Polymerization auto-initiation

The problems associated with route B also have something to do with steric hindrance. Here the critical point is the steric demand of both monomer and chain end. Incoming monomer will only be connected to the chain end, if steric hindrance is not too high. Otherwise this process will be slowed down or even rendered impossible. Depending on the kind of polyreaction applied, this may lead to termination of the reactive chain end and/or to side reactions of the monomer, like loss of coupling functionality as in some polycondensations or auto-initiation specifically in radical polymerizations. From this discussion it can be extracted that the basic problems for both routes are incomplete coverage (route A) and low molecular weight dendronized polymer (route B). 

Styrene monomer will spontaneously or auto-polymerize and must be inhibited to prevent reaction during transport and storage. Polymerization is initiated by the generation of free radicals either by the reaction of the styrene with itself ( auto-initiation ) or by means of a peroxide initiator ( chemical initiation ). Radicals rapidly propagate by reaction with monomer and ultimately terminate by coupling with another growing radical or by transferring the radical to a small molecule to start a new chain (chain transfer). 

A very unique approach to increasing the production rate of high MW polystyrene was recently developed by Dow researchers. They discovered that the rate-MW curve for auto-initiation polymerization of styrene can be significantly... 

Also indicated in Figure 7.7 is the possibility of acid-catalyzed aromatization of DH to an unreactive dimer (DA). Under neutral conditions, only traces of DA are found. However, when a small amount of CSA is added to styrene undergoing polymerization by auto-initiation, significant levels of DA are formed along with polystyrene of higher than expected MW. We believe this is strong support for the Mayo mechanism since acid would have little affect on the Flory diradical intermediate. 

When initiator is first added the reaction medium remains clear while particles 10 to 20 nm in diameter are formed. As the reaction proceeds the particle size increases, giving the reaction medium a white milky appearance. When a thermal initiator, such as AIBN or benzoyl peroxide, is used the reaction is autocatalytic. This contrasts sharply with normal homogeneous polymerizations in which the rate of polymerization decreases monotonicaHy with time. Studies show that three propagation reactions occur simultaneously to account for the anomalous auto acceleration (17). These are chain growth in the continuous monomer phase chain growth of radicals that have precipitated from solution onto the particle surface and chain growth of radicals within the polymer particles (13,18). 

However, the mechanisms by which the initiation and propagation reactions occur are far more complex. Dimeric association of polystyryllithium is reported by Morton, al. ( ) and it is generally accepted that the reactions are first order with respect to monomer concentration. Unfortunately, the existence of associated complexes of initiator and polystyryllithium as well as possible cross association between the two species have negated the determination of the exact polymerization mechanisms (, 10, 11, 12, 13). It is this high degree of complexity which necessitates the use of empirical rate equations. One such empirical rate expression for the auto-catalytic initiation reaction for the anionic polymerization of styrene in benzene solvent as reported by Tanlak (14) is given by ... 

Poly(acrylic acid) is not soluble in its monomer and in the course of the bulk polymerization of acrylic acid the polymer separates as a fine powder. The conversion curves exhibit an initial auto-acceleration followed by a long pseudo-stationary process ( 3). This behaviour is very similar to that observed earlier in the bulk polymerization of acrylonitrile. The non-ideal kinetic relationships determined experimentally in the polymerization of these two monomers are summarized in Table I. It clearly appears that the kinetic features observed in both systems are strikingly similar. In addition, the poly(acrylic acid) formed in bulk over a fairly broad range of temperatures (20 to 76°C) exhibits a high degree of syndiotacticity and can be crystallized readily (3). 

The auto-acceleration observed under such conditions is reduced ( = 1.15) and could partially result from non-steady-conditions but also from a "matrix effect" operating on the surface of unswollen polymer particles. It should be noted in this respect that the post-polymerization which is induced by the growing chains occluded in the precipitated polymer exhibits an initial rate very much lower than the rate observed during irradiation (Curve 1 in Figure 91 which suggests that the contribution of the growth of occluded chains to the over-all rate is small. 

