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  • 5 Other Inorganic Polymers that ContainPhosphorus, Boron and Sulfur

    There are many other inorganic polymers that contain main group ele-ments in their backbone [1-3]. While some of these polymers were discov-ered long ago, some others are quite recent. Representative examples ofthese polymers that contain P-N-C, P-N-S, P-N-M, P-B, P-C, S-N and B-Nlinkages are shown in Fig. 5.1.

    Cl Cl . Ph H , . Ph

    =N P=N-f- + - P B

    M = C(CI); S(Ci); S(O)CI

    O OR

    S=N-r- 4-SN-4- 4-s=NP=NM-Jn I I Jn I I I JnR R

    B

    Fig. 5.1. Representative examples of polymers containing P-N-C, P-N-S, P-B, P-C, S-N and B-N linkages

    We will first examine polymers that contain P-N-C, P-N-S and P-N-Mlinkages. These are also called poly(heterophosphazene)s since they areprepared from the corresponding heterocyclophosphazenes.

    5.1 Poly(heterophosphazene)s

    Poly(heterophosphazene)s are a recently emerging polymer family thatcontain a heteroatom such as carbon or sulfur or a transition metal as part

  • 184 5 Other Inorganic Polymers that Contain Phosphorus, Boron and Sulfur

    of the polymer backbone. Examples of these types of polymers are shownin Fig. 5.2.

    ci i-N=P- -

    ci

    CI CI

    -C=NP=NP=N

    CI CI CI

    CI CI

    S=N P=NP=N- -:i ci ci

    S=NP=N

    CI

    O RII IS N PI IR R

    O CI CIII I IS = N P = N P = N

    CI CI CI

    CI Ph Ph

    -W=N P=N P = N C1 C l Ph Ph

    Fig. 5.2. Examples of poly(heterophosphazene)s. Poly(dichlorophosphazene) isalso shown for comparison

    Polymers [(NPCl2)2(NCCl)]n, [(NPCl2)2(NSCl)]n, [(NPCl2)2(NS(O)Cl)]n,[(NPCl2)2(NWCl3)]n contain one extra heteroatom viz., C, S or W in thesix-atom repeat unit of backbone of the polymer which otherwise containsphosphorus and nitrogen atoms. Thus, each of these hetero atoms replacesevery third phosphorus atom in the polyphosphazene backbone. Polymers[(SN)(NPCl2)2]n and [(NPR2)(NS(O)R)]n can be considered as alternatingcopolymers. The repeat unit of these polymers consist of alternate P=Nand S=N (or S(O)=N) units.

    How are these polymers assembled and what are their properties? Howare they different from polyphosphazenes? These aspects will be addressedin the following sections.

    5.2 Poly(carbophosphazene)s

    The six-membered heterocyclic ring, cyclocarbophosphazene,[(NPC12)2CC1] serves as the precursor for poly(carbophosphazene)s. Thepentachlorocyclocarbophosphazene N3P2CC15 can be considered as a hy-brid of cyanuric chloride, N3C3CI3 and hexachlorocyclotriphosphazene,N3P3C16. Thus, while cyanuric chloride has three carbons and the hexa-chlorocyclotriphosphazene has three phosphorus atoms, the cyclocarbo-phosphazene has one carbon and two phosphorus atoms (Fig. 5.3) [4, 5].

  • 5.2 Poly(carbophosphazene)s 185

    ci CI

    I\K NB| |B

    / A N A \

    c.

    -CI

    Fig. 5.3. Relationship between [(NPC12)2CC1], N3C3C13 andN3P3Cl6

    The X-ray crystal structure of [(NPC12)2CC1] has been determined. Theinorganic ring is nearly planar in spite of the mismatch of sizes betweenphosphorus and carbon. The C-N distance in [(NPC12)2CC1] is 1.33 ,compared to 1.33 found in N3C3C13. The other bond-parameters are alsonot very different. Thus, the P-N distances A (av. 1.58 ) and B (av.1.61) in [(NPC12)2CC1] (Fig. 5.3) are comparable to the P-N distance of 1.58A found in N3P3C16. The N-P-N (117.4) and P-N-P (115.8) angles in[(NPC12)2CC1] are normal while the N-C-N angle is quite wide (134).

