Reaction, chain, copolymer initiation

The correlation of residence time to reaction time is critical in the ability to treat the volume of the channel as a continuous gradient in molecular mass. ATRP is particularly well-suited to this type of device because the reaction can be initiated at a fixed mixing element at the head of the channel where a catalyst and initiator can be brought together. By replacing a small molecule initiator with a polymer chain capable of being reinitiated, copolymers could be prepared. This was done in the thiolene/glass devices with a poly(n-butyl methacrylate) block... 

The possible chain transfer reactions between the initiator radical, the activated monomer, and a growing chain of the monomer applied on the one hand and the preformed polymer on the other are given under (I). The graft copolymer originates either from addition of the monomer applied to the polymer radical formed by the chain transfer (reaction II) or from recombination of two polymer radicals (reaction III), the latter one being less probable. 

The exact nature of the beginning and end of such a polymer chain is not certain. In general, the polymer can be characterized by its average degree of polymerization, i.e., the value of n, or more precisely by the distribution of n values. The heat of polymerization is 17.4 0.2 kcal/mole at 26.19°C. The reaction may be initiated by heat or by means of catalysts. Organic peroxides are typical initiators. Styrene also will polymerize in the presence of various inert materials, such as solvents, fillers, dyes, pigments, plasticizers, rubbers, and resins. Moreover, it forms a variety of copolymers with other mono- and polyvinyl monomers. 

We will now consider successively the different reactions (4) and (5) of this reaction scheme in order to examine what is their relative importance for obtaining block or graft copolymers. Initially it was admitted that the graft copolymers were produced only by reaction (4b), considering that in the competition between monomer and substrate for primary initiator radicals the addition of vinyl monomers [reaction (2)] is much easier than the abstraction of an atom of another molecule [reaction (4 a)]. This assumption is based on the higher value of the activation energy for a chain transfer reaction than for a monomer addition, at least in the case of saturated molecules (88). 

In the past two years, a number of new approaches have been reported for obtaining controlled NCA polymerizations. These approaches share a common theme in that they are all improvements on the use of classical primary amine polymerization initiators. This approach is attractive since primary amines are readily available and since the initiator does not need to be removed from the reaction after polymerization. In fact, if the polymerization proceeds without any chain breaking reactions, the amine initiator becomes the C-terminal polypeptide end-group. In this manner, there is potential to form chain-end-functionahzed polypeptides or even hybrid block copolymers if the amine is a macroinitiator. The challenge in this approach is to overcome the numerous side-reactions of these systems without the luxury of a large number of experimental parameters to adjust. 

Chain transfer to a second polymer can be exploited as a possible avenue for block copolymer synthesis. The homopolymerization of a given monomer A in the presence of a preformed polymer B or the Interaction between two homopolymers in the presence of a cationic initiator (J, 153. 154) produce in the first step of the reaction block copolymers. Synthesis of other block copolymers was... 

In common examples, essentially all of the polymer chains in each of the immiscible polymers are capable of participating in the copolymer-forming reaction by redistribution. This is in contrast to many other processes for in situ copolymer formation where only those few chains bearing reactive functionality participate. Unless the redistribution process is carefully controlled, it is difficult to stop the process to make stable, compatibUized polymer blends. If the reaction is thermally initiated, the blend processing temperatures and residence times must be strictly and reproducibly controlled within narrow Emits to achieve reproducible properties. For prolonged reaction times at a temperature above that necessary to initiate the reaction, one may obtain a broad distribution of block lengths and... 

PO/PP blends have been compatibiUzed by addition of a radical initiator. Early examples were summarized in a review by Teh et al. (1994) additional examples are hsted in Table 5.55. PP can undergo chain scission in the presence of radical initiator to give a terminal radical site. Coupling of this PP radical with a PE radical located on the PE chain may initially yield a graft copolymer. Since the new copolymer may participate in further coupling reactions, a cross-linked copolymer may eventually be formed depending upon reaction time and radical concentration among other factors. 

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