from the y-radiolysis of the monomers. By comparison to the ESR spectrum of the radicals formed by thermal initiation with azobisisobutyroni-trile in the presence of a spin trap, the radical formed is... 

In this sequence of reactions, it is the monomer that forms oxonium ion [thus Activated Monomer (AM) mechanism] and the growing chain end is neutral. As shown in the series of papers [107-115], if the conditions are created, when AM mechanism predominates, by keeping the low instantaneous ratio of [monomer]/[HO-] (slow addition of monomer to reaction mixture), back-biting is effectively eliminated. Linear polymers, free of cyclic fraction are obtained under these conditions. The mechanism and kinetics of AM polymerization of oxiranes is discussed in detail in recent monograph [6]. 

A procedure has recently been developed for the regioselective polymerization of an 3-(4-octylphenyl)thiophene with FeCl3 [253]. The slow addition of a slurry of FeCl3 in CHC13 to a solution of the monomer resulted in the formation of a polymer with 94% head-to-tail linkages. The slow addition of FeCl3 keeps the ratio of Fe3 + to Fe2 + low during the... 

That s our quick and dirty look at step-growth polymerization. The crucial feature of this type of reaction is the slow build-up of chains in a step-wise process. Now let s take a closer look at addition polymerization. 

We have seen that the rate of step-growth or condensation polymerization is relatively slow and macromolecules are only produced at high degrees of conversion. In contrast, chain or addition polymerizations occur rapidly and polymer is produced in the initial stages of the reaction. Instead of having monomers going to oligomers and then to polymers, with essentially all the molecules... 

Random hydrocarbon copolymers can also be produced by this new equilibrium polymerization method. Copolymers containing octenylene and butenylene linkages in a statistical array based on feed ratio result from the cocondensation of the two respective monomers or by the reaction of diene with unsaturated polymer. More controlled polymer stmctures have also been prepared by the slow addition of a diene solution to an unsaturated polymer containing active catalyst. Substituent effects were shown to dictate the polymerizability of monomers and in some cases selective polymerization of speciflc aUcenes in the monomer resulted in what appears as perfectly alternating copolymers. ... 

Synthesis of Siloxane-Polyimide Elastoplastics. In a typical polymerization, a 5-L, three-neck, round-bottom flask equipped with an overhead mechanical stirrer, a Dean-Stark trap with condenser and a nitrogen inlet, and a thermometer was charged with 484.00 g (0.2406 mol) of D2o-DiSiAn, 41.61 g (0.431 mol) of mPD, 19.52 g (3 wt %) of 2-hydroxypyridine, and 2 L of o-dichlorobenzene. The mixture was warmed to 100 °C for 1 h to dissolve the monomers and the catalyst. The polyamic acids precipitated and then redissolved when the mixture was warmed to 150 °C for 2 h. To the oligomer solution was added 99.13 g of BPADA dissolved in 200 mL of o-dichlorobenzene. The mixture was maintained at 150 °C for an additional 2-h period to ensure incorporation of the dianhydride and then warmed to reflux. After approximately 100 mL of a solvent-water mixture had been removed, the solution was maintained at 180 °C for 40 h. The mixture was cooled to room temperature and diluted with 1 L of methylene chloride. Polymer was isolated from the solution by a slow addition of the polymer solution to 4 L of methanol. The resulting slurry was filtered, and the polymer was redissolved in 4 L of methylene chloride, extracted three times with 2 N aqueous HCl to remove catalyst, washed with water, dried with magnesium sulfate, reprecipitated into methanol as before, filtered, and dried in vacuo at 100 °C to obtain 522 g (85%) of a rubbery material with an IV of 0.50 dL/g. IR, NMR, and Si NMR spectroscopic analysis indicated the absence of amic acid functionalities that could be present if imidization is incomplete. 

For a detailed investigation of the individual steps of the polymerization reaction, including the different intermediates of both the initiation reaction (formation of the dimer) as well as the subsequent addition polymerization reactions (formation of the trimer, tetramer,...), it is necessary to slow down or even stop the reaction by cooling the crystals. Consequently the concept of the following spectroscopic investigations is to photochemically initiate the polymerization reaction at extremely low temperatures (10 K) and to investigate the structure and kinetics of the intermediates obtained in subsequent photochemical or thermal reaction steps. The first low temperature experiments have been performed by Hori and Kispert using X-irradiation and by Bubeck et al. and Hersel et al. .ss) uV-irradiation. 

Whenever the initiation of polymerization is fast and termination is eliminated, monodispersed polymers may be formed by slow addition of monomer to a well-stirred solution of low molecular weight living polymers. This technique, suggested by the author, was developed by McCormick and Brewer (12), By water and Worsfold (17), Wenger (27), and others. Polymers of the narrowest molecular weight distribution were actually produced by this method. 

Which polymer is formed depends upon the relative rates of subsequent reactions. If chain termination then occurs with loss of X, a cyclic dimer is produced if a third isocyanate molecule is added, followed by loss of X, a cyclic trimer occurs if chain termination is relatively slow, addition of further monomers takes place with formation of a linear polymer. Conditions such as temperature, catalyst concentration, and character contribute to the reaction pattern. The tendency to cyclize no doubt plays a specially large part in isocyanate polymerization. 

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