    The preparation of [(NPC12)2CC1] is achieved by the condensation reac-tion of [Cl3P=N=PCl3]

    +[Cir with cyanamide, NC-NH2, in methylene chlo-ride. Cyanamide serves as the source of carbon and nitrogen. Although, in-termediate products have not been isolated, the formation of [(NPC12)2CC1]is believed to proceed through a condensation reaction to afford the inter-mediate C13PNPC12NCN. The latter adds hydrogen chloride across the C-Ntriple bond leading to the possible precursor C13PNPC12NC(C1)NH. Loss ofhydrogen chloride followed by ring closure leads to the carbophos-phazenes, [(NPC12)2CC1] (Fig. 5. 4) [4, 5].

    Cl

    N^CNH2

    CI

    \ l

    ' N

    CI3PN=PCI3 CI-2HCI CI2P"

    -HCI CI2P

    NH

    +2HCI

    CI

    Fig. 5.4. The mechanism of formation of [(NPC12)2CC1]

    The carbophosphazene [(NPC12)2CC1] undergoes nucleophilic substitu-tion reactions analogous to that found for N3P3C16. Even though the chem-istry of [(NPC12)2CC1] has not been studied to the same extent as that ofN3P3C16 the reactions shown in Fig. 5.5 indicate that many nucleophilessuch as primary and secondary amines, aniline, alkoxides, trifluoroethox-ide as well as phenoxide react to afford fully substituted products[NP(NHR)2C(NHR)], [NP(NR2)2C(NR2)], [NP(NHPh)2C(NHPh)],

  • 186 5 Other Inorganic Polymers that Contain Phosphorus, Boron and Sulfur

    [NP(OCH3)2C(OCH3)], [NP(OCH2CF3)2C(OCH2CF3)] and[NP(OPh)2C(OPh)]. This reactivity behavior suggests that similar nucleo-philic substitution reactions should be possible for the linear polymer[(NPCl2)2CCl]n also [6, 7].

    PhHN I1 '

    NHPh

    N

    \

    H3CO 1

    H3C0

    NHPh

    NHPh

    OCH2CF3

    NaOCH2CF3 CFaCHjOj ^OCHzCFa

    CF3CH2O OCH2CF3

    RHN NHR NR2

    Fig. 5.5. Reactions of [(NPCl^CCl] with various nucleophiles such as amines,alkoxides and aryloxides to afford fully substituted products

    The pentachlorocyclocarbophosphazene, [(NPC12)2CC1], has been foundto undergo ROP at 120 C to afford poly(chlorocarbophosphazene),[(NPCl2)2CCl]n (see Eq. 5.1) [4, 5].

    ?' (5-1)x^ r ci ci i

    120 Cck

    ci ' sci

    CII

    - - C = N P = N P = N I I ICI CI CI

    Note the considerable lowering of the polymerization temperature ofROP for [(NPC12)2CC1] in comparison to that observed for N3P3C16 (250C). Heating [(NPC12)2CC1] at higher temperatures (150-190 C) affords acrosslinked polymer. The lower temperature of ROP has been attributed toan increase in the ring strain suffered by [(NPC12)2CC1] as a result of re-placing the larger skeletal phosphorus of N3P3C16 by the smaller carbonatom.

