Polymer growth centers

Termination. By some reaction, generally involving two polymers containing active centers, the growth center is deactivated, resulting in dead polymer ... 

In ionic polymerizations termination by combination does not occur, since all of the polymer ions have the same charge. In addition, there are solvents such as dioxane and tetrahydrofuran in which chain transfer reactions are unimportant for anionic polymers. Therefore it is possible for these reactions to continue without transfer or termination until all monomer has reacted. Evidence for this comes from the fact that the polymerization can be reactivated if a second batch of monomer is added after the initial reaction has gone to completion. In this case the molecular weight of the polymer increases, since no new growth centers are initiated. Because of this absence of termination, such polymers are called living polymers. 

The situation is quite different in chain polymerization where an initiator is used to produce an initiator species R with a reactive center. The reactive center may be either a free radical, cation, or anion. Polymerization occurs by the propagation of the reactive center by the successive additions of large numbers of monomer molecules in a chain reaction. The distinguishing characteristic of chain polymerization is that polymer growth takes place by monomer reacting only with the reactive center. Monomer does not react with monomer and the different-sized species such as dimer, trimer, tetramer, and n-trier do not react with each other. By far the most common example of chain polymerization is that of vinyl monomers. The process can be depicted as... 

Each monomer molecule that adds to a reactive center regenerates the reactive center. Polymer growth proceeds by the successive additions of hundreds or thousands or more monomer molecules. The growth of the polymer chain ceases when the reactive center is destroyed by one or more of a number of possible termination reactions. 

Polymer growth is terminated at some point by destruction of the reactive center by an appropriate reaction depending on the type of reactive center and the particular reaction conditions. 198... 

This results in strong polarization of the n bond and dissociation of the Ti—C bond, thus promoting insertion into the activator aluminum-alkyl bond. Repetitive insertions of alkene molecules result in lengthening of the polymer chain. This mechanism is also termed bimetallic after the growth center complex species 44. 

The presupposition, that carboxylate anions are the active centers of polymer growth (43, 70, 72) is equally to be rejected the addition of carboxylic acid salts, e. g. sodium acetate is without any effect on the polymerization rate (103). Besides, the concentration of carboxylic groups as found in the resulting polymers used to be very low (28, 29, 60). 

Anionic polymerization of s-caprolactam is used to make cast or RIM polyamide-6. Using a premade lactam chain end and a metal catalyst, it proceeds rapidly at 100-160°C, well below the melting temperature of the polymer, Tm 220°C. The propagation differs from anionic propagation of most unsaturated monomers because the growth center at the chain end is not represented by an anionically activated group but by a neutral N-acy-lated lactam, and the anionically activated species is the incoming monomer (Table 2.26). 

Preliminary data on MMD of our samples are given in Table IV. It is evident that equimolar concentrations of activator and initiator produce PCL polymers characterized by a regularly decreasing polymolecularity index Q, from ca. 2.6 to 2.0. In Figure 1 the number of polymer molecules formed per acyllactam molecule is plotted as a function of initiator concentration. The actual values should be compared to the theoretical value of 1, which corresponds to the assumption that the number of macromolecules would be equal to the number of acyllactam molecules (26J, as in the ideal case of a step-addition of lactam anions to a constant number of growth centers. 

This process of propagation continues until another process intervenes and destroys the reactive center, resulting in the termination of the polymer growth. There may be several termination reactions depending on the type of the reactive center and the reaction conditions. For example, two growing radicals may combine to annihilate each other s growth activity and form an inactive polymer molecule this is called termination by combination or coupling ... 

It was intended to observe the initial polymer growth directly below the forming polymer shell. Therefore, the active centers on the particle sur-... 

Plasma initiation of the chain polymerization is due to formation of a primary free radical R( ), starting the traditional scheme (9-70), and by formation of positive or negative ion radicals, which are also capable of initiating the MMA polymerization. The primary free radical R( ) as well as the charged centers of polymer growth are formed from the absorbed monomers by electron/ion bombardment and UV radiation from plasma. Formation of a positive ion radical from an adsorbed MMA molectrle on the strrface under electron/ion bombardment and UV radiation can be schematically shown as the ionization process... 

Formation of a negative ion radical (which is also a center of polymer growth) from an adsorbed MMA molecule on the surface is due to direct electron attachment ... 

Similarly to process (9-72), the negative ion radical operating as a center of polymer growth also initiates a sequence of attachment processes of further MMA molectrles in the ion-molecular chain-propagation reactions involving the negative ions ... 

The RO radicals then ftmction as the centers of polymer growth. They initiate the graft polymerization, which in the specific case of MMA polymerization into PMMA is the sequence of attachment reactions of the monomer to the center of polymer growth ... 

After this diffusion phase, successively active centers in the inner part of the particles are also provided with monomer and polymer growth from the outside to the inside continues. Because of the hydraulic forces from the growing polymer, fragmentation of the Si02 support from the surface to the interior occurs (to consider as sheU-by-sheU or layer-by-layer fragmentation). Consequently, new active centers are released and the overall polymerization rate increases until the highest possibly activity is reached and the whole support is fragmented in the polymer. 

---