Monomer initiation

As with the rate of polymerization, we see from Eq. (6.37) that the kinetic chain length depends on the monomer and initiator concentrations and on the constants for the three different kinds of kinetic processes that constitute the mechanism. When the initial monomer and initiator concentrations are used, Eq. (6.37) describes the initial polymer formed. The initial degree of polymerization is a measurable quantity, so Eq. (6.37) provides a second functional relationship, different from Eq. (6.26), between experimentally available quantities-n, [M], and [1]-and theoretically important parameters—kp, k, and k. Note that the mode of termination which establishes the connection between u and hj, and the value of f are both accessible through end group characterization. Thus we have a second equation with three unknowns one more and the evaluation of the individual kinetic constants from experimental results will be feasible. 

The solvent and initiator are charged to the reactor and heated to reflux (ca 80°C). Forty percent of the monomer charge is then added. The remainder of the monomer is added in four equal increments at 24, 50, 79, and 110 min after addition of the initial monomer charge. The reaction mixture is kept at reflux overnight, then cooled and packaged (96). 

In these equations I is the initiator and I- is the radical intermediate, M is a vinyl monomer, I—M- is an initial monomer radical, I—M M- is a propagating polymer radical, and and are polymer end groups that result from termination by disproportionation. Common vinyl monomers that can be homo-or copolymeri2ed by radical initiation include ethylene, butadiene, styrene, vinyl chloride, vinyl acetate, acrylic and methacrylic acid esters, acrylonitrile, A/-vinylirnida2ole, A/-vinyl-2-pyrrohdinone, and others (2). 

When the initial monomer supply is exhausted, the anionic chain ends retain their activity. Thus, these anionic chains have been termed living polymers. If more monomer is added, they resume propagation. If it is a second monomer, the result is a block copolymer. 

Fig. 25. Drift ia monomer composition (—) and copolymer composition (-) with conversion for three initial monomer mixtures. Ratios are based on...

Propagation. The rate of emulsion polymerization has been found to depend on initiator, monomer, and emulsifier concentrations. In a system of vinyl acetate, sodium lauryl sulfate, and potassium persulfate, the following relationship for the rate of polymerization has been suggested (85) ... 

Polymerisation casting involves mixing monomer or low molecular weight polymer with a polymerisation initiator, pouring the mix into the mould and allowing polymerisation to occur in situ. A variation is to impregnate fibres with initiated monomer or other low molecular weight material and polymerise to produce composite structures. The main problem is due to the heat of polymerisation. Unless heat transfer distances are kept short or unless the reaction is carried out very slowly it can easily get out of hand. 

Other reasons for a wide propagation of polymerization in water include (1) reduction of energy consumed to separate the initial monomer in crystal form (acrylamide is produced and used in the aqueous solution form), which, in addition, is associated with the probability of its spontaneous polymerization, and (2) recovery of the organic solvents, which results in less environmental pollution and the elimination of the stage of solution of polymer reagents used, as a rule, in the form of the aqueous solutions. 

Soapless seeded emulsion copolymerization has been proposed as an alternative method for the preparation of uniform copolymer microspheres in the submicron-size range [115-117]. In this process, a small part of the total monomer-comonomer mixture is added into the water phase to start the copolymerization with a lower monomer phase-water ratio relative to the conventional direct process to prevent the coagulation and monodispersity defects. The functional comonomer concentration in the monomer-comonomer mixture is also kept below 10% (by mole). The water phase including the initiator is kept at the polymerization temperature during and after the addition of initial monomer mixture. The nucleation takes place by the precipitation of copolymer macromolecules, and initially formed copolymer nuclei collide and form larger particles. After particle formation with the initial lower organic phase-water ratio, an oligomer initiated in the continuous phase is... 

