Type monomers

Copolymerization. Copolymerization occurs when a mixture of two or more monomer types polymerizes so that each kind of monomer enters the polymer chain. The fundamental structure resulting from copolymerization depends on the nature of the monomers and the relative rates of monomer reactions with the growing polymer chain. A tendency toward alternation of monomer units is common. 

The T of a polymer can be altered by the copolymerization of two or more monomers. The approximate T value for copolymers can be calculated from a knowledge of the weight fraction W of each monomer type and the T (in degees kelvin) of each homopolymer (12). 

The ends of polymer chains are often not representative of the overall chain composition. This arises because the initiator and transfer agent-derived radicals can show a high degree of selectivity for reaction with a particular monomer type (Section 3.4). Similarly, there is specificity in chain tennination. Transfer agents show a marked preference for particular propagating species (Section 6.2.2 and 6.2.3). The kinetics of copolymerization are such that the probability for termination of a given chain by radical-radical reaction also has a marked dependence on the nature of the last added units (Section 7.4.3). 

Polymerizations of methacrylic monomers in the presence of methacrylic macromonomers under monomer-starved conditions display many of the characteristics of living polymerization (Scheme 9.36). These systems involve RAFT (Section 9.5.2). However, RAFT with appropriate thiocarbonylthio compounds is the most well known process of this class (Section 9.5.3). It is also the most versatile having been shown to be compatible with most monomer types and a very wide range of reaction conditions.382... 

One of the major advantages of radical polymerization over most other forms of polymerization, (anionic, cationic, coordination) is that statistical copolymers can be prepared from a very wide range of monomer types that can contain various unprotected functionalities. Radical copolymerization and the factors that influence copolymer structure have been discussed in Chapter 7. Copolymerization of macromonomers by NMP, ATRP and RAFT is discussed in Section 9.10.1. 

Molecular structural changes in polyphosphazenes are achieved mainly by macromolecular substitution reactions rather than by variations in monomer types or monomer ratios (1-4). The method makes use of a reactive macromolecular intermediate, poly(dichlorophosphazene) structure (3), that allows the facile replacement of chloro side groups by reactions of this macromolecule with a wide range of chemical reagents. The overall pathway is summarized in Scheme I. 

Monomer Type of polymerization Structural unit Approximate melting (Tm) or softening (Tg) temperature in Properties... 

Monomer type Example Reaction conditions Properties of polymer... 

X represents the combined number of both types of units in the polymer chain. Eq. (3) applies also to polymers stabilized (see Chap. Ill) with small amounts of monofunctional units, although here it becomes necessary to replace the extent of reaction p with another quantity, namely, the probability that a given functional group has reacted with a bifunctional monomer. Type ii polymers stabilized with an excess of one or the other ingredient will be discussed later. 

Step polymerizations of linear chains can involve either two different bifunctional monomers in which each monomer possesses only one type of functional group (commonly represented by X-X or Y-Y), or a single monomer containing both types of functional groups (common representation X-Y). However, whatever the monomer type, a linear polymer molecule contains, on average, one functional group of each species per chain (molecule). 

Each point in the phase diagram in Fig. 8.8 corresponds to a certain value of a and b, i.e., it represents the possible chemical composition of a molecular population. Variable a forms the horizontal axis, (1 +a) being the number of monomer types. The b axis represents the quality factor of the polymer catalysis. The transition region contains those populations which can have both ordered and disordered... 

Thioureas mainly find use for the vulcanisation of CR, epichlorohydrin (ECO) and some ethylene propylene diene terpolymer (EPDM) compounds. They show high crosslinking activity, with usually adequate compound flow time before onset of the crosslinking. In EPDMs, the thioureas are used as activators for low activity third monomer types and, in the presence of calcium oxide desiccants, in free state vulcanisation of extrudates, etc. The use of thioureas can overcome the retardation caused by the desiccant. In this case some care must be taken otherwise overcompensation may occur. Thioureas are not used in food product applications and are a known health hazard, particularly for pregnant women. 

The main idea of [66] was to consider a model in a sense even more random than just a random sequence of a limited (small) number of monomer types. Instead of considering just N independent random monomers, the authors assumed there are N(N - l)/2 N2 N independent arbitrary en-... 

Figure 17 Spin-lattice relaxation times for six resonances along a PB chain. Trans and cis denote sp3 hybridized carbons in the respective monomer type, trans-trans, trans-cis, cis-cis, and cis-trans. They denote sp2 hybridized carbons in a trans-group with a transgroup as neighbor, a trans-group with a cis-group as neighbor, and so on. Open bars are for simulation, and filled ones for experiment. Values are shown for 273 K (short bars) and 400 K (longer bars).

In principle block copolymers can also be produced by irradiating solutions containing a homopolymer (from monomer type A) and a monomer (type B) (Fig. 5.34). In such cases, polymerisation resembles that of graft polymerisation where the monomer (B) is thought to be initiated by the macroradical produced by the ultrasonic degradation of the homopolymer. In those cases where polymer was deliberately absent, polymerisation was either not observed to occur or proceeded at a similar rate to that in the presence of polymer. 

The initiator used is important for copolymerizations between monomers containing different polymerizing functional groups. Basic differences in the propagating centers (oxonium ion, amide anion, carbocation, etc.) for different types of monomer preclude some copolymerizations. Even when two different monomer types undergo polymerization with similar propagating centers, there may not be complete compatibility in the two crossover reactions. For example, oxonium ions initiate cyclic amine polymerization, but ammonium ions do not initiate cyclic ether polymerization [Kubisa, 1996]. 

The monomer type and ratio change the adsorption properties of the polymer on the calcium carbonate particles. The type of end group (which depends on the polymerisation process) can also influence adsorption characteristics. 

The influence of molecular weight, polymerisation process, monomer type and neutralisation have been studied. 

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