Fast initiation systems

Today 80% of PVC is manufactured by the technique of batchwise suspension polymerization (S-PVC), the remaining part being shared between emulsion and bulk polymerization. A distinctive characteristic of the S-PVC processes is the large size of the reactors, 50 to 200m3, and operation at 10 to 12 bar pressure. The reaction time has been reduced from 18 h in the 1960s to only 3.5-5 h today by making use of fast initiation systems and recipe optimization. 

Van Dyke, M. E. Clarson, S. J., Reaction Kinetics for the Anionic Ring-Opening Polymerization of Tetraphenyltetramethylcyclosiloxane Using a Fast Initiator System. J. Inorg. Organomet. Polym. 1998, 8,111-121. 

In place of a proton source, ie, a Briimsted acid, a cation source such as an alkyl haUde, ester, or ether can be used in conjunction with a Friedel-Crafts acid. Initiation with the ether-based initiating systems in most cases involves the haUde derivative which arises upon fast haUdation by the Friedel-Crafts acid, MX (2). 

Sequential addition of monomers 6 7-26-27-114) is the most obvious procedure. Once the first monomer has been polymerized, the resulting living species is used as a polymeric initiator for the polymerization of the second one. The monomers are to be added in the order of increasing electron affinity to provide efficient and fast initiation 26 U4). This condition is rather restrictive, and the number of monomer systems that can be used is limited (Table 5). Moreover, when the second monomer contains an electrophilic function (e.g. ester) which could lead to side reactions, it is necessary to first lower the nucleophilicity of the living site. This is best done by intermediate addition of 1.1-diphenylethylene25). The stabilized diphenylmethyl anions do not get involved in side reactions with ester functions, while initiation is still quantitative and fast. 

The same aplies to polymer brushes. The use of SAMs as initiator systems for surface-initiated polymerization results in defined polymer brushes of known composition and morphology. The different polymerization techniques, from free radical to living ionic polymerizations and especially the recently developed controlled radical polymerization allows reproducible synthesis of strictly linear, hy-perbranched, dentritic or cross-linked polymer layer structures on solids. The added flexibility and functionality results in robust grafted supports with higher capacity and improved accessibility of surface functions. The collective and fast response of such layers could be used for the design of polymer-bonded catalytic systems with controllable activity. 

The major approach to extending the lifetime of propagating species involves reversible conversion of the active centers to dormant species such as covalent esters or halides by using initiation systems with Lewis acids that supply an appropriate nucleophilic counterion. The equilibrium betweem dormant covalent species and active ion pairs and free ions is driven further toward the dormant species by the common ion effect—by adding a salt that supplies the same counterion as supplied by the Lewis acid. Free ions are absent in most systems most of the species present are dormant covalent species with much smaller amounts of active ion pairs. Further, the components of the reaction system are chosen so that there is a dynamic fast equilibrium between active and dormant species, as the rates of deactivation and activation are faster than the propagation and transfer rates. The overall result is a slower but more controlled reaction with the important features of living polymerization (Sec. 3-15). 

The choice of a suitable initiator represents an important step in creating a well-defined polymerization system in terms of initiation efficiency and confrol over propagation. The entire system can only be designed on a stoichiometric base when a quantitative and fast initiation occurs. This is of enormous importance, because the composition of the entire polymerization mixture needs to be varied within small increments in order to control the microstructure. The catalyst needs to be carefully selected from both chemical and practical points of view. Schrock [5,10,12,109,110] and Grubbs systems [6], both highly active in the ROMP of strained functionalized olefins, can offen be used. Since fhe preparafion and in particu-... 

Co-oxidation of indene and thiophenol in benzene solution is a free-radical chain reaction involving a three-step propagation cycle. Autocatalysis is associated with decomposition of the primary hydroperoxide product, but the system exhibits extreme sensitivity to catalysis by impurities, particularly iron. The powerful catalytic activity of N,N -di-sec-butyl-p-phenylenediamine is attributed on ESR evidence to the production of radicals, probably >NO-, and replacement of the three-step propagation by a faster four-step cycle involving R-, RCV, >NO, and RS- radicals. Added iron complexes produce various effects depending on their composition. Some cause a fast initial reaction followed by a strong retardation, then re-acceleration and final decay as reactants are consumed. Kinetic schemes that demonstrate this behavior but are not entirely satisfactory in detail are discussed. 

Around and above Tm, the decay of the B band becomes biexponential, because the prefactors (x — A2) and (A, — x) in Eq. (2.28) become comparable in magnitude. The fast initial decay corresponds to the equilibration part, and the slower second component corresponds to the decay of the equilibrated system. Of course, for this later time period, the equilibrated system behaves as a single kinetic unit (high-temperature region kBA > A + A and kAB > k + kB), and B and A bands have identical decay times given by Eq. (2.33) ... 

For example, the formation of living polymers allows the preparation of block polymers by sequential addition of monomers. It also permits the introduction of functional groups on the ends of each chain. From kinetic considerations of live polymer systems, it follows that, in a batch reaction, a fast initiation step relative to the propagation step will result in a very narrow molecular-weight distribution. It also follows that the molecular weight will be directly proportional to the mole ratio of initiator to monomer. 

The second example concerns a photo-induced e.t. with this same chemical system, to which an amine (e.g. DABCO) is added as an electron donor. The fast decay to the relaxed triplet excited state of benzophenone remains unchanged, but this is now followed by two further reactions the forward e.t. step which forms the radical ions, and the back e.t. of these ions to restore the initial system. 

Lanthanide-based initiator systems offer a fourth possibility, permitting the block copolymerization of lactones with compounds such as ethylene,tetrahy-drofuran, l-LA, trimethylene carbonate, and methyl methacrylate. Detrimental side reactions such as macrocyclic formation, transesterification, and racemiza-tion are absent and the reactions are extremely fast. 

For example, the polymerization of alkyl vinyl ethers using an HC1/ SnCU (or adduct S/SnCl4) initiating system in methylene chloride is very fast even at - 15° C to give polymers with broad and often bimodal MWDs (Figure 17D) [105], Similar effects of solvent polarity are found in the polymerizations of p-alkoxystyrenes [107], styrene [25], and iV-vinylcar-bazole [108],... 

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