Crystallographically Ordered Polymers - American Chemical Society

Chapter 6

Polymerization in Crystalline Inclusion Compounds Mario Farina, Giuseppe Di Silvestro, and Piero Sozzani

Downloaded by GEORGETOWN UNIV on August 22, 2015 | http://pubs.acs.org Publication Date: March 26, 1987 | doi: 10.1021/bk-1987-0337.ch006

Dipartimento di Chimica Organica e Industriale, Universitàdi Milano, Via Venezian 21-I 20133 Milano, Italy

The polymerization of monomers included as guests in the crystal structure of channel-like inclusion compounds extends the fields of application of solid state polymerization far beyond the melting point of pure monomers. Depending on the nature of both host and guest compounds, polymers can be obtained having a wide variety of structural features: regio- and stereospecificity can reach very high values; optical activity is induced by using chiral crystal lattices; extended-chain macroconformations are observed under specific working conditions. Inclusion polymerization proceeds by a radical mechanism and i t s living character has been demonstrated in some cases. Recent results concern the production of block and statistical copolymers and a new synthesis of polyethylene and polypropylene by high-pressure inclusion polymerization.

C r y s t a l l i n e inclusion compounds containing unsaturated monomers are e f f e c t i v e reactive systems for the production of l i n e a r polymers (1-6). This process belongs to the wider class of s o l i d state polymerization, but possesses some s p e c i f i c features which make i t worthy of a separate description. Throughout t h i s a r t i c l e , the polymerization i n inclusion compounds w i l l be referred to as " i n clusion polymerization" (other names currently used in the s c i e n t i f i c l i t e r a t u r e are channel, canal or tunnel polymerization), and the terms "clathrate" w i l l be used as synonymous with "inclusion compound". When there i s no r i s k of confusion, the more general term of "adduct" w i l l be used for clathrate: i n p r i n c i p l e a

0097-6156/87/0337-0079$06.00/0 © 1987 American Chemical Society

In Crystallographically Ordered Polymers; Sandman, D.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.


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CRYSTALLOGRAPHICALLY ORDERED POLYMERS

d i s t i n c t i o n ought to be made between true adducts i n which s p e c i f i c chemical interactions exist (hydrogen bonding, ion-dipol and other medium-strength interactions related to the chemical nature of the host and guest molecules) and clathrates where the host-guest interactions are e s s e n t i a l l y s t e r i c i n o r i g i n (7). Different classes of clathrates exist, depending on the geometry of the c a v i t i e s i n which guest molecules are confined. As regards inclusion polymerization, channel-like (or tubulate) clathrates are p a r t i c u l a r l y suited as the constraints i n space exerted by the host molecules induce the formation of polymers with no branching or crosslinking. Channels are 10-15 A apart and, at least when i n perfect c r y s t a l s , are not connected. As a consequence they behave as independent reaction vessels, each giving r i s e to an isolated macromolecule. Some of the features existing i n the included macromolecules can be retained i n the "native" polymer. D i s t i n c t i v e Features of Inclusion Polymerization In inclusion polymerization a monomeric clathrate i s transformed into a polymeric one (where monomeric and polymeric refer to the nature of the guest molecules)(Fig. 1) or into a mixture of polymer and host when the polymer does not possess the s t e r i c requirements for inclusion. The r e a l occurrence of polymerization inside the channels was demonstrated i n several ways. I t does not occur, at least for a number of monomers, when there i s a simple mixture of host and monomer without the formation of a clathrate or when the monomer i s placed i n the presence of s o l i d substances unable to form inclusion compounds. Even i n cases when polymerization does take place the structure of the polymer formed outside the channels d i f f e r s from that obtained i n proper conditions. The reaction rate i s very temperature and pressure dependent and has a sharp drop-off point beyond which reaction ceases. The boundary conditions for polymerization correspond to those which delimit the f i e l d of thermodynamic s t a b i l i t y of the monomeric clathrate, determined by vapor pressure measurements or by DSC. This coincidence enables us to state that the two phenomena, monomer inclusion and polymerization, are s t r i c t l y related. In addition, i n some t y p i c a l cases a structural change from monomer to polymer was d i r e c t l y observed inside the channels by X-ray analysis. A discussion of inclusion polymerization within the frame of s o l i d state polymerization requires the s p e c i f i c a t i o n of the points which distinguish the two processes. In inclusion polymerization the s o l i d phase consists of two components, host and guest. The former i s a c r y s t a l l i n e substance which possesses a strong tendency to polymorphism. Generally hosts are able to c r y s t a l l i z e i n



