No abstract

The MWD function and moments are derived for a "living" polymerization process which proceeds via active and "dormant" species and where the activity is directly exchanged between these chain ends in a bimolecular reaction ("degenerative transfer"). Such a mechanism is believed to be applicable to many "living" polymerizations (e.g., anionic, group transfer, cationic, and radical). For constant monomer concentration (slow addition of monomer or low conversion), the polydispersity index, PJPn, depends on the ratio of molar concentrations of monomer and initiator, = M/Io, the degree of polymerization, Pn, and the ratio of rate constants of exchange and propagation, ß = keJkv. In a limiting case (ß > 1 and Pn » 1), Pw/Pn ~1 + 2 /(ß ). The molecular weight distributions are always narrower than those obtained for a batch process, where monomer concentration decreases during polymerization.

The group transfer polymerization (GTP) of n-butyl acrylate using l-methoxy-l-(trimethylsiloxy)-2-methyl-l-propene as an initiator and HgL as a catalyst in toluene is much slower compared to nucleophilic-catalyzed GTP; half-lives are in the range of minutes to hours. Induction periods are observed which are attributed to the formation of trimethylsilyl iodide (TMSI). The polymerization follows first-order kinetics with respect to the concentrations of initiator and catalyst. With respect to concentration of monomer the reaction is of first-order internally but follows an external reaction order of 1.52 due to the higher polarity of the reaction medium at higher monomer concentrations. The degree of polymerization is controlled by the ratio [M]o/[I]o, and the molecular weight distribution is narrow and can be described by a modified Poisson distribution. Generally, MJMn < 1.2. It is proposed that the active species are formed from initiator, catalyst, and TMSI.

In the presence of trimethylsilyl iodide (TMSI) the group transfer polymerization (GTP) of -butyl acrylate (n-BuA) using l-methoxy-l-(trimethylsiloxy)-2-methyl-l-propene (MTS) as an initiator and mercuric iodide (Hgti as a catalyst in toluene is drastically accelerated. The rate of polymerization is comparable to that of the nucleophilic catalyzed GTP with half-lives in the range of seconds to minutes. The induction periods which characterize the GTP in the absence of TMSI completely disappear. A very slight curvature in the first-order time-conversion plots is attributed to some "back-biting" termination. Kinetic investigations show that the polymerization follows first-order kinetics with respect to catalyst and TMSI concentrations. The rate of polymerization strongly depends on the polarity of the reaction medium. An external reaction order of 1.52 with respect to the monomer concentration is due to the higher polarity of the reaction medium at higher monomer concentrations. The rate of polymerization is increased by 2 orders of magnitude in CHgClVtoluene mixtures. The Arrhenius plot displays a maximum at ~-60 °C, indicating a negative apparent activation energy in the temperature range from +40 to -40 °C, and a positive one at temperatures <-70 °C. The kinetic results support a mechanism in which mercuric iodide and TMSI form an active complex which acts as a nucleophilic catalyst. At lower temperatures the concentration of active species increases, overriding the decrease of the rate constant of propagation. The molecular weight is controlled by the ratio [M]o/[I]o, and the molecular weight distributions of the polymers formed are very narrow and are not effected by TMSI.

The group transfer polymerization (GTP) of n-butyl acrylate (nBuA) using 1-methoxy-1-(trimethylsiloxy)-2-methyl-l-propene (MTS) as an initiator, mercuric iodide (Hg ) as a catalyst in toluene at room temperature gves a vely good control of molecular weiat and narrow molecular weight distribution. (q, < 1.2). Kinetic studies in this system reveal that this reaction is rather slow, half-lives bemg in the range of hours. The kinetic order of the apparent rate constant of propagation with respect to initiator and catalyst concentrations were found to be near to unity. However, the first-order timeconversion plots exhibit considerable induction periods. Upon addition of trimethylsilyl iodide (TMSI) the induction periods vanish and the rates are increased by two orders of magnitude. The reaction is first-order with respect to TMSI indicating that it takes part in the reaction by activating the catalyst. The complex from HgI and TMSI acts as a nucleophilic catalyst, similar to oxyanions in the GTP of MMA in kF. It is proposed that the induction periods are due to the formation of TMSI from MTS and Hg12.