Polymerization uncontrolled

Bz s-imidoesters like DMS may be used to couple proteins to PE-containing liposomes by crosslinking with the amines on both molecules (Figure 22.24). However, single-step crosslinking procedures using homobifunctional reagents are particularly subject to uncontrollable polymerization of protein in solution. Polymerization is possible because the procedure is done with the liposomes, protein, and crosslinker all in solution at the same time. 

CHEMICAL Ability to self-polymerize Uncontrolled polymerization... 

Air—As mentioned later in Section 2.2.4.2, many substances are capable of forming peroxides in contact with the oxygen in air. Inadvertent oxidation reactions cause quality loss, potential self-heating, and possibly ignition with a resultant fire or explosion. Also, oxygen can play a key role in the depletion of inhibitors in vinyl monomers resulting in uncontrolled polymerizations. 

Stronger Lewis acids such as SnCLi, TiCLt, and CH3AICI2 yield fast but uncontrolled polymerization with broad PDI. LCP of vinyl ethers can be achieved if the other components and reaction parameters are appropriately adjusted by various combinations of lower reaction temperature, added nucleophile, added common salt, and solvent prolarity. For example, polymerization of isobutyl vinyl ether using HC1 as the initiator (or one can use the preformed adduct of monomer and HC1) with SnCLt or TiCLj in CH2CI2 is non-LCP... 

Thus, the initiators are not only the iodide-type adduct 2 but also adducts of protonic acids (HB) that carry counteranions (B ) with suitable nucleophilicity namely these protonic acids should form adducts S quantitatively with vinyl ethers but should not initiate uncontrolled polymerization by themselves. Such protonic acids include hydrogen halides and carboxylic acids. 

The activators (coinitiators) are not only molecular iodine but also other mild Lewis acids (MtX ) that are mostly metal halides such as zinc halides. The suitable metal halides and other Lewis acids should be able to effectively activate the carbon-B linkage of the adducts 5 but they should not be too strongly Lewis acidic to initiate uncontrolled polymerization in the presence of adventitious protogens like water. 

As Lewis acid activators, weak metal halides such as zinc halides are effective [98,99] too-strong Lewis acids, such as SnCl4 and EtAlCl2, induce uncontrolled polymerization [104]. In some cases, the latter metal halides also initiate polymerization in the presence of a trace of water, which is unavoidable under usual experimental conditions (under dry nitrogen or argon atmosphere, via the syringe technique). Thus, zinc halides [98,99] are used most frequently, but tin(II) halides [99] and some acetyla-cetonate complexes [120] can also be effective. 

Under these reaction conditions, the use of ammonium and related onium salts with nucleophilic anions has been found effective at converting the HCl/SnCU-initiated, uncontrolled polymerizations into controlled/ living processes [105], Similar results are reported for TiCl4-based polymerizations [174,175], Effective salts include tetraalkylammonium and phosphonium salts R4N+Y and R4P+Y (Y = I, Br, Cl, CH3COO R = CH3, C2Hs, 71-C4H9, etc.). As added nucleophiles do in nonpolar solvents, the added salts retard the polymerization, narrow the MWD of the polymers, and render their M values directly proportional to conversion and close to the calculated values (one living chain per initiator molecule). 

Electron transfer to or from a conjugated tr-system can also induce pericyclic reactions leading to skeletal rearrangements. A typical example is the Diels-Alder cycloaddition occurring after radical-cation formation from either the diene or the dienophile [295-297], The radical cation formation is in most cases achieved via photochemically induced electron transfer to an acceptor. The main structural aspect is that the cycloaddition product (s Scheme 9) contains a smaller n-system which is less efficient in charge stabilization than the starting material. Also, the original radical cations can enter uncontrollable polymerization reactions next to the desired cycloaddition, which feature limits the preparative scope of radical-type cycloaddition. 

This reaction has been carried out many times without incident in a 1-1. stainless-steel rocker bomb. A pressure gauge was attached during two runs and recorded a maximum of 750 p.s.i. However, an attempt in another laboratory to scale up the reaction using a 3-1. autoclave resulted in a bulged vessel. Uncontrolled polymerization may be hazardous. 

The chemistry of 2-alkoxydihydro-3-//-pyrans such as the starting material of this problem is usually plagued by uncontrolled polymerizations, almost inevitable hydrolysis in the presence of traces of moisture, and so on. However, no matter how difficult these compounds are to handle, they are intriguing enough to deserve close attention. 

A nonaqueous workup procedure has been reproted for the preparation of arylboronic esters [ArBCOR a)]-" Uncontrollable polymerization or oxidation of much of the boronic acid occurred during the final stages of the isolation procedure, but could be avoided by in situ conversion to the dibutyl ester by adding the crude product to 1-butanol. The samarium(lll)-catalyzed hydroboration of olefins with catecholborane is a good synthesis of boronate esters." ... 

