Types of Initiators

The thermal, homolytic dissociation of initiators is the most widely used mode of generating radicals to initiate polymerization-for both commercial polymerizations and theoretical studies. Polymerizations initiated in this manner are often referred to as thermal initiated or 

Other peroxides used to initiate polymerization are acyl atkylsulfonyl peroxides (Va), dialkyl peroxydicarbonates (Vb), diperoxyketals (Vc), and ketone peroxides (Vd).  

Aside from the various peroxy compounds, the main other class of compound used extensively as initiators are the azo compounds. 2,2 -Azobisisobutyronitrile (AIBN) is the most important member of this class of initiators, although other azo compounds such as 

2 -azobis(2,4-dimethylpentanenitrile), 4,4 -azobis(4-cyanovaleric acid), and l,l -azobis(cy-lohexanecarbonitrile) are also used [Sheppard, 1985]. The facile dissociation of azo compounds is not due to the presence of a weak bond as is the case for the peroxy compounds. The C—N bond dissociation energy is high ( 290 kJ mol-1), hut the driving force for homolysis is the formation of the highly stable nitrogen molecule. 

Among other initiators that have been studied are disulfides 

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Solution Polymerization. Plant scale polymerizations ia water are conducted either adiabaticaHy or isotherm ally. Molecular weight control, exotherm control, and reduction of residual monomer are factors which limit the types of initiators employed. Commercially available high molecular weight solution polyacrylamides are usually manufactured and sold at about 5% soHds so that the viscosities permit the final product to be pumped easily. 

The type of initiator utilized for a solution polymerization depends on several factors, including the solubiUty of the initiator, the rate of decomposition of the initiator, and the intended use of the polymeric product. The amount of initiator used may vary from a few hundredths to several percent of the monomer weight. As the amount of initiator is decreased, the molecular weight of the polymer is increased as a result of initiating fewer polymer chains per unit weight of monomer, and thus the initiator concentration is often used to control molecular weight. Organic peroxides, hydroperoxides, and azo compounds are the initiators of choice for the preparations of most acryUc solution polymers and copolymers. 

Thermally activated initiators (qv) such as azobisisobutyroaittile (AIBN), ammonium persulfate, or benzoyl peroxide can be used in solution polymeriza tion, but these initiators (qv) are slow acting at temperatures required for textile-grade polymer processes. Half-hves for this type of initiator are in the range of 10—20 h at 50—60°C (13). Therefore, these initiators are used mainly in batch or semibatch processes where the reaction is carried out over an extended period of time. 

Other nonpolymeric radical-initiated processes include oxidation, autoxidation of hydrocarbons, chlorination, bromination, and other additions to double bonds. The same types of initiators are generally used for initiating polymerization and nonpolymerization reactions. Radical reactions are extensively discussed in the chemical Hterature (3—15). 

Much effort has been expended toward the improvement of the properties of polyacetylenes made by the direct polymerization of acetylene. Variation of the type of initiator systems (17—19), annealing or aging of the catalyst (20,21), and stretch orientation of the films (22,23) has resulted in increases in conductivity and improvement in the oxidative stabiHty of the material. The improvement in properties is likely the result of a polymer with fewer defects. 

Two types of initiators are internal and external. Internal initiators result from failures within a plant or the plant s support utilities. Thus, vessel rupture, human error, cooling failure, and loss of offsite power are internal events. All others are external events earthquakes, tornados, fires (external or internal), and floods (external or internal). Event trees can be used to analyze either type of initiator. 

Photoinitiation of polymerization has played an important role in the early developments of polymer chemistry. The main features of this type of initiation are ... 

The most important practical application of the organometallic complex photoinitiators is the possibility of using these types of initiators in modifying the pre-existing polymer chain, e.g., block, graft, and crosslinked copolymers preparation. 

Grafting presents a means of modifying the cellulose molecule through the creation of branches of synthetic polymers, which impart to the cellulose certain desirable properties without destroying the properties of cellulose. The polymerization of vinyl monomers may be initiated by free radicals or by certain ions. Depending on the monomer, one or the other type of initiation may be preferred. The grafting process depends on the reactivity of the monomer used, the type of initiation, and cellulose accessibility [1,2]. 

In catalytic polymerization the reactivity of the propagation center depends on the catalyst composition. Therefore, the dependence of the molecular structure of the polymer chain mainly on the catalyst composition, and less on the experimental conditions, is characteristic of catalytic polymerization. On the other hand, in polymerization by free-radical or free-ion mechanisms the structure of a polymer is determined by the polymerization conditions (primarily temperature) and does not depend on the type of initiator. 

