Vinyl polymerisation initiation

When heated to 120°C, AIBN decomposes to form nitrogen and two 2-cyanoisopropyl radicals. The ease with which AIBN forms radicals, and the fact that the rate of information does not vary much in various solvents has resulted in wide use of AIBN as a free-radical initiator. AIBN is used commercially as a catalyst for vinyl polymerisation (see Initiators). 

The efficiency of the intitiator is a measure of the extent to which the number of radicals formed reflects the number of polymer chains formed. Typical initiator efficiencies for vinyl polymerisations lie between 0.6 and 1.0. Clearly the efficiency cannot exceed 1.0 but it may fall below this figure for a number of reasons, the most important being the tendency of the newly generated free radicals to recombine before they have time to move apart. This phenomenon is called the cage effect . 

We have already come across a few examples in which oxidation-reduction reaction can initiate vinyl polymerisation. Such a polymerisation reaction is known as redox polymerisation. In such reactions the oxidant is generally referred to as initiator and the reductant as activator. 

Most initiators in typical vinyl polymerisations have efficiencies between 0.6 and 1 (i.e., between 60 and 10 per cent of all the radicals formed ultimately initiate Polymer chains). A fraction of the radicals may disappear under certain circumstances also through different reactions, i.e., through direct combination with atmospheric oxygen or other inhibiting substances present in the system. 

More recently certain transitional metal ions have been used for initiation of vinyl polymerisation. It has been explained by Santappa et al. and Nayak et al. that the termination in case of the metal ion initiation of polymerisation to be linear, may be shown as below ... 

The heuristic value of equation (3) is that we can see what is required for a good initiator D and I(R ) must be as large a possible, and AHS(R+) should be small compared to AHS(RI>1+) in other words, as an efficient initiator we need a large cation whose parent radical has a high ionisation potential, and which forms a strong bond to monomer. On the last two counts the trityl cation must be one of the least suitable. These considerations indicate that the first four ions listed in Table 1 should merit detailed study as initiators for vinyl polymerisations. 

While the latter measurements, strictly speaking, evaluate the state of dissociation of the initiator ions only, they also provide at least a guide as to the degree of dissociation of the propagating polymeric ion pairs. In the case of vinyl polymerisations, where no living cations have been observed to date, direct evaluation of the dissociation constant Kd, of the growing ion pair is not possible. However, in a number of cyclic monomer polymerisations living characteristics are observed, and direct measurements have been possible (27). 

Perhaps the most versatile cation for initiation of polymerisation is the tri-phenylmethyl species (Ph3C+) which is reactive towards both electron rich olefins and cyclic monomers. The more stable cycloheptatrienyl cation (C H,) has proved very useful in vinyl polymerisations but has been little used in ring opening reactions (/5). Not surprisingly, therefore, the dissociation of numerous salts of these two cations have been widely studied, and data for each is summarised in Tables 2 and 3 respectively. 

In general the initiation reaction for a vinyl polymerisation by any cation A+ may be represented by the simple equilibrium... 

Even more stable salts than those already mentioned can be prepared from oxonium, R30+, sulphonium, R3S+, and ammonium, R4N+ ions. Not surprisingly, however, these are in general too stable to initiate vinyl polymerisations though Et30+BF4 has been reported to polymerise alkyl vinyl ethers (96). Both oxonium and strained sulphonium ion salts are very efficient initiators of ring opening polymerisations as we shall see later (Section IV). 

The polymerisation of p-methoxystyrene in methylene chloride has been studied utilising both Ph3C+SbCl (21) and QH SbCl (67) as initiators. With this and other bulky monomers (relative to vinyl ethers) initiation with Ph3C+ appears to be less efficient than by CjH, and a possible explanation for this has already been advanced (Section IDA). 

Subira et al. recently carried out a very detailed and thorough study of the polymerisation of isolufyl vinyl ether initiated by trityl hexadiloroantimonate in methylene chloride at various temperatures. The mqor difference between this investigation and the previous ones consisted in the fact that for the first time the rate of initiation was measured spectroscc ically at the same time as the rate of monomer consumption (calorimetry). These determinations clearly diowed that the bdiavicmr of the system was by no means as simple as postulated by previous workers. Initiation was in fact found to be... 