    The polymer [(NPCl2)2CCl]n is hydrolytically sensitive and rapidly re-acts with moisture with concomitant chain degradation. Its 31P-NMR spec-trum consists of a single resonance observed -3.7 ppm. This is upfield

  • 5.2 Poly(carbophosphazene)s 187

    shifted in comparison to the cyclic precursor (+36.5 ppm). A similar shiftis observed for [NPCl2]n (8 = -18.4 ppm) in comparison to N3P3C16 (8 =+19.3 ppm). The observation of a single resonance in the 31P-NMR spec-trum suggests a head-tail arrangement in the polymer backbone (Fig. 5.6).Alternative arrangements such as head-head would have resulted in morenumber of 31P resonances.

    ciI

    CI CI CIC=NPNP=NC=NP=NP=N

    I I I I I ICI CI CI CI CI CI

    Head-Tail arrangement of the polymer

    CI CI CI CI

    C = N P = N P = N P = N P = N C : = N I I I I I ICI CI CI CI CI CI

    Head-Head arrangement of the polymer

    Fig. 5.6. Head-tail and head-head arrangements of [(NPQ2)2CCl]n

    The glass transition temperature of [(NPCl2)2CCl]n is -21 C. This isconsiderably higher than that observed for [NPCl2]n (Tg = -63 C).Thissuggests that skeletal rigidity increases upon replacing phosphorus by car-bon. One possible reason for this is the more efficient rc-bonding formedbetween C and N in comparison to P and N.

    The chlorine substituents of poly(chlorocarbophosphazene) can be re-placed with other nucleophiles to afford poly(organocarbophosphazene)swhich are hydrolytically more stable in comparison to the parent polymer(Fig. 5.7). There are no examples of poly(alkyl/arylcarbophosphazene)s.The molecular weights of poly(organocarbophosphazene)s are moderatelyhigh (Mw = 50,000-150,000).

    ci ciC=N P = N P = N I I ICI CI CI

    OPh OPh

    C = N P = N P = N I I IOPh OPh OPh

    NHPh NHPh

    C = N P = N P = N - -

    . NHPh NHPh NHPh

    Fig. 5.7. Formation of poly(organocarbophosphazene)s

  • 188 5 Other Inorganic Polymers that Contain Phosphorus, Boron and Sulfur

    The 31P-NMR data and glass transition temperatures of somepoly(organocarbophosphazene)s are given in Table 5.1. Notice the near in-variance of the 31P NMR chemical shifts for a given class of polymer.Thus, for example, the 31P-NMR chemical shifts for most aryloxy poly-mers are seen at about -10 ppm.

    The trends in the glass transition temperatures are similar to what havebeen observed in polyphosphazenes. Thus, replacing the chlorines witharyloxy substituents increases the Tg. One of the highest Jg's is observedfor the anilino derivative, [{NP(NHPh)2} {CNHPh}]n (+112 C).

    Table 5.1. 31P-NMR data and glass transition temperatures of selectedpoly(organocarbophosphazene)s

    S.No

    1

    2

    3

    4

    5

    6

    Polymerr Cl Cl !

    I I4 - C = N P = N P = N 4 -

    Cl Cl Cl

    I ?Ar ?Ar 14-C=NP=NP=N-4-

    OAr OAr OAr n

    Ar = Ph

    r OAr OAr -,I I

    -HC=NP=NP=N-j-

    OAr OAr OAr n

    Ar = -C6H4-p-CF3

    r NHPh NHPh-,

    4-c=N P=N P=N-f-

    NHPh NHPh NHPh "

    r NHR NHR I

    4-C=NP=NP=N-4-

    NHR NHR NHR n

    R = nPr

    r NR2 NR2 1^ C - N - P = N - P = N ^

    NR2 NR2 NR2 "

    R = CH3

    P

    -3.7

    -10.4

    -10.8

    -9.1

    6.7

    0.5

    ^ZkCJL.-21

    +18

    +31

    +112

    +74

    +28

  • 5.3 Poly(thiophosphazene)s 189

    5.3 Poly(thiophosphazene)s

    Poly(thiophosphazene)s contain S(IV) along with phosphorus and nitrogenatoms in their backbone [8, 9]. Similar to poly(dichlorophosphazene) andpoly(chlorocarbophosphazene), poly(chlorothiophosphazene) is also pre-pared by the ring-opening polymerization method. The monomer for thepreparation of this polymer is the pentachlorocyclothiophosphazene,N3P2SCI5.

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