Polymerization chain propagation

In contrast to step polymerizations, chain polymerizations require an initiator (I) to produce reactive centers. Because monomer (M) reacts exclusively with the active center (M ) and not with another monomer molecule, the polymerization rate is usually first order in monomer. In addition to initiation and propagation, chain polymerizations may also undergo transfer and termination reactions (e.g., with reagent A) in which inactive chains (P) are formed [Eq. (3)]. 

FIGURE 1111 Cham propagation in polymerization of styrene The growing polymer chain has a free radical site at the benzylic carbon It adds to a molecule of styrene to extend the chain by one styrene unit The new polymer chain is also a benzylic radical it attacks another molecule of styrene and the process repeats over and over again... 

The departure of dependence of Rp on the concentration of CHP from 0.5 order might be ascribed to induction decomposition of ROOH type to form ROO- radical, which has very low activity to initiate monomer polymerization [40], but can combine with the propagation chain radical to form the primary radical termination. For the same reason, the order of concentration of TBH was also lower than 0.5 when the TBH-DMT system was used as the initiator in MMA bulk polymerization. But in the BPO-DMT initiation system as shown in Table... 

Addition polymerization is employed primarily with substituted or unsuhstituted olefins and conjugated diolefins. Addition polymerization initiators are free radicals, anions, cations, and coordination compounds. In addition polymerization, a chain grows simply hy adding monomer molecules to a propagating chain. The first step is to add a free radical, a cationic or an anionic initiator (I ) to the monomer. For example, in ethylene polymerization (with a special catalyst), the chain grows hy attaching the ethylene units one after another until the polymer terminates. This type of addition produces a linear polymer ... 

Anionic polymerization is better for vinyl monomers with electron withdrawing groups that stabilize the intermediates. Typical monomers best polymerized by anionic initiators include acrylonitrile, styrene, and butadiene. As with cationic polymerization, a counter ion is present with the propagating chain. The propagation and the termination steps are similar to cationic polymerization. 

Certain monomers may act as inhibitors in some circumstances. Reactivity ratios for VAc-S copolymerization (r< 0.02, rVu -2.3) and rates of cross propagation are such that small amounts of S are an effective inhibitor of VAc polymerization. The propagating chain with a terminal VAc is very active towards S and adds even when S is present in small amounts. The propagating radical with S adds to VAc only slowly. Other vinyl aromatics also inhibit VAc polymerization.174... 

More recent work has shown that the observed variation in propagation rate constants with composition is not sufficient to define the polymerization rates.5" 161,1152 There remains some dependence of the termination rate constant on the composition of the propagating chain. Thus, the chemical control (Section 7.4.1) and the various diffusion control models (Section 7.4.2) have seen new life and have been adapted by substituting the terminal model propagation rate constants (ApXv) with implicit penultimate model propagation rate constants (kpKY -Section 7.3.1.2.2). 

The influence of changes in these other variables on MWD in a homopolymerization has not yet been tested, but whatever perturbations are introduced to the feed in a radical polymerization in a laboratory-scale CSTR, they are unlikely to introduce dramatic changes in the MWD of the product because of the extremely short life-time of the active propagating chains in relation to the hold-up time of the reactor. This small change in MWD could be advantageous in a radically initiated copolymerization where perturbations in monomer feeds could give control over polymer compositions independent of the MWD. This postulate is being explored currently. 

Chain polymerization involves three steps. To start the reaction, a catalyst that can generate an active site, such as a free radical (R ), is used. In the initiation step, the radical adds to the double bond, and the radical site is moved to the end carbon. This new radical reacts with another molecule to give a larger radical, and the propagation reaction is imderway. Usually, the number of monomers in the chain is greater than 1000. In the above formulae. 

The authors concluded that the side reactions normally observed in amine-initiated NCA polymerizations are simply a consequence of impurities. Since the main side reactions in these polymerizations do not involve reaction with adventitious impurities such as water, but instead reactions with monomer, solvent, or polymer (i.e., termination by reaction of the amine-end with an ester side chain, attack of DMF by the amine-end, or chain transfer to monomer) [11, 12], this conclusion does not seem to be well justified. It is likely that the role of impurities (e.g., water) in these polymerizations is very complex. A possible explanation for the polymerization control observed under high vacuum is that the impurities act to catalyze side reactions with monomer, polymer, or solvent. In this scenario, it is reasonable to speculate that polar species such as water can bind to monomers or the propagating chain-end and thus influence their reactivity. 

Species present during polymerization Monomer, polymer and propagating chains (at very low concentration) Monomer, dimer, trimer, oligomer, polymer... 

Monomers, such as ethylene, propylene, isobutylene, and isoprene, containing the carbon-carbon double bond undergo chain polymerization. Polymerization is initiated by radical, anionic or cationic catalysts (initiators) depending on the monomer. Polymerization involves addition of the initiating species R, whether a radical, cation, or anion, to the double bond followed by its propagation by subsequent additions of monomer... 

Radical Polymerization. Radical chain polymerization involves initiation, propagation, and termination. Consider the polymerization of ethylene. Initiation typically involves thermal homolysis of an initiator such as benzoyl peroxide... 

Successful application of radical polymerization requires the appropriate choice of the specific initiator to achieve the desired initiation rate at the desired reaction temperature and the realization that higher polymerization rates achieved by increasing the initiation rate (either by increasing [I] or kmolecular weights. Higher radical concentrations result in more propagating chains but each propagates for a shorter time. 

The enantiomorphic site selectivity of (R)-(—)-(263) allows highly syndiotactic PLA to be prepared via the polymerization of meso-LA (Mn = 12,030, Mn calc= 13,540, Mw/Mn= 1.05).802 The ring opening of meso-LA by (R)-(—)-(263) occurs to produce a syndiotactic propagating chain bound to the metal via an (S)-lactic acid unit.803 Attempts to produce syndiotactic PLA via the ROP of meso-LA using rac-( )-(263) instead afforded heterotactic-biased material. 

The use of these initiators to polymerize LA814 and methylglycolide815 has been reported to proceed in a well-controlled fashion. Block copolymers such as PCL-b-PLA have also been prepared. Elimination of PrOH from the reaction of (270) with preformed hydroxyl terminated polymers, followed by lactone polymerization, yields diblocks of CL with polystyrene or polybutadiene.816 The preparation of an ABA triblock has also been reported (A = CL, B = LA) since propagating chains of PLA do not initiate CL ring opening, (270) was pretreated with hydroxy terminated (PCL-b-PLA)-OH 814... 

The polymerization of olefins and di-olefins is one of the most important targets in polymer science. This review article describes recent progress in this field and deals with organo-transition metal complexes as polymerization catalysts. Recent developments in organometallic chemistry have prompted us to find a precise description of the mechanism of propagation, chain transfer, and termination steps in the homogeneously metal-assisted polymerization of olefins and diolefins. Thus, this development provides an idea for designing any catalyst systems that are of interest in industry. 

The additional complexity present in block copolymer synthesis is the order of monomer polymerization and/or the requirement in some cases to modify the reactivity of the propagating center during the transition from one block to the next block. This is due to the requirement that the nucleophilicity of the initiating block be equal or greater than the resulting propagating chain end of the second block. Therefore the synthesis of block copolymers by sequential polymerization generally follows the order dienes/styrenics before vinylpyridines before meth(acrylates) before oxiranes/siloxanes. As a consequence, styrene-MMA block copolymers should be prepared by initial polymerization of styrene followed by MMA, while PEO-MMA block copolymers should be prepared by... 

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