Initial additional agents

Features of the method The solutions and prodecures are uncomplicated, and can often be adapted for barrel plating. Addition agents are usually necessary in order to restrict the high initial rate of plating, which might... 

The very small number of growing polymer chains, when compared to the monomer concentration results in a very low overall concentration of free control agent and leads to inefficient capping of chain ends. One solution to this problem is the addition of a free or unbound control agent to the polymerization medium. This can take the form of a low molecular weight alkoxyamine, ATRP initiator, RAFT agent or, alternatively, free deactivator such as nitroxide or Cu(II). This species is often called a sacrificial agent. This solution also leads to the formation of free polymer that must ultimately be removed from the brush. 

Attack on Nitrogen. A variety of cyclic derivatives of phosphorous acid have been converted into spirophosphoranes (51), using diethyl azodicarboxylate as the condensing agent,42 probably by initial addition to nitrogen to give (50). Several... 

The precise structure of the initiating agent and initial addition to the monomer varies according to the reaction conditions, monomer, and initiator. For illustration, we will look at the products formed from BPO. Generally, BPO is reported to break forming two benzoyl free radicals (Equation 6.3). In reality, the initial step is complex and involves the formation of a number of products as shown in Figure 6.1 for styrene. The major reaction involves direct... 

Initiators/initiation Effect is irreversible Only one exposure may suffice Multiple exposures may be additive Cannot identify initiated cells Agents are considered carcinogens Agents are usually mutagenic No measurable threshold dose Must be administered before the promoter Does not result in neoplasia unless promoter is subsequently applied... 

The discontinuation of pyrimethamine-sulfadiazine after successful initial therapy may be considered in persons who have a sustained CD4 cell count of greater than 200 cells/mcL for at least 6 months and have completed initial therapy and are asymptomatic. A regimen of sulfadiazine 500 to 1000 mg orally 4 times daily plus pyrimethamine 25 to 50 mg orally and leucovorin 10 to 25 mg orally once daily is recommended for maintenance therapy. Pyrimethamine plus clindamycin also can be considered for maintenance therapy, but this regimen may be associated with a higher relapse rate. A further advantage to the combination of pyrimethamine plus sulfadiazine is that an additional agent for prophylaxis of PCP is unnecessary, which is not the case with the pyrimethamine plus clindamycin combination. ... 

One of the most common technologies for the synthesis of polymer polyols by a radical mechanism is based on the stepwise addition of a mixture of vinylic monomers (polyether polyol, initiator, transfer agent (mixture I)) to a second mixture (mixture II) of polyether polyol (identical with the polyether used for mixture I) and NAD (macromer or nonreactive NAD) under a nitrogen protective atmosphere, in the polymerisation reactor at 115-... 

However, with some metal ions, the mechanism may involve initial addition of HO to the metal. For example, the initial product with Cu " is known to be [CuOH]. In other instances, the reaction proceeds via initial addition to a ligand, followed by reaction with the metal center. For example, Ru(bpy)3 reacts via addition of HO to an aromatic ring. The metal center then acts as an internal reducing agent, yielding HO and the oxidized complex. 

After the addition step is complete, the mercury is usually replaced by hydrogen, using a reducing agent. The net result is the addition of hydrogen and the nucleophile to the alkene, and the reaction is known as oxymercuration reduction. The regioselectivity is in the same sense as is observed for proton-initiated additions. ... 

Hydroxylation of phenol by hydrogen peroxide over solid acids exhibits an autocatalysis that has never been described in earlier works. The induction period is dependent on the acidity and is reduced by initial addition of dihydroxybenzenes or other electron-transfer agents. A new mechanism, initiated by the slow formation of dihydroxybenzenes in the induction period, should be considered. Comparison of various catalysts shows that the reaction is also dependent on the structure of the solid. Zeolites with too small a porosity are not active, according to a large space demand of the reaction. Catalysis by titanium silicalites does not show such behaviour the reactivity is low but regular. Thus, our results show that valuable comparison between catalysts cannot be deduced from tests performed by stopping the reaction at a determined time, but that kinetic studies are essential. 

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