DEGRADATION REACTIONS OF POLYMERS 371 If Ch represents the chromophoric group, the following reactions can occur, where the exponents 1, 2, and 3 denote the singlet, doublet, and triplet states and the asterisk denote an excited state. 1 Ch +hν −→ 1 Ch ∗ (9.1) 1 Ch ∗ +hν −→ 1 Ch ∗∗ (9.2) 1 Ch +hν −→ 2 Ch ∗ +e − (9.3) 1 Ch ∗ −→ 1 Ch ∗ +energy (9.4) 1 Ch ∗ −→ 3 Ch ∗ +energy (9.5) 1 Ch ∗∗ −→ 1 Ch ∗ +energy (9.6) 1 Ch ∗ −→ 1 Ch +hν (9.7) 3 Ch ∗ −→ 1 Ch +hν (9.8) Equations (9.1) and (9.2) correspond to the absorption photons by the chromophore, (9.3) to its photoionisation, (9.4), (9.5) and (9.6) to the emission of a non-radiative energy, and finally (9.7) and (9.8) to a radiative emission. In the case of ethylene/carbon monoxide copolymer, the degradation occurs generally with the triplet state which exhibits the maximum lifetime and corre- sponds to a free-radical carried by the carbonyl group. This radical evolves to give either • an α-scission O O + • • • (with all the subsequent reactions that implies) or • a β-scission O OH O + • • • 9.4.2.4. As for any organic molecule, the sensitivity to thermal degradation of a polymer is in close relationship with the energy of its bonds. Polymers are, however, less stable thermally than its homologous simple molecule. This is due to the disordered motion of the chains above the glass transition temperature and the energy associated with it, which can be concentrated on a particular bond of the macromolecular backbone. 372 REACTIVITY AND CHEMICAL MODIFICATION OF POLYMERS H, etc. or R 2 R 2 R 2 R 1 R 1 R 1 −R • 1 R 2 R 2 R 1 R 1 R 2 R 2 R 2 R 2 R 1 R 1 + R 2 R 2 R 1 R 1 R 2 + • • • This additional energy supply applies only to monomer units that are sufficiently far away from chain ends and does not affect significantly the degradation of side groups. The latter can, however, cause the degradation of the main chain, when the bond linking the main chain to its substituents is weak, the process then corresponds to a chain reaction. Hydrocarbon chains can also undergo a homolytic rupture of their carbon– carbon bonds that are particularly weakened by head-to-head sterically hindered irregular sequences: A A A A A A A A AA + •• Two situations can occur, depending upon the temperature applied with respect to the polymer ceiling temperature (see Section 8.2.1). When this temperature is below its ceiling temperature, the free radicals generated undergo a rearrange- ment with stabilization of the species formed until disappearing by combination or disproportionation: AA AA + H • • • A H A ,etc. Pol-H + Pol DEGRADATION REACTIONS OF POLYMERS 373 With chains containing hetero-elements in their backbone, each polymer is a par- ticular case. For example, in the case of cellulose, the following degradation is observed above 180 ◦ C: O O OH CH 2 OH OH O O Cell OH CH 2 OH OH O OH CH 2 OH OH O + O OH CH 2 OH OH O Cell O OH OH HO CH 2 O + O Cell OH CH 2 O OH HO O Cell O OH CH 2 OH OH O + O Cell O OH CH 2 OH OH O O Cell • • • • •• When the temperature applied approaches or exceeds the ceiling temperature, a homolytic rupture occurs that may result in the total depolymerization and regen- eration of the monomer for several polymers. Methyl methacrylate is recovered in this way from waste of the corresponding polymer: CH 3 CH 3 O O O O CH 3 CH 3 n CH 3 O O CH 3 CH 3 O O CH 3 n + • • To get a better insight into the mechanism/type of degradation, thermogravimetric analysis is the most appropriate technique that is generally used in association with a technique of determination of molar masses. Figure 9.2 shows two typical behaviors. From the knowledge of structural parameters that determine the polymer ther- mostability, the structure of the ideal thermostable polymer can be designed as follows: • The interatomic chemical bonds should be strong. • The chains should be rigid and generate strong molecular interactions in order to exhibit little mobility. 