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d i f f e r e n t forms, some of which retain a d e f i n i t e amount of solvent or other substances. The role of the host i n inclusion polymerization i s manifold: i t s t a b i l i z e s the s o l i d phase f a r beyond the melting point of the monomeric guest, i t selects i n a more or less s p e c i f i c way the guests according to their shape and dimensions, i t imposes on them a regular arrangement inside the channels and keeps the growing chains i n well separate channels thus preventing the formation of branched or crosslinked structures. Moreover, i t can produce the i n i t i a t i n g species for polymerization and s t a b i l i z e the growing chain-ends. These l a s t points were c l e a r l y demonstrated in a s p e c i f i c case, however i t i s probable that they are generally valid. The most common hosts f o r inclusion polymerization are: urea, thiourea, perhydrotriphenylene (PHTP), deoxycholic acid (DCA), apocholic acid (ACA) and tris(o-phenylenedioxy)cyclotriphosphazene (TPP)(Fig. 2). They have the common feature of forming channel-like clathrates, but d i f f e r i n many s p e c i f i c properties. For instance, urea and thiourea have a r i g i d structure i n which the host molecules are connected by hydrogen bonds and possess a high s e l e c t i v i t y towards the guests. In urea channels are rather narrow whereas i n thiourea they are wider; as a consequence, linear molecules include only i n urea and branched or c y c l i c molecules i n thiourea. On the contrary, channels existing i n PHTP clathrates are very f l e x i b l e and can accomodate l i n e a r , branched and c y c l i c molecules. The a b i l i t y of a molecule to act as a guest depends primarily on how i t f i l l s the clathrate c a v i t i e s . In t h i s respect the chemical nature of the guest and host i s only of limited importance: polar hosts, such as urea and thiourea, and apolar ones, such as PHTP, can include hydrocarbons, alcohols and ethers, carboxylic acids and esters, etc., i . e . a number of molecules of very d i f f e r e n t p o l a r i t y . Knowing the thermal s t a b i l i t y of clathrates permits the prediction of experimental conditions f o r polymerization (8). A detailed analysis of this problem requires the examination of a l l the involved phases, p a r t i c u l a r l y the s o l i d and l i q u i d phases. Equations f o r phase e q u i l i b r i a were derived from within the framework of the regular solution theory; they contain an interaction parameter W, (whose value i s always p o s i t i v e or zero for ideal solutions), which measures the tendency of host and guest to segregate i n the l i q u i d phase. The melting or decomposition point i s very sensitive to the value of W, especially when i t exceeds 2 RT, i . e . when a m i s c i b i l i t y gap i s observed i n the l i q u i d phase. For t h i s reason the PHTP-hydrocarbon clathrates melt congruently between 115 and 180°C, whereas the urea-hydrocarbon


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Figure 1. Schematic drawing of i n c l u s i o n polymerization. (Reproduced with permission from reference 12. Copyright 1982 Huthig and Wepf Verlag, Basel.)

Urea NH -CO-NH 2 2

Thiourea NH -CO-NH 2 :

Perhydrotriphenylene PHTP

Deoxycholic acid DCA

Tris-(o-phenylendioxy)cyclotriphosphazene TPP

Apocholic acid ACA

Figure 2. Hosts used i n i n c l u s i o n polymerization.