Some reagents react with the initiating radical to give unreactive substances, a process known as inhibition. A common inhibitor for vinyl polymerisations is hydroquinone, which reacts by the transfer of two hydrogen radicals to the initiator radicals (Fig. 2.4). This gives quinone and unreactive initiator and has the net effect of causing a lag time in the polymerisation and a decrease in the initiator concentration. Monomers are often stored in the presence of inhibitor in order to prevent polymerisation. The amount and type of inhibitor may vary depending on the monomer batch and the manufacturer. For inter-laboratory comparisons of materials to be possible, it is therefore important to remove the inhibitor and purify the monomers prior to use [13]. 

Since peroxides are known to act as initiators for vinyl polymerisation, it follows that under suitable conditions a vinyl monomer will react with peroxide group to produce a graft copolymer with the substrate, e.g. 

The practical technique to obtain polymer polyols by radical polymerisation is to add an homogeneous mixture of vinylic monomer, initiator, chain transfer agent and a part of polyether polyol, to the rest of polyether polyol containing the NAD (macromer or nonreactive NAD), at 115-125 °C. The mechanism of solid polymer particle formation during radical polymerisation of vinylic monomers in liquid polyethers, in the presence of a nonreactive NAD, in the form of very stable dispersions, is described next. 

Polyacrylamide gels are prepared by copolymerisation of acrylamide monomer (CH2=CHCO NH2) with a cross linking agent, usually N, N-methylene bisacrylamide, CH2(NHCOCH = CH2)2, in the presence of a catalyst accelerator-chain initiator mixture. This mixture may consist of freshly prepared ammonium persulphate as catalyst (0.1 to 0.3% w/v) together with about the same concentration of a suitable base, for example, dimethylamino propionitrile (DMAP) or N, N, N, N tetramethylene diamine (TEMED) as initiator. TEMED is most frequently used and proportional increases in its concentration speed up the rate of gel polymerisation. Photochemical polymerisation may be brought about by riboflavin in the presence of UV radiation. Gelation is due to vinyl polymerisation as shown below ... 

In this century it has been shown that mastication of rubber produces free radicals with high chemical reactivity. Thus, for example, macroradicals from cis-polyisoprene initiate vinyl polymerisation when oxygen is rigorously excluded from a high shear mixer, giving rise to block copolymers, typically containing rubber and methyl methacrylate (Scheme 3.2). In the presence of oxygen this process is inhibited and the... 

Polyester Resin—Thermosetting resins produced by dissolving unsaturated, generally linear, alkyd resins in a vinyl type monomer such as styrene and capable of being crosslinked by vinyl polymerisation using initiators and promoters. 

By far the greatest part of PVC production across the world is now made by the suspension process. Vinyl chloride monomer (derived from a reaction between ethylene (derived from oil) and chlorine (derived from common salt) is dispersed in deionised water with the help of small quantities of chemical dispersants and polymerisation initiators (typically peroxide compounds). At moderately raised temperature (50 C) and pressure (0.7 MPa) polymerisation proceeds and the polymer can be removed from the resulting slurry by de-watering and steam stripping the unconverted vinyl chloride monomer. 

The relationship between the surface characteristics of colloidal copper particles and the kinetic parameters of heterogeneous catalytic initiation in aqueous polymerisation was investigated. The dependence of the rate of emulsion polymerisation initiated by the colloidal copper on the nature of the monomer used (styrene, methyl methacrylate, vinyl acetate and N-vinyl pyrrolidone) and method of manufacture of the copper powder was examined. The characteristics of styrene emulsion polymerisation initiated by modified and unmodified colloidal copper particles are reported and the ability of... 

For a typical condensation polymerisation, the molar mass distribution function is generally in the range 3—20, but is sometimes even greater. On the other hand, in vinyl polymerisation the values typically wiU be in the range 1.05—3.0. The narrowest molar mass distributions are observed with anionic and certain cationic initiated polymerisations. Molar mass effects are observed with aU polymer systems but they are more important in the physical properties of amorphous polymers than in their crystalline analogues. 

