Copolymers, formation

We begin our discussion of copolymers by considering the free-radical polymerization of a mixture of two monomers. Mi and M2. This is already a narrow view of the entire field of copolymers, since more than two repeat units can be present in copolymers and, in addition, mechanisms other than free-radical chain growth can be responsible for copolymer formation. The essential features of the problem are introduced by this simpler special case, so we shall restrict our attention to this system. 

The tendency toward alternation is not the only pattern in terms of which copolymerization can be discussed. The activities of radicals and monomers may also be examined as a source of insight into copolymer formation. The reactivity of radical 1 copolymerizing with monomer 2 is measured by the rate constant kj2. The absolute value of this constant can be determined from copolymerization data (rj) and studies yielding absolute homopolymerization constants (ku) ... 

Table 7. Relative Ease of Copolymer Formation for 1 1 Ratios of Acrylic and Other Monomers...

Blends have also been prepared by dissolving DMPPO in a monomer and then polymerizing the monomer. An example is an epoxy—DMPPO blend prepared by curing a solution of DMPPO in Epon 828 at 85°C with an alurninum—tetramethylguariidine catalyst. Some copolymer formation is observed. The solutions can be appHed to glass cloth before curing to produce prepregs for composites in appHcations such as printed circuit boards (67). 

Mixtures of monomers can be used to balance properties. This is possible due to the ease of copolymer formation via free-radical polymerization. The glass transition temperature of acrylic copolymers can be predicted from the weight fraction of the component monomers and the glass transition temperatures of the respective homopolymers [20]. Eq. 3 (commonly known as the Fox equation) is reported ... 

Through the addition of a low-molecular weight component that may promote copolymer formation and crosslinking or co-crosslinking reactions. 

In a third type of block copolymer formation. Scheme (3), the initiator s azo group is decomposed in the presence of monomer A in a first step. The polymer formed contains active sites different from azo functions. These sites may, after a necessary activation step, start the polymerization of the second monomer B. Actually, route (3) of block copolymer formation is a vice versa version of type (1). It has been shown in a number of examples that one starting bifunctional azo compound can be used for block copolymer synthesis following either path. Reactions of type (3) are tackled in detail in Section III of this chapter. 

As was explained, block copolymer formation by azo initiators always involves at least one radically polymeri-... 

Monomers that are strong electron donors may undergo spontaneous oupolymeri/.aliun with strong electron acceptor monomers by a radical mechanism. In certain cases homopolymers formed by an ionic mechanism accompany copolymer formation.312,j2s... 

In summary, these exploratory data suggest that the chromatographic method used could be a valuable tool for study of this polymerization reaction. Reasonable data were obtained for amount and composition of the copolymer. Formation of graft polymer and/or nitrile rich polymer was detected. More detailed chromatographic study of this batch polymerization could lead to a practical on-line monitoring method... 

The catalysts described in Table XII cannot be used to make tailored-block copolymers because of reaction (19). The latter continues in the absence of monomer resulting in detachment of chains from the transition metal centers forming hydride (XX). Introducing a second monomer would lead to realkylation of the chain centers giving a homopolymef of the second monomer. Hence mixtures of homopolymers would be obtained with little block-copolymer formation. 

For example, we have described that nearly monodisperse PEs can be formed by 2/ MAO (1 min polymerization, atmospheric pressure 25 °C Mn 52,000, MJMn 1.12 50 °C Mn 65,000, MJMn 1.17) and 38 (Fig. 25)/MAO (1 min polymerization, atmospheric pressure W25n°C M 8000, M /M 1.05 50 °C M 9000, M IM 1.08) [28, 68, 69]. Additionally, Coates and coworkers subsequently reported that Ti-FI catalysts 34 (Fig. 22) and 39 (Fig. 25) can form nearly monodisperse PEs under controlled conditions [70]. With these Ti-FI catalysts, however, synthesizing high molecular weight and narrow molecular weight distribution PEs is generally difficult (e.g., 5 min polymerization, atmospheric pressure, 50 °C 2Mn 132,000, MJMn 1.83 38Mn 24,000, MJMn 1.46) [28, 68]. Moreover, normally, these catalysts cannot be applied to block copolymer formation. 

Studies have shown that this reaction is a result of transesterification between PBT and PC. Transesterification is influenced by many factors, including PBT end groups and catalyst residues. While a little copolymer formation is not a bad thing for the performance of the blend, uncontrolled reaction is unacceptable since the same material could never be made twice. Fortunately, methods to control this chemistry were developed. Generally, addition of certain phosphites is used to quench the transesterification and related reactions [45, 46], Since phosphites are also used as antioxidants and color stabilizers, the quencher was often... 

Lu and coworkers have synthesized a related bifunctional cobalt(lll) salen catalyst similar to that seen in Fig. 11 that contains an attached quaternary ammonium salt (Fig. 13) [36]. This catalyst was found to be very effective at copolymerizing propylene oxide and CO2. For example, in a reaction carried out at 90°C and 2.5 MPa pressure, a high molecular weight poly(propylene carbonate) = 59,000 and PDI = 1.22) was obtained with only 6% propylene carbonate byproduct. For a polymerization process performed under these reaction conditions for 0.5 h, a TOF (turnover frequency) of 5,160 h was reported. For comparative purposes, the best TOF observed for a binary catalyst system of (salen)CoX (where X is 2,4-dinitrophenolate) onium salt or base for the copolymerization of propylene oxide and CO2 at 25°C was 400-500 h for a process performed at 1.5 MPa pressure [21, 37]. On the other hand, employing catalysts of the type shown in Fig. 12, TOFs as high as 13,000 h with >99% selectivity for copolymers withMn 170,000 were obtained at 75°C and 2.0 MPa pressure [35]. The cobalt catalyst in Fig. 13 has also been shown to be effective for selective copolymer formation from styrene oxide and carbon dioxide [38]. 

A similar procedure was described by Eboatu and Ferguson. An object of analysis was the complex obtained by template polymerization of acrylic acid in the presence of poly(vinyl pyrrolidone). The polycomplex was dispersed in dry benzene and treated with diazomethane. The insoluble portion was filtered. The filtrate containing poly(methyl acrylate) was concentrated and finally dried. The insoluble fraction was scrubbed with methanol to extract polyCvinyl pyrrolidone). The residue was further washed with methanol and then dried. These three portions were characterized by IR spectroscopy. It was found that only about 70% separation of the complex is achieved. The occurrence of inseparable portion is attributed to a graft copolymer formation. For the separated... 

Fig. 26. Polymerization of acrylonitrile by polylmethyl methacrylate) mastication. Schematic representation of block copolymer formation and...

Copolymers of 4-methyl-l-pentene with an n-olefin are obtained when 4-methyl-l-pentene is initially polymerized in the absence of the comonomer and then the comonomer is added to the reaction vessel to permit copolymer formation. Initially, pure 4-methyl-l-pentene homopolymer is produced in order to reduce the solubility of the later formed copolymer so that eventually the copolymer does not contain more than about 5% weight percent of homopolymer. 

Most often, the extent of completely alternating copolymer formation, expressed as 100% C02 linkages or 50% C02 content, is very high. With regards to the selectivity of the coupling reaction for copolymer versus cyclic carbonate production, two observations are consistently found, regardless of the catalyst. First, aliphatic epoxides are more prone to cyclic carbonate formation than alicyclic epoxides for example, PO affords propylene carbonate more readily than CHO provides cyclohexene carbonate. Second, in either instance, since it has been shown that the activation barriers for cyclic carbonate production are higher... 

---