Polymerizations producing periodic copolymers

Until recently periodic copolymers (consisting of more than two monomers) have been a more or less theoretical domain, treating special kinds of polycondensations rather than polymerizations. Not long ago, donor—acceptor complexes (often zwitterions) composed of two or more compounds (monomers) were discovered, homopolymerizing as a whole to yield periodic polymers. 

Saegusa et al. synthesized a number of nucleophilic (MN) and electrophilic (Me) monomers (see Chap. 2, Sect. 5.2), yielding zwitterions which can mutually combine by their oppositely charge ends [237]. 

By suitable choice of monomers, zwitterions composed of three units can be prepared, either from one MN and two kinds of ME 

Zwitterions formed in this way can mutually combine yielding a ternary, periodic copolymer. An example of this kind of copropagation is the copolymerization of the three units 

The number of this kind of periodic copolymer is rapidly increasing [238]. The existence of DA complexes naturally offers a much broader choice for the synthesis of alternating copolymers. 

---

The resulting mixture is then reacted with lactide and subjected to an elevated temperature for a period of time sufficient to produce a copolymer. As a general rule, the reaction temperature for this polymerization will be within the range of 110°C to 160°C. At reaction temperatures tvithin this range, the polymerization will be complete within a period of about 1 to 4 hours. 

Except in very special cases (azeotropic copolymerizations), copolymerization via radical mechanism shows a drift in the composition of the copolymers produced through the polymerization process. Emulsion copolymerization obeys this rule too, although the special features of its mechanism can change the drift process. The most common way to obviate that composition drift is to use the semi-continuous process where, after polymerization has been initiated with a small percent of the total charge (say 10 to 20 %) like in the batch process, most of the charge is added continuously at a much smaller rate (Ra) than the rate (Rp) at the end of the batch period, so that the added charge is polymerized quite instantaneously (J, 2). Then,the composition drift is limited to the initial period and most of the product does possess actually a constant composition. 

Blackwell and coworkers [19] have extensively studied the case of liquid crystalline main-chain copolymers produced by the random polymerization of two or three different monomers. The X-ray scattering patterns of these materials usually display a set of non-periodic diffuse streaks. These diffuse streaks were analyzed by extending the model of disorder of Hosemann to take into account chemical disorder. The description of the data by this model gave the correlation length of the ordering, information on the conformations of the repeat units and proved the random sequence of the copolymers. Subsequent molecular modelling studies revealed the detailed conformation of the repeat units. 

The use of fluidized beds for gas-phase polymerization started in 1968 with the UNIPOL process, which was developed by Union Carbide to make high-density polyethylene. This process has now been adapted to produce other grades of polyethylene as well as polypropylene and various copolymers. The fluid bed is composed of porous particles, which are aggregates of polymer containing fine grains of titanium, chromium, or other metal catalyst. Polymerization takes place at the polymer-catalyst interface, and the particles grow larger over a period of several hours. Some of the polymer is withdrawn continuously or at intervals to maintain the bed... 

---