374 REACTIVITY AND CHEMICAL MODIFICATION OF POLYMERS degradation by depolymerization m t M n t degradation by random chain breaking m t t M n Figure 9.2. Variations of both molar mass and mass of polymer samples with time during degradation. (1) depolymerization; (2) random chain breaking. For example, the structure of the polyimide that is shown below meets these criteria; it is approximately thermostable up to 450 ◦ C: N O O N O O O n 9.4.2.5. Polymers are sensitive to mechanical degradation that occur through homolytic scissions, similarly to those caused by thermal degradation. The mas- tication of certain polymers in the molten state, in particular of natural rubber before addition of stabilizing additives, may cause a drastic reduction of their molar masses; in the case of polypropene, thermal and mechanical degradations both com- bine their effects to degrade it. The mechanical energy can also be provided by an ultrasonic generator. 9.4.2.6. When several sources of energy are combined to cause degradation, their synergism can be sometimes spectacular. For instance, degradations by photo- oxidation and thermal oxidation are particularly effective—in particular, in the degradation of polyethylene films. It is frequent that additives that are incorporated in polymers to either generate or improve a given property impart an accelerating effect on the degradation process. The corresponding mechanisms are often complex but are not different from the basic phenomena described above. STABILIZATION OF POLYMERS 375 9.5. STABILIZATION OF POLYMERS The side effects due to degradation can be alleviated through a precise knowledge of the mechanism involved. For instance, the autocatalytic thermal dehydrochlo- rination of poly(vinyl chloride) and poly(vinylidene chloride) suggests that basic additives can well stabilize them and prevent their degradation by neutralizing the HCl gradually formed. Bases utilized for this purpose can be either organic molecules (N,N  - diphenylurea, dihydropyridin, polyols, etc.) or salts or metallic oxides such as 3PbO·PbSO 4 ·H 2 O as well as barium, cadmium, calcium, zinc, lead carboxylates, or thiolates. Instead of preventing the dehydrochlorination process, which generates colored conjugated sequences, it may be more appropriate to use additives that react with the chromophoric polyene formed and reduce the length of the conjugated sequences. A hypsochromic effect is observed in this case, which decreases the absorption in the visible range. When the degradation of a polymer gives rise to free radicals, the addition of compounds that can trap and neutralize these radicals is the logical solution. For example, polyolefins are stabilized by addition of substituted phenols or polyphe- nols, which act as antioxidants: O + H OH + • • The phenoxy radical formed are too stable to propagate the reaction of degradation. Carbon black is also an excellent antioxidant that is commonly used when its col- oration is not a drawback for the application contemplated. It is systematically used to stabilize polyalkadienes as it contributes to the reinforcement of their mechanical properties in addition to its capacity to stabilize against heat and oxidation. There are two categories of products that can protect against photodegradation: • The first corresponds to UV radiation absorbers that possess a molecular struc- ture enabling them to absorb sunlight up to λ =360 nm. These compounds are thus used as screen that absorb photons and dissipate thermally the cor- responding energy; derivatives of benzophenone are often used in this case. When the application contemplated permits, carbon black can also be utilized as a “total screen.” • The second category is that of free radicals traps. For example, hindered amines are excellent light-stabilizing compounds: their oxidation generates stable free radicals called nitroxide that are able to efficiently trap reactive free radicals. Very recently, such nitroxide radicals were used to prevent a 376 REACTIVITY AND CHEMICAL MODIFICATION OF POLYMERS polymer from degrading by a free radical mechanism as shown below: Tetramethylpiperidyloxyl (TEMPO) N−O • ~~~~~~pol • + TEMPO • ~~~~~~pol-TEMPO All the additives used should exhibit a very high compatibility with the poly- mer to stabilize in order to prevent their migration to the surface and their subsequent elimination; the durability of their effect depends on this factor. LITERATURE E. Mar ´ echal, Chemical Modification of Synthetic Polymers,inComprehensive Polymer Science, G. C. Eastmond, A. Ledwith, S. Russo, and P. Sigwalt. Pergamon Press., Oxford, p. 1, 1989. J. C. Arthur, Jr., Chemical Modification of Cellulose and its Derivatives in Comprehensive Polymer Science, G. C. Eastmond, A. Ledwith, S. Russo, and P. Sigwalt (Eds.), Pergamon Press, Oxford, p. 49, 1989. M. Laz ´ ar,T.Bleha,andJ.Rychl ´ y, Chemical Reactions of Natural and Synthetic Polymers, Ellis Horwood Ltd., Chichester, 1989. N. A. Plat ´ e, A. D. Limanovich, and O. V. Noah, Macromolecular Reactions, Wiley, Chichester, 1995. 10 MACROMOLECULAR SYNTHESIS 10.1. INTRODUCTION Chemical methods that are designed to afford polymers of controlled architecture/ structure, corresponding to the expectation of the experimenter, are referred to as macromolecular synthesis. The whole set of these methods is also called macro- molecular engineering, and in many aspects this domain of polymer chemistry is close to that of polymerization reactions and/or that of polymer chemical modifi- cation (see Chapters 7, 8, and 9). The variety of potentially accessible macromolecular structures/architectures is endless, and only the most representative ones are described here. Recent develop- ments in the field of “living” and/or “controlled” polymerizations further expand the possibilities of macromolecular engineering. Arbitrarily, this chapter is divided into three parts devoted respectively to the synthesis of the following basic structures/architectures: • End-functionalized polymers [i.e., α-and(α,ω-di) functionalized polymers], including macromonomers • Block and graft copolymers • Polymers with complex topology. Organic and Physical Chemistry of Polymers, by Yves Gnanou and Michel Fontanille Copyright  2008 John Wiley & Sons, Inc. 377 378 MACROMOLECULAR SYNTHESIS 10.2. END-FUNCTIONALIZATION OF POLYMER CHAINS (SYNTHESIS OF REACTIVE PRECURSORS) Remark. It is important to point out the difference between functionalized polymers, which are polymers carrying functional groups, and functional polymers, which are those exhibiting a specific property and used for a par- ticular application or a given function. Functionalized polymers may also be functional polymers. When an accurate control of the structure targeted is not necessary, it is relatively easy to obtain functionalized chain ends. Widely used in industry, two methods of facile and straightforward functionalization are briefly described below. The simplest method to obtain α,ω-difunctionalized polymers is to resort to step-growth polymerization and control the degree of polymerization of the formed polycondensates by means of the stoichiometry (r) of the initial reactants: + 2a XY + a < b a X– –X b Y– –Y Y– –( – ) n –Y The Carothers relation (Section 7.2.1) for nonstoichiometric conditions gives X n = 1 +r r(1 −2p) + 1 where r =a/b is the stoichiometric imbalance and p is the extent of the reaction. Alternatively, end-functionalization of growing chains can also be obtained via transfer in chain addition polymerization. For instance, dihydroxy polybutadiene telechelics (i.e. carrying one hydroxyl group at each chain-end) are industrially produced by free radical polymerization of butadiene initiated by hydrogen peroxide which simultaneously gives rise to transfer reactions: 2 HOH 2 O 2 HO + n HO∼∼polybutadiene∼∼∼ H 2 O 2 HO + HO∼∼∼polybutadiene∼∼OH etc. n As a matter of fact, the average functionality of polymers prepared under these conditions is slightly higher than 2, due to transfer reactions to polymer chains. The “telomerization” of vinyl monomers is another example of chain end- functionalization by transfer. In the latter case, both propagation and transfer exhibit comparable rates, which affords α,ω-difunctionalized