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adducts show incongruent decomposition points at lower temperatures (9). In any case the temperature at which a c r y s t a l l i n e clathrate containing the monomer s t i l l exists i s f a r higher than the melting temperature of the pure guest. As a consequence, inclusion polymerization offers two d e f i n i t e advantages over common s o l i d state polymerizations: i t permits the use of simple monomers, widely used i n other f i e l d s of polymer chemistry, and extends the range of experimental conditions 100 - 200 Κ above the temperature used for the polymerization of a given monomer i n the s o l i d state. A topochemical condition f o r polymerization i s the proper approach of successive monomers at the growing chain-end within the channels. In t h i s respect, conjugated dienes l i k e butadiene, isoprene, etc. possessing reactive atoms i n terminal positions, are very suited to inclusion polymerization. However, even bulkier monomers such as substituted styrenes or methyl methacrylate can polymerize i f the space available inside the channels permits a favorable orientation and/or conformation of the monomer. The most studied examples are: butadiene, v i n y l chloride, bromide and f l u o r i d e , and a c r y l o n i t r i l e i n urea; 2,3-dimethylbutadiene and 2,3-dichlorobutadiene i n thiourea; butadiene, isoprene, c i s - and trans-pentadiene, trans-2-methylpentadiene, ethylene and propylene in PHTP; butadiene, c i s - and trans-pentadiene, c i s - and trans-2methylpentadiene i n DCA and ACA; butadiene, v i n y l chloride, 4-bromostyrene, divinylbenzene, a c r y l o n i t r i l e and methyl methacrylate i n TPP. The common method f o r inducing polymerization i n clathrates i s their exposure to high-energy radiation. Generally, when a preformed clathrate i s irradiated, radicals deriving from both host and guest are produced. In the case of PHTP a d i f f e r e n t process can be used, that of i r r a d i a t i n g the pure host and the subsequent inclusion of the monomeric guest; i n t h i s way, at the s t a r t only r a d i c a l s derived from the host are present. As we s h a l l see l a t e r , polymerization proceeds by a l i v i n g r a d i c a l mechanism, thus permitting the formation of block copolymers by subsequent inclusion of two monomers i n the same host. The most notable feature of inclusion polymerization i s the high degree of constitutional and s t e r i c control found f o r many host-guest systems. This i s p a r t i c u l a r l y true f o r urea, thiourea, PHTP and TPP on one side, and diene monomers on the other. For instance, butadiene, 2,3-dimethylbutadiene, trans-pentadiene, trans-2-methylpentadiene give 1,4-trans crystalline polymers; polypentadiene and poly-2-methylpentadiene, which possess an asymmetric carbon atom i n every monomeric unit, are highly i s o t a c t i c . A notable exception, which w i l l be discussed l a t e r , concerns isoprene. For other hosts, l i k e DCA and ACA, structural control i s lower and strongly dependent on the bulkiness of the monomer and on the presence of additives.




84

PHTP i s a c h i r a l host which can be resolved into enantiomers; DCA and ACA are (or derive from) naturally occurring o p t i c a l l y active compounds. Using these hosts inclusion polymerization can be performed i n a c h i r a l environment and can be used for the synthesis of o p t i c a l l y active polymers. This l i n e of research has been very f r u i t f u l , both on the synthetic and on the theoretical plane. I t has been ascertained that asymmetric inclusion polymerization occurs by a "through space" and not by a "through bond" induction: only s t e r i c host-guest interactions (physical i n nature) and not conventional chemical bonds are responsible for the transmission of c h i r a l i t y (10). Stereochemical control extends to conformational aspects too. SAXS and DSC r e s u l t s are consistent with the presence of an extended chain macroconformation i n some "native" polymers, i . e . i n polymers extracted from the clathrate without dissolving or swelling (11,12). A l l the important aspects of inclusion polymerization have been reviewed several times i n recent years (3-6). Interested readers can refer to the l i t e r a t u r e c i t e d i n order to have a more complete knowledge of the s p e c i f i c examples. In the next pages, we s h a l l discuss some points which have been recently developed i n our laboratory. An Electron Spin Resonance Study The mechanistic study of a polymerization process generally requires the combined use of different methods. Among them we r e c a l l the determination of the chemical and stereochemical relationships between polymer and monomer structures, the detection of reactive intermediates and the k i n e t i c approach. The first method has a wide application due to the quantity and quality of the possible information; i t generally involves an NMR (or X-ray) analysis of products and reagents. Kinetics can hardly be applied to inclusion polymerization due to the high dispersion of experimental data. In the few cases where the detection of reactive intermediates can be successfully r e a l i z e d , a unique and deep knowledge of the intimate reaction mechanism has been obtained. ESR spectroscopy has sometimes been used for the study of r a d i c a l polymerization i n the c r y s t a l l i n e s o l i d state or i n frozen solutions (13). In the f i e l d of inclusion polymerization an ESR study of the urea-butadiene system was reported a long time ago (14). PHTP offers two d e f i n i t e advantages over other hosts: the large number of monomers which can polymerize i n i t s clathrates (a property related to the high f l e x i b i l i t y of the c r y s t a l structure) and the possibility to perform polymerizations using a



