Vinyl polymers molecular weights

Often a chain-transfer agent is added to vinyl acetate polymerizations, whether emulsion, suspension, solution, or bulk, to control the polymer molecular weight. Aldehydes, thiols, carbon tetrachloride, etc, have been added. Some emulsion procedures call for the recipe to include a quantity of preformed PVAc emulsion and sometimes antifoamers must be added (see Foams). 

The net result is a decrease in initiator efficiency and an attendant increase in polymer molecular weight. In fact, all of our work on radical polymerization of phenol-containing vinyl monomers suggests that inhibition and transfer problems are at most minor, if AIBN is used as initiator and oxygen is carefully excluded from the reaction mixtures (9). 

This reaction may account in part for the oligomers obtained in the polymerization of pro-pene, 1-butene, and other 1-alkenes where the propagation reaction is not highly favorable (due to the low stability of the propagating carbocation). Unreactive 1-alkenes and 2-alkenes have been used to control polymer molecular weight in cationic polymerization of reactive monomers, presumably by hydride transfer to the unreactive monomer. The importance of hydride ion transfer from monomer is not established for the more reactive monomers. For example, hydride transfer by monomer is less likely a mode of chain termination compared to proton transfer to monomer for isobutylene polymerization since the tertiary carbocation formed by proton transfer is more stable than the allyl carbocation formed by hydride transfer. Similar considerations apply to the polymerizations of other reactive monomers. Hydride transfer is not a possibility for those monomers without easily transferable hydrogens, such as A-vinylcarbazole, styrene, vinyl ethers, and coumarone. 

In the absence of H2 and other transfer agents, polymer molecular weight is limited by various P-hydride transfers—from normal (1,2-) and reverse (2,1-) propagating centers, before and after rearrangement [Lehmus et al., 2000 Resconi et al., 2000 Rossi et al., 1995, 1996 Zhou et al., 2001] (Sec. 8-4i-2). Vinylidene, vinylene, and trisubstituted double-bond end groups are formed in 1-alkene polymerizations, vinyl and vinylene in ethylene polymerization. [Vinyl groups are also produced in some 1-alkene polymerizations, not by P-hydride transfer, but by P-alkyl transfer (Sec. 8-4i-2).]... 

A chain-transfer agent is added to vinyl acetate polymerizations to control the polymer molecular weight,... 

One way to distinguish between the two possibilities is to study the isotope effect on the kinetics of vinyl acetate polymerization and on the polymer molecular weight. The deuterium isotope eftect has been ascribed to the difference in the zero point energies of the stretching vibrations of the C-H and C-D bond (11). The rate of a reaction in which deuterium is transferred is slower than that of the corresponding reaction for hydrogen, since the C-D bond has a lower zero point energy. 

We have assumed so far that K and a are fixed for a given polymer type and solvent and do not vary with polymer molecular weight. This is not strictly true. Oligomers (less than about 100 repeating units in most vinyl polymers) often conform to... 

In another series of experiments, changes in polymer molecular weight with initial monomer concentration were investigated at —180° C. The concentration of monomer was varied by dilution with propane, and the ethyl chloride-vinyl chloride mixture was used as polar solvent for the AICI3 catalyst. 

Blends. The type I reaction produces free radicals which, in the presence of oxygen, initiates photooxidation which also results in a decrease in the polymer molecular weight. An indication of the relative importance of the type I reaction in these systems can be estimated from the amount of chain scission induced in a blend of the copolymers with homopolymer polystyrene. For these experiments, one part of 5% vinyl ketone copolymer was blended with four parts of styrene homopolymer to retain an overall ketone monomer concentration of 1%. 

The UCFT measured in these systems as a function of the concentration of polymer (molecular weight 8 000) added to the dispersion medium (2 M NaNOj) is presented in Fig. 9.5 for three different volume fractions of latex. The UCFT in all cases increased to a maximum value and then declined. Separate measurements, using the phase separation method of Comet and Ballegooijen (1966), suggested that in 2M NaNOs the apparent 0-temperature for the poly(vinyl alcohol) of this degree of hydrolysis was... 

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