Polymerisation chain growth reaction

These reactions are usually exothermic, thus producing energy. 

The number of molecules combined, n, may vary at the low end between 10 - 20. The products are then called telomers or oligomers. For pol5miers, n is between 1000 and 100000 or more. The polymer growth occurs very rapidly, in seconds or minutes. Thus, fully formed macromolecules exist almost from the beginning of the reaction. However, the overall time required for a high conversion of monomer to pol5mier is often several hours. 

Depending on the activation (type of reaction initiation), a differentiation is made between radical and ionic polymerisation  

Initiators very often need special care since they are either potentially explosive like peroxides or react vigorously with water and are flammable such as metal alkyls. Usually, initiator concentrations vary and are between 0.1-0.5 wt-%. Dissociation products of the radical initiator are removed from the polymer or built-in, while decomposed metal alkyl residues of the initiator remain in the product and sometimes have an influence on end use properties. 

The actual polymerisation may be carried out in bulk, in water or in organic solvents or dispersants. 

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Table 3.7 Isoprene polymerisation by TiCl4-Al(t-C4H9)3 catalyst, kp is the chain growth reaction rate constant is the concentration of AC k, are chain-to-monomer and chain-to-aluminium-organic compound transfer reaction rate constants respectively, w is the initial polymerisation rate, method 1) traditional, 2) hydrodynamic impact on a separately prepared catalytic system, 3) catalytic system formation in a turbulent mode, and 4) preliminary formation of a reaction mixture in a turbulent mode...

When styrene is polymerised, polyst5rene is formed. The polymerisation of styrene is a chain growth reaction and it is induced by any known initiation techniques such as heat, free radical, anionic or cationic addition. The product polystyrene is a white polymer with high clarity and good physical and electrical properties. 

This second reaction leads to the small amount of branching (usually less than 5%) observed in the alcohol product. The alpha olefins produced by the first reaction represent a loss unless recovered (8). Additionally, ethylene polymerisation during chain growth creates significant fouling problems which must be addressed in the design and operation of commercial production faciUties (9). 

A mass of polymer will contain a large number of individual molecules which will vary in their molecular size. This will occur in the case, for example, of free-radically polymerised polymers because of the somewhat random occurrence of ehain termination reactions and in the case of condensation polymers because of the random nature of the chain growth. There will thus be a distribution of molecular weights the system is said to be poly disperse. 

Most of these addition polymerisation are chain growth polymerisations as a particular Polymer chain is formed in a single chain reaction. 

When carbon monoxide is bubbled through a methanol solution of (dppp)Pd(triflate)2 a carbomethoxy-palladium species is formed, which can undergo insertion of alkenes and hence this is a feasible alternative initiation route to chain-growth polymerisation (Figure 12.4) [13], To ensure a clean formation of the carbomethoxy species, however, exclusion of water is a prerequisite. If during the preparation water was present the formation of a palladium hydride complex (dppp)PdFT was observed (reaction (1), Figure 12.2). 

Copolymerisation is a polymerisation reaction in which a mixture of more than one monomeric species is allowed to polymerise and form a copolymer. The copolymer can be made not only by chain growth polymerisation but by step growth polymerisation also. It contains multiple units of each monomer used in the same polymeric chain. For example, a mixture of 1, 3 - butadiene and styrene can form a copolymer. 

In the presence of an organic peroxide Initiator, the alkenes and their derivatives undergo addition polymerisation or chain growth polymerisation through a free radical mechanism. Polythene, teflon, orlon, etc. are formed by addition polymerisation of an appropriate alkene or Its derivative. Condensation poiymerisation reactions are... 

A very promising variant on this type of condensation polymerisation is to use monomers that possess groups X and Y which can be eliminated from the same molecule (Scheme 8.1, Route C). This circumvents the need for careful control of reaction stoichiometry as an equal number of the different functional groups are built in to the monomer. Furthermore, in certain cases, polymerisation of monomers of this type can follow a chain-growth polycondensation type of... 

The first step in catalytic reactions of the Heck type is the oxidative addition of the organic halide to Pd(0) species to form an intermediate organopalladium halide constituting the Pd(II) species. This is followed by insertion of the olefinic bond and subsequent /khydrogen elimination [scheme (30)]. The catalyst is recycled by the reaction of the Pd(II)-hydride species with a base [scheme (31)]. It is worth noting here that palladium species, L2(X)Pd—ArCH=CH2, do not propagate the chain growth polymerisation of the CH2=CHArX monomer via its olefinic bond in the discussed process. 

The structural development model in Figure 19 is a combination of all the information obtained from TEM observation, WAXD and NMR measurements.46 The results of the morphological changes during annealing demonstrate that the actual polymerisation commenced when the temperature was 20 °C higher than the preparation Tpoiy for each of the nascent powders. This low temperature difference corresponds to an exothermic reaction near the active sites on the catalyst, where continuous chain growth occurs. Rapid crystallisation occurs for... 

Finally, in the presence of a non polymerisable olefin the above initiation reaction is not followed by chain growth, but only by elimination (transfer) or alkylation (termination) reactions involving an active ion pair incapable of propagating. 

In our particular type of step-addition polymerisation, monomers, dimers, trimers, oligomers and polymers are the reactive species which participate in the chain growth. Initially, the monomers react with monomers and give dimers, dimers react with monomers and dimers and give trimers and tetramers, respectively. The high MW polymer is formed only in the last stages of the poly addition reaction, at high conversion rates. Chain transfer and termination reactions are absent. 

TTie extension of tandem catalysis to polymer chemistry is, however, not trivial. In order to reach high molecular weight polymers, each reaction has to proceed with almost perfect selectivity and conversion. Obviously, combining different catalytic reactions limits the choice of suitable reactions since they must also be compatible with each other. We recently introduced Iterative Tandem Catalysis (ITC), a novel polymerisation method in which chain growth during polymerisation is effectuated by two or more intrinsically different catalytic processes that are both compatible and complementary. If the catalysts and monomers are carefully selected, ITC is able to produce chiral polymers from racemic monomers, as was shown by us for the ITC of 6-MeCL and the DKR polymerisation of sec-diols and diesters. ... 

Free radicals are reactive enough to attack double bonds, thus initiating polymerisation (Scheme 6.9c). The reaction is able to satisfy the bonding requirements of the initial free radical but creates a new free radical in the process, which can attack another ethene molecule in turn, leading to continued chain growth (Schemes 6.9d and 6.9e). The reactions can be written as follows ... 

The theoretical form of the MWD can be calculated for step-growth polymerisations, since the reaction is at equilibrium. The number fraction fi of molecules with i repeat units is equal to the probability that a molecule chosen at random has i repeat units. Since each step in forming a chain is mutually exclusive, we can multiply the probabilities that the first unit is... 

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