The continuous availability of trillions of independent microreactors greatly multiplied the initial mixture of extraterrestrial organics and hydrothermal vent-produced chemicals into a rich variety of adsorbed and transformed materials, including lipids, amphiphiles, chiral metal complexes, amino add polymers, and nudeo-tide bases. Production and chiral amplification of polypeptides and other polymeric molecules would be induced by exposure of absorbed amino adds and organics to dehydration/rehydration cydes promoted by heat-flows beneath a sea-level hydro-thermal field or by sporadic subaerial exposure of near-shore vents and surfaces. In this environment the e.e. of chiral amino adds could have provided the ligands required for any metal centers capable of catalyzing enantiomeric dominance. The auto-amplification of a small e.e. of i-amino adds, whether extraterrestrially delivered or fluctuationally induced, thus becomes conceptually reasonable. 

The bulk polymerization of vinyl acetate is primarily of interest for laboratory studies, although a few large-scale procedures have been reported. Since the heat of polymerization is quite high (21 kcal/mole) and the boiling point of the monomer is relatively low (72.7°C) (Table I), not only must the reaction temperature be monitored closely, but the reaction temperature must be kept low, unless pressure equipment is used. The low temperatures mean that the usual initiators of free-radical polymerization will act rather slowly. To further complicate bulk polymerizations, the polymerization process is strongly auto-catalytic [17, 68]. 

There are several variants of the DCR theory differing from one another by the way in which account is taken of the physical factors influenced by the diffusion control on the description of elementary reactions rate. As a rule, the main factor influenced is the bimolecular chain termination process. The constant rate of chain termination is considered as a fnnction of the macroradical s mobility, their length [9-14], free volume [12,15-17] or characteristic viscosity of monomer-polymeric system. However, with the aim of explaining the auto deacceleration stage, the efficiency of initiation and constants of rate chain propagation are also considered to be functions of the macroradical s mobility [12,15,18]. 

In many polymerizations, a marked increase in rate is observed toward the end of the reaction instead of the expected gradual decrease caused by the depletion of the monomer and initiator. This auto-acceleration is a direct result of the increased viscosity of the medium, and the effect is most dramatic when polymerizations are carried out in the bulk phase or in concentrated solutions. The phenomenon, sometimes known as the Trommsdorff-Norrish or gel effect, is caused by the loss of the steady state in the polymerization kinetics. 

Bulk polymerization is used in the production of polystyrene, poly(methyl-methacrylate), and poly (vinyl chloride). The reaction mixture contains only monomer and initiator, but because the reaction is exothermic, hot spots tend to develop when heat removal is inefficient. Auto-acceleration occurs in the highly viscous medium, making control difficult and impeding efficient monomer conversion. To overcome some of the disadvantages, low conversions are used, after which the... 

Mechanism of deterioration of polymers by auto-oxidization (B-3) Mechanism of free radical type polymerization by grafting initiators (B-4) Diffusion controlled reaction of free radical decay in solid polymers... 

Kinetics of Photopolymerization. For each of these monomers, the polymerization rate is relatively low at low monomer concentration (less than 1 Wt%). This is because dilution reduces initiator efficiency and prevents auto-acceleration (Tromsdorff effect), which is typical of bulk photopolymerizations. The polymerization rate increases with increasing monomer concentration (IS), Similar observation was made by following the double bond conversion of the diacrylate (IS). The maximum polymerization rate and the conversion at maximum rate increase with increasing monomer concentration, suggesting that, at low monomer concentration, the mobility in the mixtures is high and decrease of rate at later stages results from a depletion of monomers and a decrease in the mobility of the polymer-rich phase as crosslink density increases (5). The polymerization rate is also dependent upon the architecture of the monomer. In dilute solution, differences in mobility are less, and factors such as the electronic structure of the monomers is important. 

The polymerisation of styrene with vegetable oils involves free radical initiated polymerisation. A free radical type initiator, such as benzoyl peroxide, azobisisobutryronitrile and ditertiarybutyl peroxide is normally used to accelerate the copolymerisation reaction (Rg. 8.4). Linseed, tung,soybean, sunflower and oiticica oils and dehydrated castor oil (DCO) are widely used in the preparation of styrenated-oil products. " The free radical polymerisation of methyl methacrylate or n-butyl methacrylate, using polymeric oil peroxy initiators from the auto-oxidation of linseed oil, soybean oU, and Unoleic acid has been carried out successfully. 

What is the proposed mechanism of initiation of polymerization, by the Lewis arid like aluminum chloride or bromide through an auto ionization process ... 

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