It is the aim of this chapter to describe the nature, selectivity, and efficiency of initiation. Section 3.2 summarizes the various reactions associated with initiation and defines the terminology used in describing the process. Section 3.3 details the types of initiators, indicating the radicals generated, the byproducts formed (initiator efficiency), and any side reactions (e.g. transfer to initiator). Emphasis is placed on those initiators that see widespread usage. Section 3.4 examines the properties and reactions of the radicals generated, paying particular attention to the specificity of their interaction with monomers and other components of a polymerization system. Section 3.5 describes some of the techniques used in the study of initiation. 

There is potential for this behavior to be utilized in devising methods for the control of the types of initiating radicals formed and hence the polymer end groups. 

Type of Initiation Catalytic Cr Ni Metal Rings Spark Hot Wire Inert Shock... 

Type of Initiation Catalytic (Ni Cr oxides in coke granules) Spark Hot Wire Expl Shock... 

The type of initiator used affects the molecular weight and conversion limits in a reactor of fixed size and the molecular weight distribution of the material produced at a given conversion level. The initiator type also dictates the amount of initiator which is necessary to yield a given conversion to polymer, the operating temperature range of the reactor and the sensitivity of the reactor to an unstable condition. Clearly, the initiator is the most important reaction parameter in the polymer process. 

In order to bring about crosslinking of polyesters with styrene one of two types of initiator systems is used, which differ in the temperature at which they are effective. For curing at elevated temperatures, peroxides are used which decompose thermally to yield free radicals. Among those peroxides employed are benzoyl peroxide, 2,4-dichlorobenzoyl peroxide, di-t-butyl peroxide, and dodecyl peroxide. Mixtures of polyester prepolymer, styrene, and such initiators are reasonably stable at room temperatures but undergo fairly rapid crosslinking at temperatures between 70 °C and 150 °C, depending on which particular peroxide is used. 

Many commercially important polymers are actually mixtures of two or more polymer components that differ from one another in composition (for copolymers) or in microstructure (for homopolymers). Such mixtures may be the deliberate result of polymer blending, polymer synthesis, or the presence of different types of initiators or catalytic sites that produce different polymer chains. The ung spectral data of the whole polymer in such systems would include contributions from all its components, and as such should be treated with care. 

An alternative method of preparing the saturated cyclic amines via cyclopolymerization of diallylamine or diallylammonium chloride was unsuccessful. Common free radical initiators such as 2,2 -azobisisobutyronitrile, ammonium persulfate, benzoyl peroxide were found to be ineffective. Several procedures reported in the literature were followed, and unfortunately all of them have resulted only a small amount of low molecular weight oligomers. Further research for polymerization conditions and types of initiation is still required. 

The explosion limits have been determined for liquid systems containing hydrogen peroxide, water and acetaldehyde, acetic acid, acetone, ethanol, formaldehyde, formic acid, methanol, 2-propanol or propionaldehyde, under various types of initiation [1], In general, explosive behaviour is noted where the ratio of hydrogen peroxide to water is >1, and if the overall fuel-peroxide composition is stoicheiometric, the explosive power and sensitivity may be equivalent to those of glyceryl nitrate [2],... 

A variety of initiators have been used for cationic polymerization. The most useful type of initiation involves the use of a Lewis acid in combination with small concentrations of water or some other proton source. The two components of the initiating system form a catalyst-cocatalyst complex which donates a proton to monomer... 

In the early studies on luminol and related hydrazides the systems used were composed of either sodium or potassium hydroxide, as base, hydrogen peroxide as the oxidizing agent (more recently molecular oxygen, hypochlorite, iodide, and permanganate have also been used), and some type of initiator or activator. This initiator was frequently hypochlorite, persulfate, a transition metal... 

This type of initiation, by reaction of monomer and catalyst only, is sometimes called direct initiation, to distinguish it from an initiation which requires the intervention of a co-catalyst. The discovery of co-catalysis proved this view to be inapplicable for many systems, and theoretical reasons against it (at least for hydrocarbons) were also put forward [42]. 

The aim of this paper is a rational approach to designing the optimum initiator for the cationic polymerisation of alkenes. Our method is based on thermodynamic calculations making use of the best available data and estimates, and it is a development of the ideas of Fairbrother [2] and Plesch [3]. Some of our conclusions have been tested experimentally and found valid. The whole complex of problems concerned with this type of initiation is treated in a very comprehensive book [4],... 