The departure of dependence of Rp on the concentration of CHP from 0.5 order might be ascribed to induction decomposition of ROOH type to form ROO- radical, which has very low activity to initiate monomer polymerization [40], but can combine with the propagation chain radical to form the primary radical termination. For the same reason, the order of concentration of TBH was also lower than 0.5 when the TBH-DMT system was used as the initiator in MMA bulk polymerization. But in the BPO-DMT initiation system as shown in Table... 

Based on the ESR studies of Ce(IV) ion-BzyAcAc-MNP, Ce(IV) ion BzAc-MNP systems as mentioned before, the grafting reaction of P(St-CH2-AcAc) will take place on the methene carbon of 1,3-dikeone via the abstraction of hydrogen by the Ce(I V) ion to form radicals and then initiate monomer graft copolymerization. The initiation mechanism of graft copolymerization is proposed in Scheme (10). 

Since 1 is a monomer with low activity, copolymers 2 obtained at any stage of the copolymerization process, irrespective of the monomer ratio in the initial mixture, always contain a smaller amount of monomeric units of 1 than that in the corresponding monomer mixture. 1 being prone to enter the chain-transfer reaction, the increase of its content in the initial monomer mixture reduces substantially the reaction rate and decreases the molecular mass of the copolymers. It was found that copolymers 2 which contain 2—8% of monomeric units of 1 and are suitable for obtaining fibres must have a molecular mass between 45 000 and 50000. Such copolymers can be obtained with a AN 1 ratio in the initial mixture between 95 5 and 85 15. Concentrated solutions of copolymers, especially those with a molecular mass smaller than the above limit, are characterized by a very low stability which is a substantial shortcoming of these copolymers. 

Transfer to initiator can be a major complication in polymerizations initiated by diacyl peroxides. The importance of the process typically increases with monomer conversion and the consequent increase in the [initiator] [monomer] ratio.9 105160 162 In BPO initiated S polymerization, transfer to initiator may be lire major chain termination mechanism. For bulk S polymerization with 0.1 M BPO at 60 °C up to 75% of chains are terminated by transfer to initiator or primary radical termination (<75% conversion).7 A further consequence of the high incidence of chain transfer is that high conversion PS formed with BPO initiator tends to have a much narrower molecular weight distribution than that prepared with other initiators (e.g. AIBN) under similar conditions. 

Chain transfer is the reaction of a propagating radical with a non-radical substrate (X-Y, Scheme 6.1) to produce a dead polymer chain and a new radical (Y ) capable of initiating a polymer chain. The transfer agent (X-Y) may be a deliberate additive (e.g. a thiol) or it may be the initiator, monomer, polymer, solvent or an adventitious impurity. 

Terminal model reactivity ratios may be estimated from the initial monomer feed composition and the dyad concentrations in low conversion polymers using the following relationships (eqs. 45, 46). 

Similarly, penultimate model reactivity ratios can be estimated from initial monomer feed composition and triad concentrations using eqs. 50-53. 

Seed No. Initial Monomer Concentration (moles/kgm) Initial BuLi Concentration (moles/kgm) M n D n M w M n Used In Runs Nos. 

The mechanical properties, degradation, and surface characteristics of poly(diol citrates) could be controlled by choosing different diols and by controUing synthesis conditions such as cross-linking temperature and time, vacuum, and initial monomer molar ratio. 

Figure 6 shows the temperature proflle that should be used with the initiator monomer system described in the caption to reduce the monomer concentration from 0.47 mol/L to 0.047 mol/L. The optimal nonisothermal policy consists of decreasing temperature from a temperature above the optimal isothermal temperature to one below it. The rate of polymerization could be increased, as expected, by an initially higher temperature, but the temperature must be decreased to avoid depletion of initiator and depolymerization. However, the amount of time saved by this policy does not seem to be significant in comparison to the isothermal policy for this case. 

M , = equilibrium monomer concentration Mo = initial monomer concentration (mol/L)... 

Fig.1. AB vinyl inimers (initiator-monomers) used for SCVP and SCVCP...

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