Tridentate nitrogen ligands form 1 1 uncharged complexes with copper, in contrast to bidentate ligands that form tetrahedral ionic complexes.81 Well-controlled polymers of styrene and MA are obtained with a substituted terpyridine (L-18), whereas the unsubstituted derivative induced a heterogeneous and uncontrolled polymerization.111 A similar controlled polymerization of styrene was reported for a substituted terpyridine-based complex of Cu(II) (Cu-5) coupled with Al(O i-Pr)3.112 A tridentate ligand with two pyridines and one amine (L-19) gave narrow MWDs for styrene, MA, and MMA (MJMn = 1.1 —... 

Merrifield resin) such as SL-2 and SL-3 also resulted in uncontrolled polymerizations of acrylates, methacrylates, and styrene. A better control can be achieved with the use of SL-4 for a mixture of CuBr2 and CuBr in the polymerization of MA, where the molecular weights became closer to the calculated values, and the MWDs became narrower (Mw/Mn = 1.6—2.0).145 The residual copper in the reaction mixture was estimated about 3% of the initial amount, which indicates that the metal was effectively bound to the resin. 

Preventing polymerization is the key to successful styrene storage. Special handling and storage procedures are required to maintain the styrene product quality and to avoid a potentially dangerous situation involving uncontrolled polymerization. 

It must be noted tliat, even with an extremely hydrophobic catalyst, catalyst-free particles would be formed by radical exit and nucleation of new particles, and thus uncontrolled polymerization would occur. On the other hand, it is also proposed that with more hydrophilic ligands (such as Meg-TREN, or pentamethyl diethylenetriamine (PMDETA) [221]) all the catalyst is located in the aqueous phase, and uncontrolled polymerization occurs in the hydrophobic polymer particles. However, these ligands are also ineffective in controlling the aqueous polymerization of water-soluble monomers (see above), hinting that ligand dissociation might also occur in these systems [215]. 

Liquid. Mild, sweet odor resembling that of chloroform. dj° 1.2129. mp -122.5. bpT40 31.7" ttjf 1.4249. Flash pt — 15. Practically insol in water. Sol in organic solvents. At temps above 0° and especially in the presence of oxygen or other suitable catalysts polymerizes to a plastic. Several inhibitors to preserve the monomer have been invented. Uncontrolled polymerization may lead to explosive reaction products with oxygen or ozone Reinhardt, CJtem. Eng,... 

Exploration of the template controlled free-radical oligomerization of other activated olefins began with standard monomers utilized in bulk polymer synthesis and the template 63. Vinyl acetate and acrylonitrile led only to uncontrolled polymerization, while vinylene carbonate did not react under the standard experimental conditions. More exotic monomers, such as vinyl trifluoroacetate and rert-butyl acrylate, were also unsuccessful. Only methyl acrylate polymerization was arrested by template 64 to provide the macrocyclized product 96 in modest yield as a mixture of five diastereomers (Scheme 8-25). Subsequent studies with the more effective thiophenyl-bearing template 63 at lower temperatures improved this yield to 35%. The diastereomer distribution was reminiscent of the methyl methacrylate-derived product, although no stereochemical assignments were made in this case either. 

A few minutes after cis-trans isomerization the 11-cw-retinal is recovered by an isomerase and recombined with a rhodopsin protein. The isomerization all-tram 11-cw is a dark reaction and occurs with activated retinol esters (Fig. 5.2.7). A nucleophilic group of an enzyme is reversibly added in a Michael reaction to Cll and the 5 kcal needed for the unfavorable isomerization comes from the cleavage of the activated ester. The resulting diene with the nucleophile added to the terminal allylic position is then hydrated and the nucleophile eliminated again. The trans-cis isomerization has thus been achieved in a controllable nucleophilic addition-elimination cycle, which is typical for biological reactions. The stereochemistry of the system is not disturbed. The other chemically plausible isomerization, namely via homolysis of the double bond and biradical formation, is not used in biological systems because it may lead to uncontrolled polymerization and side reactions with the protein (Rando, 1990). 

A second PIB precursor with = 32,000 was used for the synthesis of another series of triblock copolymers. However, solubility problems arose during the polymerization of MMA since the precursor precipitated in THF bielow -55 °C. The polymerization of MMA with the high molecular weight PIB macroinitiator resulted in polymers with multimodal MWD due to uncontrolled polymerization of MMA at... 

During a normal chemical reaction to create a particular polymer from a monomer, a catalyst is used to control the reaction. A catalyst is any substance that in a small amount noticeably affects the rate of a chemical reaction, without itself being consumed or undergoing a chemical change. For example, phosphoric acid is used as a catalyst in some polymerization reactions. Once an uncontrolled polymerization starts at an incident scene, it will not be stopped until it has completed its reaction, no matter what responders may try to do. If the polymerization occurs inside a tank, the tank may rupture violently. If a container of a monomer is exposed to fire, it is important to keep the container cool. Heat from an exposure fire may start the polymerization reaction. In Figure 5.18, the monomer vinyl chloride is shown along with the process of polymerization of the vinyl chloride molecules. 

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