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preirradiated matrix and without subjecting the monomer i t s e l f to i r r a d i a t i o n . The results we obtained can be summarized as follows (15): 1) the nature of radicals was determined at various stages of polymerization and under d i f f e r e n t conditions; 2) the l i v i n g character of inclusion polymerization was d e f i n i t e l y confirmed; 3) the way of insertion of unsymmetric monomers was established; 4) an indication on the conformational mobility o f the chain end was obtained. Irradiation of pure PHTP produces saturated hydrocarbon r a d i c a l s which are stable f o r weeks and months; the spectral complexity i s probably due to the presence of a mixture of d i f f e r e n t r a d i c a l s . After monomer inclusion, the spectrum changes i n a few minutes and converts to that of an a l l y l type r a d i c a l . As an example, i n the case of butadiene the spectrum recorded at room temperature consists of s i x l i n e s , spaced 14 gauss, with the intensity r a t i o 1:5:10:10:5:1, i n agreement with structure 1):

3 a a a -CH -CH=CH-CH^ CI C2 C3 C4 x

2

t

2

I t i s assumed that the f i v e ol and ρ hydrogens are coupled with the unpaired electron by the same constant and that the a hydrogen has a much smaller coupling constant owing to i t s lower spin density. The spectrum i s temperature dependent: at -150°C i t appears as an i l l - r e s o l v e d curve, but reverts rapidly to the o r i g i n a l shape at room temperature. This fact indicates a notable conformational mobility around the C1-C2 bond. Isoprene, pentadiene, 2,3-dimethylbutadiene and 2-methylpentadiene give spectra corresponding to the following r a d i c a l s (from 2 to 5) respectively) (Fig. 3): 1

2

-CH -C(CH )=CH-CH* CI C2 C3 C4

2)

-CH -C(CH )=C(CH )-CH' CI C2 C3 C4

4)

2

-CH -CH=CH-CH'(CH ) CI C2 C3 C4 3

-CH -C(CH )=CH-CH *(CH ) CI C2 C3 C4

3)

5)