Before the publication of Colclough and Dainton s work, Gantmakher and Medvedev [11] had revived Hunter and Yohe s theory of direct initiation, but restricted it to solvents of moderately high dielectric constant. They maintained that in such solvents neither a protonic acid nor an alkyl halide co-catalyst is required. The experiments of Colclough and Dainton make this appear highly unlikely, although they do not disprove it completely. It is important to realise that several types of initiation could co-exist in the same system even if in certain systems co-catalysis by alkyl halides were proved, this does not exclude the existence of a concurrent direct initiation by the Hunter-Yohe, Gantmakher-Medvedev mechanism. 

The kinetic arguments which these authors adduce to support their theory are not decisive - they are based on the steady state hypothesis which is by no means always valid for cationic polymerisations. There is no independent evidence for the various types of initiation which they give, and they are too general to be useful. Without going into a detailed discussion we would merely point out that under identical conditions (CH2C12 - TiCl4 -... 

This brings us to the complicated and difficult question concerning the mechanism of initiation by titanium tetrachloride. There is extensive evidence, obtained with many different systems, that under certain circumstances titanium tetrachloride will only act as initiator in the presence of a co-initiator. There is also good evidence from different laboratories that in other circumstances a co-initiator is apparently not required, and in at least one system both types of initiation go on simultaneously [15]. 

Various groups of workers have attempted to determine the k, by the use of common-ion salts so as to repress the dissociation of the ion-pairs at the growing end. In this context the two types of initiator pose different, but related, problems. If the initiator is a protonic acid AH, as in most of the stopped-flow experiments, the greatest part of the AH does not participate in the initiation (see Section 4.3.2). In these systems, therefore, conjugate ions A2H can be formed from the added salt Bs+A" and therefore the effective [A ] < [Bs+A ]0. In the usual plots [Equation (23) below] of kp (defined in Section 2.5.3) versus [Bs+A ]01/2 this uncertainty will not affect the intercept k, at [Bs+A ]0 1/2 = 0, but it will make the slope, from which the kp can be found, smaller, so that the resulting kp will be an underestimate. 

An explosive device is initiated or detonated by an explosive train — an arrangement of explosive components by which the initial force from the primary explosive is transmitted and intensified until it reaches and sets off the main explosive composition. Most explosive trains contain a primary explosive as the first component. The second component in the train will depend on the type of initiation process required for the main explosive composition. If the main explosive composition is to be detonated, the second component of the train will burn to detonation so that it imparts a shockwave to the main composition. This type of explosive train is known as a detonator. Detonators can be initiated by electrical means, friction, flash, or percussion. 

The stability of vinylidene chloride copolymers generated using different polymerization initiators has also been examined. The two common types of initiators for radical polymerization are azo compounds and peroxides. A common azo initiator is azoisobutronitrile or AIBN. The initiation of vinylidene chloride polymerization using AIBN is illustrated in scheme 3. 

Homo- and copolymerizations involving the monomers depicted in Scheme 1 are chain reactions which can be initiated, at least potentially, by typical free-radical, cationic or anionic promoters. TTie object of the studies reported below is to establish first of all which monomers adapt best to each type of initiation, then what peculiarities (if any) are caused by the presence of the furan ring, compared to the known behaviour of the corresponding aliphatic and/or aromatic homologues and finally to establish the structure-properties relationships of the materials obtained. 

For instance, the CROP of EtOx using four different acetyl halide type of initiators showed that the rate of polymerization increases with the decreased basicity of the counter ion acetyl iodide < acetyl bromide < acetyl chloride. The apparent rates of polymerization of EtOx with different initiators are listed in Table 2. 

Whether a particular monomer can be converted to polymer depends on both thermodynamic and kinetic considerations. The polymerization will be impossible under any and all reaction conditions if it does not pass the test of thermodynamic feasibility. Polymerization is possible only if the free-energy difference AG between monomer and polymer is negative (Sec. 3-9b). A negative AG does not, however, mean that polymerization will be observed under a particular set of reaction conditions (type of initiation, temperature, etc.). The ability to carry out a thermodynamically feasible polymerization depends on its kinetic feasibility—on whether the process proceeds at a reasonable rate under a proposed set of reaction conditions. Thus, whereas the polymerization of a wide variety of unsaturated monomers is thermodynamically feasible, very specific reaction conditions are often required to achieve kinetic feasibility in order to accomplish a particular polymerization. 

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