The HTV -vinyl polymerisation is initiated using a platinum salt that is heated to a temperature above 100 °C. The resulting matrix -with both Si-O and C-C... 

Free Radical Initiators for the Suspension Polymerisation of Vinyl Chloride, Technical Publication 30.90, Lucidol Division, Pennwalt Corp., Buffalo, N.Y., 1976. 

Figure 5 illustrates the type of encapsulation process shown in Figure 4a when the core material is a water-immiscible Hquid. Reactant X, a multihmctional acid chloride, isocyanate, or combination of these reactants, is dissolved in the core material. The resulting mixture is emulsified in an aqueous phase that contains an emulsifier such as partially hydroly2ed poly(vinyl alcohol) or a lignosulfonate. Reactant Y, a multihmctional amine or combination of amines such as ethylenediamine, hexamethylenediamine, or triethylenetetramine, is added to the aqueous phase thereby initiating interfacial polymerisation and formation of a capsule shell. If reactant X is an acid chloride, base is added to the aqueous phase in order to act as an acid scavenger. 

The principal use of the peroxodisulfate salts is as initiators (qv) for olefin polymerisation in aqueous systems, particularly for the manufacture of polyacrylonitrile and its copolymers (see Acrylonitrile polymers). These salts are used in the emulsion polymerisation of vinyl chloride, styrene—butadiene, vinyl acetate, neoprene, and acryhc esters (see Acrylic ester polymers Styrene Vinyl polymers). 

Organic peroxides are used in the polymer industry as thermal sources of free radicals. They are used primarily to initiate the polymerisation and copolymerisation of vinyl and diene monomers, eg, ethylene, vinyl chloride, styrene, acryUc acid and esters, methacrylic acid and esters, vinyl acetate, acrylonitrile, and butadiene (see Initiators). They ate also used to cute or cross-link resins, eg, unsaturated polyester—styrene blends, thermoplastics such as polyethylene, elastomers such as ethylene—propylene copolymers and terpolymers and ethylene—vinyl acetate copolymer, and mbbets such as siUcone mbbet and styrene-butadiene mbbet. 

Films from prepolymer solutions can be cured by heating at 150°C. Heating the prepolymer in molds gives clear, insoluble moldings (38). The bulk polymerisation of DAP at 80°C has been studied (35). In conversions to ca 25% soluble prepolymer, rates were nearly linear with time and concentrations of bensoyl peroxide. A higher initiator concentration is required than in typical vinyl-type polymerisations. 

Chain transfer also occurs to the emulsifying agents, leading to their permanent iacorporation iato the product. Chain transfer to aldehydes, which may be formed as a result of the hydrolysis of the vinyl acetate monomer, tends to lower the molecular weight and slow the polymerisation rate because of the lower activity of the radical that is formed. Thus, the presence of acetaldehyde condensates as a poly(vinyl alcohol) impurity strongly retards polymerisation (91). Some of the initiators such as lauryl peroxide are also chain-transfer agents and lower the molecular weight of the product. 

Continuous Polymerization. A typical continuous flow diagram for the vinyl acetate polymerisation is shown in Figure 12. The vinyl acetate is fed to the first reactor vessel, in which the mixture is purged with an inert gas such as nitrogen. Alternatively, the feed may be purged before being introduced to the reactor (209). A methanol solution containing the free-radical initiator is combined with the above stream and passed directiy and continuously into the first reactor from which a stream of the polymerisation mixture is continuously withdrawn and passed to subsequent reactors. More initiator can be added to these reactors to further increase the conversion. 

Addition polymerisation is effected by the activation of the double bond of a vinyl monomer, thus enabling it to link up to other molecules. It has been shown that this reaction occurs in the form of a chain addition process with initiation, propagation and termination steps. 

Most vinyl monomers will polymerise by free-radical initiation over a wide range of monomer concentration. Methyl methacrylate can even be polymerised... 

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