In a l l cases the spectrum does not change with temperature and


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i t s m u l t i p l i c i t y i s one degree lower than expected. This fact i s interpreted as evidence of r e s t r i c t e d rotation around the C1-C2 bond inside the channel: i n the frozen conformation one of the hydrogens, that marked by an asterisk i n F i g . 4, i s roughly perpendicular to the ρ o r b i t a l of the a l l y l system and does not give a s i g n i f i c a n t coupling. Under t h i s hypothesis, substitution of t h i s hydrogen by a methyl group should not change the spectral multiplicity: this i s exactly what happens during the polymerization of 2-cis,4-trans-hexadiene whose spectrum i s indistinguishable from that of trans-pentadiene. The s t a b i l i t y of the radicals and the ease of formation of PHTP clathrates permits the r e a l i z a t i o n of an unusual experiment. After recording the spectrum i n the presence of a given monomer, the ESR tube i s degassed and a second monomer i s introduced. A fast transformation of the spectrum i s observed and the new r a d i c a l corresponds to that predicted f o r the second monomer. When this experiment i s performed on a preparative scale, i t leads to the formation of a block copolymer. This point i s of c r u c i a l importance for an understanding of the reaction mechanism because i t unequivocally proves the l i v i n g radical nature of this polymerization. Radicals are not simply side-products formed during i r r a d i a t i o n but are the true active chain-ends; they change with the change of the included monomer and are stable i n d e f i n i t e l y . ESR spectra of unsymmetric monomers l i k e isoprene, pentadiene and 2-methylpentadiene indicate, within the l i m i t s of the method, the presence of only one (or of p r e v a i l i n g l y one) of the two possible r a d i c a l s , that originated by the 1,4 and not by the 4,1 i n s e r t i o n . As a rule, we can state that those radicals are preferentially formed which allow f o r extensive coupling with the hydrogens of the methyl groups, i n other words those r a d i c a l s that contain methyl groups bonded to C2 or C4 and not to CI or C3. This finding has a direct implication on polymerization. In the case of pentadiene and of 2-methylpentadiene, both electronic and topochemical factors are i n favor of a highly regular 1,4 insertion: the reactive chain-end consists of a secondary carbon (C4) and the monomer approaches with i t s less hindered side (CI). For isoprene, where the topochemical factors are less active, 1,4 insertion prevails, but a s i g n i f i c a n t amount of the opposite 4,1 insertion i s present, as revealed by NMR spectra. Regioselectivity i n Isoprene Polymerization When isoprene was polymerized f o r the f i r s t time i n PHTP clathrates, a 1,4-trans structure was assigned to the polymer on the basis of i t s IR spectrum (16). In spite of i t s high s t e r i c


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purity and i n contrast with samples obtained by other methods, i t does not appear to be c r y s t a l l i n e . ^ T h i s discrepancy was solved several years ago by examining the C NMR spectrum (10,17): the presence of four signals i n the saturated methylene region indicates the presence of head-to-head, t a i l - t o - t a i l besides head-to-tail sequences. The two adjacent methylene carbons are anisochronous i n head-to-tail sequences (they correspond to carbons CI and C4) and isochronous (at least at the dyad level) f o r head-to-head (CI and CI) and t a i l - t o - t a i l (C4 and C4).

-CH -C(CH )=CH-CH -CH -C(CH )=CH-CH C4 CI


2

3

2

2

3

2

DD dyad head-to-tail junctions

-CH -CH=C(CH )-CH -CH -CH=C(CH )-CH Cl C4

II dyad

-CH -CH=C(CH )-CH -CH -C(CH )^CH-CP^Cl CI

ID dyad

head-to-head junction

-CH -C(CH )=CH-CH -CH -CH=C(CH )-CH C4 C4

DI dyad

tail-to-tail junction

2

3

2

2

3

3

2

2

2

2

2

2

3

2

3

3

2

A quantitative determination of r e g i o s e l e c t i v i t y i s carried out on t h i s basis by using the experimental signal intensities. Nothing, however, could be inferred regarding the d i s t r i b u t i o n of sequences from the analysis of the spectrum run at 25.2 MHz. The improvement of resolution obtained at 50.3 MHz enabled us to investigate the polymer m i c r o s t r u c t u r j i n greater d e t a i l (18) ( F i g . 5). As was shown i n our study on C NMR spectra of 1,4-trans methylsubstituted polybutadienes, unsaturated carbons are i n a more favorable position f o r examining sequences of three monomeric units. As a matter of fact, the signal of the o l e f i n i c C2 carbon of polyisoprene s p l i t s into four signals, f a l l i n g between 134.9 and 135.2 ppm, thus indicating a t r i a d s e n s i t i v i t y . The main peak, which i s placed upfield, i s clearly related to head-to-tail, head-to-tail sequences (ht,ht); the smallest, placed downfield, i s unequivocally assigned to head-to-head, t a i l - t o - t a i l (hh,tt) sequences. The two peaks of equal intensity placed i n the middle belong to t t , h t and ht,hh sequences. 3




1 3

Figure 5. 50.3 MHz C NMR spectrum of polyisoprene obtained i n PHTP. Expanded spectra correspond to C2 ( l e f t , t r i a d s e n s i t i v i t y ) and CI (right, tetrad s e n s i t i v i t y ) .


6.