chains of low degree of polymerization also called telomers. For instance, the free radical polymerization of vinyl monomers in halogenated solvents produces α,ω-halogenated oligomers: END-FUNCTIONALIZATION OF POLYMER CHAINS (SYNTHESIS OF REACTIVE PRECURSORS) 379 R + CCl 4 + n A CCl 3 CCl 3 + n' + CCl 3 etc. R n Cl 3 C n' Cl A A Cl A Experimental conditions are generally selected in such a way that n and n  are limited to few units. Some of these telomers can subsequently serve as precursors for polycondensation reactions. To obtain better defined structures than the latter ones, it is advisable to rely on “living” and/or “controlled” polymerizations for the production of functionalized chains: the end-standing functions can be introduced at the initiation step upon selection of an appropriate initiator or at the end of the polymerization through a deactivating molecule carrying the desired functional group. 10.2.1. Functionalization Through Initiator Due to the high reactivity of the propagating active centers, it is often neces- sary to protect the functional group carried by the initiator. For instance, hydroxyl functions in initiators for anionic polymerization require protection, and acetal func- tions are generally used to this end. Such acetal functional groups are introduced at the ends of polystyrene chains by means of an acetal-containing alkyllithium initiator that triggers “living” anionic polymerization; upon coupling, such “living” carbanionic polystyrene with dimethyldichlorosilane, α,ω-bisacetal chains can be indeed generated: (CH 3 ) 2 SiCl 2 O O Cl Li O O 2 2 n n Li powder O O Li n 2(n + 1) Si CH 3 H 3 C O O O O 380 MACROMOLECULAR SYNTHESIS α,ω-Dihydroxyl chains, as well as other end functions (–C–I), can be subsequently obtained by chemical modification of these acetal functional groups. By a similar process, macromonomers—that are chains carrying a polymerizable group at one of their ends—can be synthesized. The homopolymerization of such macromonomers affords “comb-like” polymers and their statistical copolymeriza- tion with simple monomers graft copolymers whose branches have all roughly the same size. The preparation of α-norbornenylpolystyrene is illustrated below: ether, –30°C for ring-opening metathesis polymerization CCl 4 , 60°C Li powder P(C 6 H 5 ) 3 H n (i) benzene (ii)TMEDA (iii) MeOH L i Cl OH 10.2.2. Functionalization by Deactivation of ‘‘Living’’ Chain Ends This method is generally preferred to the preceding one because it allows one to synthesize end-functionalized chains with a variety of functional groups. One of the possible pathways is to react a “living” chain with a bifunctional deactivating molecule used in large excess to minimize the coupling of two growing chains. For instance: PSLi + COCl 2 (in excess) PS–COCl + LiCl However, the reaction of growing active centers with a deactivating molecule, serv- ing as precursor of the functional group to be introduced, is the most commonly used method. The preparation of ω-hydroxy polybutadiene by deactivation of “liv- ing” polybutadiene carbanions by ethylene oxide is an illustration of this strategy: O H + ∼∼polybutadiene–CH 2 –CH=CH–CH 2 –CH 2 –CH 2 –OLi ∼∼polybutadiene–CH 2 –CH=CH–CH 2 –Li ∼∼polybutadiene–CH 2 –CH=CH–CH 2 –CH 2 –CH 2 –OH Macromonomers can be obtained by similar means, as illustrated below with the example of ω-methacryloyl poly(ethylene oxide) macromonomer: [...]... combining the properties of both the amorphous and of crystalline states at the macroscopic level Organic and Physical Chemistry of Polymers, by Yves Gnanou and Michel Fontanille Copyright  2008 John Wiley & Sons, Inc 401 402 THERMOMECHANICAL PROPERTIES OF POLYMERS When comparing macromolecular systems with simple molecules, one may, a priori , think that the thermomechanical properties of polymers would be... that are peculiar and completely different from those of other architectures—in particular, star polymers The size of these dendrimer-like polymers is not solely controlled by the number of generations, but also by the length of