Molecule-chain transfer agent

The newly formed short-chain radical A then quickly reacts with a monomer molecule to create a primary radical. If subsequent initiation is not fast, AX is considered an inhibitor. Many have studied the influence of chain-transfer reactions on emulsion polymerisation because of the interesting complexities arising from enhanced radical desorption rates from the growing polymer particles (64,65). Chain-transfer reactions are not limited to chain-transfer agents. Chain-transfer to monomer is ia many cases the main chain termination event ia emulsion polymerisation. Chain transfer to polymer leads to branching which can greatiy impact final product properties (66). 

Various techniques have been studied to increase sohds content. Hydroxy-functional chain-transfer agents, such as 2-mercaptoethanol [60-24-2], C2HgOS, reduce the probabihty of nonfunctional or monofunctional molecules, permitting lower molecular-weight and functional monomer ratios (44). Making low viscosity acryhc resins by free-radical initiated polymerization requires the narrowest possible molecular-weight distribution. This requires carehil control of temperature, initiator concentration, and monomer concentrations during polymerization. 

Typical chain transfer reactions involve the abstraction of an atom from a neutral saturated molecule, which may be solvent or a chain transfer agent added to the polymerisation mixture specifically to control the final size and distribution of molar masses in the polymer product. The chain transfer reaction may be represented as in Reaction 2.7. 

Polybromochloromethanes containing two or even one C— Br bond are thus seen to be more efficient chain transfer agents than bromoform with three bromine atoms in molecule. 

While in most of the reports on SIP free radical polymerization is utihzed, the restricted synthetic possibihties and lack of control of the polymerization in terms of the achievable variation of the polymer brush architecture limited its use. The alternatives for the preparation of weU-defined brush systems were hving ionic polymerizations. Recently, controlled radical polymerization techniques has been developed and almost immediately apphed in SIP to prepare stracturally weU-de-fined brush systems. This includes living radical polymerization using nitroxide species such as 2,2,6,6-tetramethyl-4-piperidin-l-oxyl (TEMPO) [285], reversible addition fragmentation chain transfer (RAFT) polymerization mainly utilizing dithio-carbamates as iniferters (iniferter describes a molecule that functions as an initiator, chain transfer agent and terminator during polymerization) [286], as well as atom transfer radical polymerization (ATRP) were the free radical is formed by a reversible reduction-oxidation process of added metal complexes [287]. All techniques rely on the principle to drastically reduce the number of free radicals by the formation of a dormant species in equilibrium to an active free radical. By this the characteristic side reactions of free radicals are effectively suppressed. 

When the polymerization of PS with 17 (X=SPr) was carried out in the presence of thiol as the chain transfer agent, a polymer of uniform molecular weight was obtained with the number of the polymer molecules equal to the sum of those of the molecules of the initiator and thiol (immortal character). 

Polymerization of emulsion SBR is started by free radicals generated by the redox system in cold SBR and by persulfate or other initiator in hot SBR. The initiators are not involved in the molecular structure of the polymers. Almost all molecules are terminated by fragments of the chain transfer agent (a mercaptan). Schematically, the molecules are RSM H. where RS is the C H S pan of a dodccyl mercaptan molecule M is the monomer involved n is the degree of polymerization, and H is a hydrogen atom formerly attached to the sulfur of a mercaptan. In the case of free-radical-initiated polymerization of butadiene, by itself to form homopolymers or with other monomers for fonn copolymers, the butadiene will be about 18% 16% fix-1.4 and 66% trms-1,4-... 

The growing polymer chain may react with stable molecules present in the polymerization system initiator, solvent, monomer, polymer, and chain transfer agent. In chain transfer reactions, the growing polymer chain loses the radicals to the other molecules to form the shorter polymer chains (i.e., dead polymers). For a chain transfer to monomer,... 

In all cases, chain transfer may occur to monomer, solvent, polymer, or a chain transfer agent. For this to be a transfer reaction rather than termination, the molecule generated (A ) must be reactive enough to reinitiate polymerization [Eq. (10)]. 

Transfer reactions to small molecules (T) including monomer, solvent, chain transfer agent (CTA), etc., are all very similar and are generalized as follows ... 

Initiation has been discussed in the previous section. In order for propagation to occur, many product molecules need to be formed from just one radical produced in the initiation step, and this occurs only if the propagation steps are exothermic. Termination steps include coupling, disproportionation, and abstraction. Abstraction by a chain-transfer agent removes a radical from a propagation step, and a new radical is generated in its place. If this newly generated radical is sufficiently stable, the chain transfer results in termination. 

This may lead to a core and shell morphology. Process E in Table I, termination in the aqueous phase, may also lead to homogeneous nuc-leation if the dead molecules are sufficiently water insoluble. When particles have been formed, transfer reactions to monomer (or chain transfer agent if present) will lead to monomer radicals, which may be desorbed into the aqueous phase. This is also indicated in Fig. 1. The monomer radicals may act in a way similar to the initiator radicals. Below is given a detailed description of the different processes listed in Table I and of the theories that have been advanced for these processes-... 

Problem 32.6 (a) Chain transfer can cause branching of a polymer molecule. Show how this could happen. What is the chain-transfer agent (b) Rather short branches (4 or 5 carbons) are attributed to back-biting. What do you think is meant by this term Show the chemical reactions probably involved. 

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