-CH -C(CH )=CH-CH -CH -C(CH )=CH-CH -CH -C(CH )=CH-CH t h t h

DDD

triad

-CH -CH=C(CH )-CH -CH -C(CH )=CH-CH -CH -C(CH )=CH-CH h h t h

IDD

triad

-CH -C(CH )=CH-CH -CH -C(CH )=CH-CH -CH -CH=C(CH )-CH 2 3 3 2 2 3 2

DDI

triad

-CH -CH=C(CH )-CH -CH -C(CH )=CH-CH -CH -CH=C(CH )-CH h h t t

IDI

triad

2

3

2

2

2

3

2

2

3

2

2

t


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h

t

t

A s t a t i s t i c a l analysis of the sequence d i s t r i b u t i o n can be performed i n terms of d i r e c t and inverted units (D and I ) , i . e . of units written with carbon CI at the l e f t or at the r i g h t , respectively. Dyads DD and I I , which d i f f e r i n the sense of observation, correspond to head-to-tail, ID to head-to-head and DI to t a i l - t o - t a i l junctions respectively. In the same way t r i a d s of D or I units are related to longer sequences. Remember that DDD and III, IDD and IID, DDI and DII, IDI and DID cannot be distinguished from each other. An interpretation according to a f i r s t - o r d e r Markov chain requires the use of two conditional p r o b a b i l i t i e s , p^ and P î t h i s scheme reduces to a simple Bernoulli d i s t r i b u t i o n when p ^ + ρ = 1. Equations giving the r e l a t i v e frequency of the various sequences are reported i n Table 1. I t should be pointed out that two solutions, i n which the values of p ^ and p^ are exchanged, are possible for t h i s system. The choice of the correct value i s not possible on s t a t i s t i c a l grounds only, but requires independent experimental data obtained, f o r example, from ESR analysis. The ESR spectrum of the growing chain-end i n isoprene polymerization consists, as already explained, i n an a l l y l r a d i c a l with the unpaired electron delocalized on C2 and C4: -CH -C(CH )=CH-CH , corresponding to a p r e v a i l i n g i n s e r t i o n of D (or 1,4) units. As a consequence, for our discussion we s h a l l use a p^ value greater than P « In the s p e c i f i c case of a polymerization carried out at -40°C values of p^ and p^ are 0.806 and 0.103 respectively. As a f i r s t approximation, insertion of d i r e c t and inverted units follows a Bernoulli process (ρ + ρ = 0.909). Similar r e s u l t s were also obtained for the s t a t i s t i c a l analysis of inversion i n poly-2-chloro butadiene (19) and i n fluorinated polymers (20). However i n our case a s l i g h t bias towards the formation of pairs of I units i s systematically observed over the entire temperature range. D I

D

2

D

3

2

D I

β ι


90


Table I. P r o b a b i l i s t i c Relationships i n Polyisoprene (ht) = DD • I I = 1 - 2 p

(hh) = ID = ( t t ) = DI = p (ht.ht) = DDD

p

i D

i D

III = 1 + P

+

D I

P

I D

/(p

P )

l D +

D I

/(P

P

D I

M

• P )

I D

D I

- 4 p

l D

P

D I

/(P

+ P >

I D

M

(ht.hh) = IDD + IID = (tt.ht) = DDI + DII =


=

2

P

P

ID D I

/ ( P

(hh.tt) = IDI + DID = ρ

(hh.tt) + ((hh.tt) p

ID

+

V

- ID DI P

P

ρ

2

2

y

- ((hh.tt) + (ht.hh)) ( h h . t t ) )