the branches of each generation The example shown below illustrates the synthesis of dendrimers of poly(ethylene oxide) Dendrimers made of both polystyrene and poly(ethylene... in mesophases, block copolymers exhibit specific features and properties that are exploited at industrial level The morphology of these mesophases depends primarily, among other parameters, on the nature of the comonomers and the relative length of the blocks, on the dispersion of molar masses, on the possible presence of residual homopolymers, and on the overall architecture of the copolymer (including... eight-shaped bicyclics, and so on 10.4.2 Star Polymers Comb-like polymers (see Section 10.3.2), whose backbone is of small degree of polymerization, tend to adopt a conformation that reminds that of star polymers; as the degree of polymerization of their backbone increases, their conformation undergoes a transition toward a worm-like type Star polymers including branches (if not isometric) of controlled size... proportion of grafts introduced in such graft copolymers is determined by the [PS− ,Li+ ]/[MMA] ratio, and the reaction is fast and total Many other examples of graft copolymers have been described following the same principle 10.3.2.3 Copolymerization of Macromonomers “Comb-type” polymers can be obtained by homopolymerization of a macromonomer (see Section 10.2.1) In the latter case, all monomeric units of. .. groups, these α- or α,ω-difunctionalized polymers are difficult to characterize by conventional methods of analysis due to the very low concentration of the said functional groups The use of techniques of high sensitivity and precision such as mass spectrometry (MALDI-TOF) is helpful with respect to the identification of these functional groups 10.3 BLOCK AND GRAFT COPOLYMERS As shown in the chapter devoted... the physical state of an amorphous polymer 404 THERMOMECHANICAL PROPERTIES OF POLYMERS The behavior of semicrystalline polymers is more difficult to describe Indeed, they not only contain amorphous and crystalline zones but also exhibit a more or less ideal chain packing depending on the molecular irregularities present and the crystallization conditions On the other hand, depending upon the degree of. .. molecules whose degree of polymerization (4, 6, or 8) determines the resulting number of branches 10.4.2.3 Combination of ‘‘Convergent’’ and ‘‘Divergent’’ Methods Contrary to the stars described previously, those resulting from the combination of convergent and divergent methods are characterized by a large fluctuation of their functionality Their core is obtained by copolymerization of a monofunctional linear... example of this method is given by the synthesis of styrene-dimethylsiloxane block copolymers: CH3 PS−, Li+ + Cl CH3 LiCl + Si PS CH3 PS Si CH3 CH3 CH3 CH3 CH2 CH2 Si Si + O PDMS CH3 CH3 H2PtCl6 PDMS O Si H CH3 The synthesis of SBS triblock copolymer obtained by coupling of two “living” SB block copolymers by means of Cl2 Si(CH3 )2 is another well-known example 10.3.2 Synthesis of Graft Copolymers... character of the resulting condensation polymer, the notion of “degree of branching” (DB) was POLYMERS WITH COMPLEX TOPOLOGY 3 97 defined as D+ T D+ L+ T DB = For a perfect dendrimer, DB is equal to 1; and, by contrast, for a linear polycondensate, DB is close to 0 For XY2 -type monomers, the theory predicts a degree of branching of the resulting polymer equal to 0.5, assuming the isoreactivity of all Y . topology. Organic and Physical Chemistry of Polymers, by Yves Gnanou and Michel Fontanille Copyright  2008 John Wiley & Sons, Inc. 377 378 MACROMOLECULAR SYNTHESIS 10.2. END-FUNCTIONALIZATION OF. the synthesis of the following basic structures/architectures: • End-functionalized polymers [i.e., α -and( α,ω-di) functionalized polymers] , including macromonomers • Block and graft copolymers • Polymers. use of techniques of high sensitivity and precision such as mass spectrometry (MALDI-TOF) is helpful with respect to the identification of these functional groups. 10.3. BLOCK AND GRAFT COPOLYMERS As