2

=

(hh.tt) + (ht.hh) P

d i

= (hh,tt)/P

iD

A careful examination of the CH region shows other elements useful f o r a s t r u c t u r a l investigation. When observed i n well resolved spectra the head-to-head signal at 38.51 ppm s p l i t s into two peaks, which can be assigned to sequences of four monomeric units (see F i g . 5). Only three tetrads centered on a head-to-head (or ID) junction exist f o r polyisoprene: DIDD (or i t s equivalent IIDI), DIDI and HDD. The l a s t two are centrosymmetric ( i n t h e i r zig-zag planar representation) and each gives r i s e to a single signal f o r the central CI carbons. The f i r s t i s not symmetric and presents two heterotopic carbons. As a consequence, we can expect four signals at most. However, differences i n structure are so f a r from the observed nucleus that some of the resonances are not resolved at 50.3 MHz. I f we take into account only α' , β and ε' contributions ( i n the sense indicated i n a previous paper of ours) (17) we predict the existence of two signals only, one formed by the CI carbon of the inverted unit (I) of tetrad DIDD and by the two equivalent carbons i n the middle of DIDI tetrad, the other by the CI carbon of the D central unit of DIDD and the two central carbons of HDD. The temperature dependence of the r e g i o s e l e c t i v i t y observed i n isoprene inclusion polymerization was investigated between -60 and +70°C. As expected, the number of defects increases with increasing temperature and ranges from 8 to 26%, expressed as f r a c t i o n of inverted units. I f we plot log P / P and log ρ /ρ n T

n n


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vs. 1/T the difference i n activation energy can be calculated f o r the insertion of an inverted unit with respect to a direct one, either when the growing r a d i c a l i s D* or i " ; Ε amounts to 4.5 and —1 a 3.9 kJ mol respectively. The closeness of these values i s again in favor of a roughly Bernoullian d i s t r i b u t i o n of monomeric units. The detailed analysis reported f o r polyisoprene was made possible by the extreme s e l e c t i v i t y of polymerization i n PHTP clathrates which i n h i b i t s the presence of any other structure, both at the constitutional (1,2 or c y c l i c units vs. 1,4) and at the s t e r i c l e v e l ( c i s vs. trans). Copolymerization Inclusion compounds allow the r e a l i z a t i o n of copolymerization in the c r y s t a l state (1-6). This i s a further difference with respect to t y p i c a l s o l i d state reactions. Both block- and s t a t i s t i c a l copolymers can be obtained: the former involves a two-step process, with subsequent inclusion and polymerization of two different monomers (21); the l a t t e r requires the simultaneous inclusion of two guests. This phenomenon has a much wider occurrence than thought at f i r s t , especially when a not very selective host such as PHTP i s used. Research with this host started with mixtures of 2-methylpentadiene and 4-methylpentadiene, two almost exactly superimposable molecules (22), but was successfully extended to very d i s s i m i l a r monomers, such as butadiene and 2,3-dimethylbutadiene. When ternary clathrates form, the s t r u c t u r a l problem concerning the arrangement of guests i n the channel remains undefined. From t h i s point of view useful information can be derived just from copolymerization, which acts as an unconventional probe f o r structural analysis. I f we admit that interchange between included monomers i s slow, the sequence of monomer units i n the copolymer corresponds to that of the guests i n the channel before polymerization. The polymer chain behaves as a recording tape or a permanent copy of an otherwise elusive intermolecular arrangement (23). The investigation carried out on homopolymerization and copolymerization of diene monomers included i n PHTP allowed us to make evident that the same stereochemical control exists for the two processes (17,24). In both cases 1,4-trans units only are produced, thus permitting a straightforward sequential analysis. The only difference concerns r e g i o s e l e c t i v i t y , which is somewhat lower in copolymerization. For instance, poly(butadiene-co-pentadiene) contains 1 - 3% of head-to-head pentadiene-pentadiene dyads with adjacent tertiary carbons,


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-CH(CH )-CH(CH )-, which are not detectable i n the corresponding homopoîymer. ?he disorder existing among the guests probably extends to the host l a t t i c e thus making i t possible to generate small amougts of usually forbidden sequences. The C NMR spectra of diene homopolymers and copolymers were interpreted on the basis of a f u l l y consistent set of additive parameters (17,24). The presence of methyl groups influences the chemical s h i f t of the chain carbon atoms even when they are four or f i v e bonds away s (δ and e parameters). As a consequence the spectrum generally responds to the structure of t r i a d s of monomeric units, the unsaturated carbons often being more sensitive to the polymer microstructure than the saturated ones. The methyl spectrum i s comparatively less detailed, i n contrast with what happens i n other known cases (polypropylene). As an example, poly(2-methylpentadiene-co-4-methylpentadiene) presents 16 signals at low f i e l d , corresponding to the eight triads observed on the C2 and C3 carbons ( F i g . 6). Quantitative spectra can give an estimate of the probability parameters according to the copolymerization theory. Poly(2-methylpentadiene-co-4-methylpentadiene) possesses a Bernoullian d i s t r i b u t i o n of monomer units, moreover polymer composition i s the same as that of the monomer mixture used f o r the formation of the inclusion compound. I t can therefore be considered as an ideal azeotropic copolymer. In other instances, we observed that ρ + p ^ i s often lower than 1, (or that the product r ^ . r ^ i s greater than 1 ). This fact indicates a tendency toward the formation of blocks of l i k e units. A single example of terpolymerization i s known. I t was performed using a PHTP clathrate containing pentadiene, 2-methylpentadiene and 4-me^hylpentadiene (24). Its actual formation was demonstrated by C NMR: the spectrum can i n the main be interpreted on the basis of the three known copolymers, but there are four signals that should be assigned to mixed t r i a d s . No indication of s t e r i c disorder has been observed i n spite of the presence of three d i f f e r e n t guests i n the same channel. High Pressure Inclusion Polymerization F i n a l l y , we wish to comment b r i e f l y on a recent development i n inclusion polymerization. As already discussed, this reaction can be carried out on the pure clathrate or i n the presence of an excess monomer. Consequently, the vapor pressure of a v o l a t i l e monomer during polymerization ranges from the decomposition pressure of the clathrate to the vapor pressure of the saturated solution of the host i n the guest, which i s generally very close to that of the pure l i q u i d monomer. For example, the vapor pressure



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of pure butadiene, one of the most v o l a t i l e guests, i s 14 torr at -75°C, the temperature at which polymerization i n urea i s usually carried out, and 2520 torr at +25°C, as i s more often used with PHTP. In these conditions decomposition pressure i s only 605 torr and polymerization s t i l l occurs below the atmospheric pressure. A d i f f e r e n t problem arises when simple monomers l i k e ethylene and propylene are subjected to polymerization. A l l e f f o r t s made so far have been unsuccessful, mainly because the correct conditions for inclusion were not known and for the widely diffused erroneous opinion that these molecules cannot act as guests f o r structural reasons. We directed our work to a knowledge of the decomposition curve of t h e i r PHTP clathrates by assuming that the inclusion compound exists i n the temperature range i n which the equilibrium pressure of the s o l i d phase i s lower than that of the pure monomer. For both guests the adduct has a d e f i n i t e existence and so conditions f o r polymerization could be determined (25). We succeeded i n polymerizing ethylene i n PHTP i n a steel tube at 20°C and 50 atm and propylene at 20°C and 10 atm. As f a r as we know these are the f i r s t examples of inclusion polymerization performed under high pressure. Polypropylene obtained i n PHTP i s predominantly syndiotactic (26). I t s microstructure was studied at the pentad l e v e l by NMR: stereosequence d i s t r i b u t i o n agrees with a f i r s t - o r d e r Markov chain with the following parameters: ρ = 0.238; ρ = 0.437. ESR rm mr experiments combined with the synthesis of an isoprene-b-propylene copolymer indicates that even i n the case of o l e f i n i c compounds inclusion polymerization proceeds by a r a d i c a l mechanism. In the case of propylene t h i s conclusion appears to be very peculiar, especially i f compared with a l l the other polymerization methods known f o r t h i s monomer.

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Figure 6. 25.2 MHz C NMR spectrum of the unsaturated region of 50:50 poly(2-methylpentadiene-co-4-methylpentadiene) prepared i n PHTP. Acknowledgments We are indebted to Consiglio Nazionale delle Ricerche (CNR), Rome, Italy and Ministero d e l l a Pubblica Istruzione, Rome, I t a l y , f o r f i n a n c i a l support.


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Crystallographically Ordered Polymers - American Chemical Society

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